diff --git a/src/Compilers/CSharp/Test/Emit/Emit/EditAndContinue/EditAndContinueClosureTests.cs b/src/Compilers/CSharp/Test/Emit/Emit/EditAndContinue/EditAndContinueClosureTests.cs
index 631a347e2793e..a618d8dcb2e43 100644
--- a/src/Compilers/CSharp/Test/Emit/Emit/EditAndContinue/EditAndContinueClosureTests.cs
+++ b/src/Compilers/CSharp/Test/Emit/Emit/EditAndContinue/EditAndContinueClosureTests.cs
@@ -67,6 +67,7 @@ static object F(object o)
                     Row(7, TableIndex.MethodDef, EditAndContinueOperation.Default),
                     Row(4, TableIndex.TypeDef, EditAndContinueOperation.AddMethod),
                     Row(8, TableIndex.MethodDef, EditAndContinueOperation.Default),
+                    Row(6, TableIndex.Param, EditAndContinueOperation.Default),
                     Row(4, TableIndex.CustomAttribute, EditAndContinueOperation.Default),
                     Row(1, TableIndex.NestedClass, EditAndContinueOperation.Default));
             }
@@ -185,7 +186,8 @@ int F(object o)
             CheckEncLogDefinitions(reader1,
                 Row(2, TableIndex.StandAloneSig, EditAndContinueOperation.Default),
                 Row(1, TableIndex.MethodDef, EditAndContinueOperation.Default),
-                Row(4, TableIndex.MethodDef, EditAndContinueOperation.Default));
+                Row(4, TableIndex.MethodDef, EditAndContinueOperation.Default),
+                Row(1, TableIndex.Param, EditAndContinueOperation.Default));
         }
 
         [Fact]
@@ -257,7 +259,8 @@ int F(object o)
                 Row(4, TableIndex.StandAloneSig, EditAndContinueOperation.Default),
                 Row(1, TableIndex.MethodDef, EditAndContinueOperation.Default),
                 Row(4, TableIndex.MethodDef, EditAndContinueOperation.Default),
-                Row(6, TableIndex.MethodDef, EditAndContinueOperation.Default));
+                Row(6, TableIndex.MethodDef, EditAndContinueOperation.Default),
+                Row(1, TableIndex.Param, EditAndContinueOperation.Default));
         }
 
         [Fact]
@@ -532,6 +535,7 @@ int F(int a)
                 Row(2, TableIndex.StandAloneSig, EditAndContinueOperation.Default),
                 Row(1, TableIndex.MethodDef, EditAndContinueOperation.Default),
                 Row(3, TableIndex.MethodDef, EditAndContinueOperation.Default),
+                Row(1, TableIndex.Param, EditAndContinueOperation.Default),
                 Row(4, TableIndex.CustomAttribute, EditAndContinueOperation.Default));
         }
 
@@ -586,6 +590,7 @@ int F(int a)
                 Row(2, TableIndex.StandAloneSig, EditAndContinueOperation.Default),
                 Row(1, TableIndex.MethodDef, EditAndContinueOperation.Default),
                 Row(3, TableIndex.MethodDef, EditAndContinueOperation.Default),
+                Row(1, TableIndex.Param, EditAndContinueOperation.Default),
                 Row(4, TableIndex.CustomAttribute, EditAndContinueOperation.Default));
         }
 
@@ -641,7 +646,8 @@ int F(int a)
             CheckEncLogDefinitions(reader1,
                 Row(2, TableIndex.StandAloneSig, EditAndContinueOperation.Default),
                 Row(1, TableIndex.MethodDef, EditAndContinueOperation.Default),
-                Row(4, TableIndex.MethodDef, EditAndContinueOperation.Default));
+                Row(4, TableIndex.MethodDef, EditAndContinueOperation.Default),
+                Row(1, TableIndex.Param, EditAndContinueOperation.Default));
         }
 
         [Fact]
@@ -786,6 +792,7 @@ int F(int a)
                 Row(2, TableIndex.StandAloneSig, EditAndContinueOperation.Default),
                 Row(1, TableIndex.MethodDef, EditAndContinueOperation.Default),
                 Row(3, TableIndex.MethodDef, EditAndContinueOperation.Default),
+                Row(1, TableIndex.Param, EditAndContinueOperation.Default),
                 Row(4, TableIndex.CustomAttribute, EditAndContinueOperation.Default));
         }
 
@@ -866,7 +873,9 @@ class C : D
                 Row(8, TableIndex.MethodDef, EditAndContinueOperation.Default),
                 Row(9, TableIndex.MethodDef, EditAndContinueOperation.Default),
                 Row(10, TableIndex.MethodDef, EditAndContinueOperation.Default),
-                Row(11, TableIndex.MethodDef, EditAndContinueOperation.Default));
+                Row(11, TableIndex.MethodDef, EditAndContinueOperation.Default),
+                Row(2, TableIndex.Param, EditAndContinueOperation.Default),
+                Row(3, TableIndex.Param, EditAndContinueOperation.Default));
         }
 
         [Fact]
@@ -990,7 +999,14 @@ void F()
                 Row(18, TableIndex.MethodDef, EditAndContinueOperation.Default),
                 Row(19, TableIndex.MethodDef, EditAndContinueOperation.Default),
                 Row(20, TableIndex.MethodDef, EditAndContinueOperation.Default),
-                Row(21, TableIndex.MethodDef, EditAndContinueOperation.Default));
+                Row(21, TableIndex.MethodDef, EditAndContinueOperation.Default),
+                Row(6, TableIndex.Param, EditAndContinueOperation.Default),
+                Row(7, TableIndex.Param, EditAndContinueOperation.Default),
+                Row(8, TableIndex.Param, EditAndContinueOperation.Default),
+                Row(9, TableIndex.Param, EditAndContinueOperation.Default),
+                Row(10, TableIndex.Param, EditAndContinueOperation.Default),
+                Row(11, TableIndex.Param, EditAndContinueOperation.Default),
+                Row(12, TableIndex.Param, EditAndContinueOperation.Default));
         }
 
         [Fact]
@@ -1076,6 +1092,12 @@ void F()
                 Row(18, TableIndex.MethodDef, EditAndContinueOperation.Default),
                 Row(19, TableIndex.MethodDef, EditAndContinueOperation.Default),
                 Row(20, TableIndex.MethodDef, EditAndContinueOperation.Default),
+                Row(5, TableIndex.Param, EditAndContinueOperation.Default),
+                Row(6, TableIndex.Param, EditAndContinueOperation.Default),
+                Row(7, TableIndex.Param, EditAndContinueOperation.Default),
+                Row(8, TableIndex.Param, EditAndContinueOperation.Default),
+                Row(9, TableIndex.Param, EditAndContinueOperation.Default),
+                Row(10, TableIndex.Param, EditAndContinueOperation.Default),
                 Row(15, TableIndex.CustomAttribute, EditAndContinueOperation.Default),
                 Row(16, TableIndex.CustomAttribute, EditAndContinueOperation.Default));
         }
@@ -1153,6 +1175,8 @@ void F()
                 Row(10, TableIndex.MethodDef, EditAndContinueOperation.Default),
                 Row(12, TableIndex.MethodDef, EditAndContinueOperation.Default),
                 Row(15, TableIndex.MethodDef, EditAndContinueOperation.Default),
+                Row(5, TableIndex.Param, EditAndContinueOperation.Default),
+                Row(6, TableIndex.Param, EditAndContinueOperation.Default),
                 Row(14, TableIndex.CustomAttribute, EditAndContinueOperation.Default));
         }
 
@@ -1240,6 +1264,10 @@ public void F()
                 Row(8, TableIndex.MethodDef, EditAndContinueOperation.Default),
                 Row(10, TableIndex.MethodDef, EditAndContinueOperation.Default),
                 Row(12, TableIndex.MethodDef, EditAndContinueOperation.Default),
+                Row(2, TableIndex.Param, EditAndContinueOperation.Default),
+                Row(3, TableIndex.Param, EditAndContinueOperation.Default),
+                Row(4, TableIndex.Param, EditAndContinueOperation.Default),
+                Row(5, TableIndex.Param, EditAndContinueOperation.Default),
                 Row(5, TableIndex.CustomAttribute, EditAndContinueOperation.Default),
                 Row(7, TableIndex.CustomAttribute, EditAndContinueOperation.Default),
                 Row(9, TableIndex.CustomAttribute, EditAndContinueOperation.Default));
@@ -1322,6 +1350,10 @@ public void F()
                 Row(6, TableIndex.MethodDef, EditAndContinueOperation.Default),
                 Row(9, TableIndex.MethodDef, EditAndContinueOperation.Default),
                 Row(10, TableIndex.MethodDef, EditAndContinueOperation.Default),
+                Row(2, TableIndex.Param, EditAndContinueOperation.Default),
+                Row(3, TableIndex.Param, EditAndContinueOperation.Default),
+                Row(4, TableIndex.Param, EditAndContinueOperation.Default),
+                Row(5, TableIndex.Param, EditAndContinueOperation.Default),
                 Row(5, TableIndex.CustomAttribute, EditAndContinueOperation.Default));
         }
 
@@ -1399,6 +1431,8 @@ public void F()
                 Row(4, TableIndex.MethodDef, EditAndContinueOperation.Default),
                 Row(6, TableIndex.MethodDef, EditAndContinueOperation.Default),
                 Row(9, TableIndex.MethodDef, EditAndContinueOperation.Default),
+                Row(2, TableIndex.Param, EditAndContinueOperation.Default),
+                Row(3, TableIndex.Param, EditAndContinueOperation.Default),
                 Row(5, TableIndex.CustomAttribute, EditAndContinueOperation.Default));
         }
 
@@ -2635,7 +2669,9 @@ static object F()
                 Row(2, TableIndex.StandAloneSig, EditAndContinueOperation.Default),
                 Row(2, TableIndex.MethodDef, EditAndContinueOperation.Default),
                 Row(6, TableIndex.MethodDef, EditAndContinueOperation.Default),
-                Row(7, TableIndex.MethodDef, EditAndContinueOperation.Default));
+                Row(7, TableIndex.MethodDef, EditAndContinueOperation.Default),
+                Row(2, TableIndex.Param, EditAndContinueOperation.Default),
+                Row(3, TableIndex.Param, EditAndContinueOperation.Default));
         }
 
         [Fact]
@@ -4235,7 +4271,8 @@ .maxstack  2
                 Row(2, TableIndex.StandAloneSig, EditAndContinueOperation.Default),
                 Row(1, TableIndex.MethodDef, EditAndContinueOperation.Default),
                 Row(4, TableIndex.MethodDef, EditAndContinueOperation.Default),
-                Row(5, TableIndex.MethodDef, EditAndContinueOperation.Default));
+                Row(5, TableIndex.MethodDef, EditAndContinueOperation.Default),
+                Row(1, TableIndex.Param, EditAndContinueOperation.Default));
         }
 
         [Fact]
@@ -4323,7 +4360,8 @@ .maxstack  2
                 Row(2, TableIndex.StandAloneSig, EditAndContinueOperation.Default),
                 Row(1, TableIndex.MethodDef, EditAndContinueOperation.Default),
                 Row(4, TableIndex.MethodDef, EditAndContinueOperation.Default),
-                Row(5, TableIndex.MethodDef, EditAndContinueOperation.Default));
+                Row(5, TableIndex.MethodDef, EditAndContinueOperation.Default),
+                Row(1, TableIndex.Param, EditAndContinueOperation.Default));
         }
 
         [Fact]
@@ -4429,7 +4467,8 @@ .maxstack  2
                 Row(1, TableIndex.MethodDef, EditAndContinueOperation.Default),
                 Row(4, TableIndex.MethodDef, EditAndContinueOperation.Default),
                 Row(6, TableIndex.MethodDef, EditAndContinueOperation.Default),
-                Row(7, TableIndex.MethodDef, EditAndContinueOperation.Default));
+                Row(7, TableIndex.MethodDef, EditAndContinueOperation.Default),
+                Row(1, TableIndex.Param, EditAndContinueOperation.Default));
         }
 
         [Fact]
@@ -4478,7 +4517,8 @@ public void TopLevelStatement_Closure()
             CheckEncLogDefinitions(reader1,
                 Row(2, TableIndex.StandAloneSig, EditAndContinueOperation.Default),
                 Row(1, TableIndex.MethodDef, EditAndContinueOperation.Default),
-                Row(3, TableIndex.MethodDef, EditAndContinueOperation.Default));
+                Row(3, TableIndex.MethodDef, EditAndContinueOperation.Default),
+                Row(1, TableIndex.Param, EditAndContinueOperation.Default));
         }
     }
 }
diff --git a/src/Compilers/CSharp/Test/Emit/Emit/EditAndContinue/EditAndContinueStateMachineTests.cs b/src/Compilers/CSharp/Test/Emit/Emit/EditAndContinue/EditAndContinueStateMachineTests.cs
index d0cf473e1e686..7e8adb0b172ed 100644
--- a/src/Compilers/CSharp/Test/Emit/Emit/EditAndContinue/EditAndContinueStateMachineTests.cs
+++ b/src/Compilers/CSharp/Test/Emit/Emit/EditAndContinue/EditAndContinueStateMachineTests.cs
@@ -637,6 +637,9 @@ static async Task<int> F(int a)
                         Row(6, TableIndex.MethodDef, EditAndContinueOperation.Default),
                         Row(9, TableIndex.MethodDef, EditAndContinueOperation.Default),
                         Row(12, TableIndex.MethodDef, EditAndContinueOperation.Default),
+                        Row(1, TableIndex.Param, EditAndContinueOperation.Default),
+                        Row(2, TableIndex.Param, EditAndContinueOperation.Default),
+                        Row(3, TableIndex.Param, EditAndContinueOperation.Default),
                         Row(1, TableIndex.CustomAttribute, EditAndContinueOperation.Default),
                         Row(2, TableIndex.CustomAttribute, EditAndContinueOperation.Default),
                         Row(6, TableIndex.CustomAttribute, EditAndContinueOperation.Default),
@@ -1107,6 +1110,7 @@ static IEnumerable<int> F(int p)
                         Row(1, TableIndex.MethodDef, EditAndContinueOperation.Default),
                         Row(4, TableIndex.MethodDef, EditAndContinueOperation.Default),
                         Row(5, TableIndex.MethodDef, EditAndContinueOperation.Default),
+                        Row(1, TableIndex.Param, EditAndContinueOperation.Default),
                         Row(1, TableIndex.CustomAttribute, EditAndContinueOperation.Default),
                         Row(7, TableIndex.CustomAttribute, EditAndContinueOperation.Default));
 
@@ -1211,6 +1215,7 @@ static IEnumerable<int> F(int p)
                         Row(1, TableIndex.MethodDef, EditAndContinueOperation.Default),
                         Row(4, TableIndex.MethodDef, EditAndContinueOperation.Default),
                         Row(5, TableIndex.MethodDef, EditAndContinueOperation.Default),
+                        Row(1, TableIndex.Param, EditAndContinueOperation.Default),
                         Row(1, TableIndex.CustomAttribute, EditAndContinueOperation.Default),
                         Row(7, TableIndex.CustomAttribute, EditAndContinueOperation.Default));
 
@@ -1322,6 +1327,7 @@ static IEnumerable<int> F(int p)
                         Row(1, TableIndex.MethodDef, EditAndContinueOperation.Default),
                         Row(4, TableIndex.MethodDef, EditAndContinueOperation.Default),
                         Row(5, TableIndex.MethodDef, EditAndContinueOperation.Default),
+                        Row(1, TableIndex.Param, EditAndContinueOperation.Default),
                         Row(1, TableIndex.CustomAttribute, EditAndContinueOperation.Default),
                         Row(7, TableIndex.CustomAttribute, EditAndContinueOperation.Default));
 
diff --git a/src/Compilers/CSharp/Test/Emit/Emit/EditAndContinue/EditAndContinueTests.cs b/src/Compilers/CSharp/Test/Emit/Emit/EditAndContinue/EditAndContinueTests.cs
index 429fe7ebad6c4..2edb50bd5a7f4 100644
--- a/src/Compilers/CSharp/Test/Emit/Emit/EditAndContinue/EditAndContinueTests.cs
+++ b/src/Compilers/CSharp/Test/Emit/Emit/EditAndContinue/EditAndContinueTests.cs
@@ -1101,6 +1101,221 @@ static void Main() { }
                 Handle(2, TableIndex.AssemblyRef));
         }
 
+        [Fact]
+        public void ModifyMethod_ParameterAttributes()
+        {
+            var source0 =
+@"class C
+{
+    static void Main() { }
+    static string F(string input, int a) { return input; }
+}";
+            var source1 =
+@"class C
+{
+    static void Main() { }
+    static string F([System.ComponentModel.Description(""input"")]string input, int a) { return input; }
+    static void G(string input) { }
+}";
+            var source2 =
+@"class C
+{
+    static void Main() { }
+    static string F([System.ComponentModel.Description(""input"")]string input, int a) { return input; }
+    static void G([System.ComponentModel.Description(""input"")]string input) { }
+}";
+
+            var compilation0 = CreateCompilation(source0, options: TestOptions.DebugExe, targetFramework: TargetFramework.NetStandard20);
+            var compilation1 = compilation0.WithSource(source1);
+            var compilation2 = compilation1.WithSource(source2);
+
+            var methodF0 = compilation0.GetMember<MethodSymbol>("C.F");
+            var methodF1 = compilation1.GetMember<MethodSymbol>("C.F");
+            var methodG1 = compilation1.GetMember<MethodSymbol>("C.G");
+            var methodG2 = compilation2.GetMember<MethodSymbol>("C.G");
+
+            // Verify full metadata contains expected rows.
+            var bytes0 = compilation0.EmitToArray();
+            using var md0 = ModuleMetadata.CreateFromImage(bytes0);
+            var reader0 = md0.MetadataReader;
+
+            CheckNames(reader0, reader0.GetTypeDefNames(), "<Module>", "C");
+            CheckNames(reader0, reader0.GetMethodDefNames(), "Main", "F", ".ctor");
+            CheckNames(reader0, reader0.GetMemberRefNames(), /*CompilationRelaxationsAttribute.*/".ctor", /*RuntimeCompatibilityAttribute.*/".ctor", /*Object.*/".ctor", /*DebuggableAttribute*/".ctor");
+            CheckNames(reader0, reader0.GetParameterDefNames(), "input", "a");
+
+            CheckAttributes(reader0,
+                new CustomAttributeRow(Handle(1, TableIndex.Assembly), Handle(1, TableIndex.MemberRef)),
+                new CustomAttributeRow(Handle(1, TableIndex.Assembly), Handle(2, TableIndex.MemberRef)),
+                new CustomAttributeRow(Handle(1, TableIndex.Assembly), Handle(3, TableIndex.MemberRef)));
+
+            var generation0 = EmitBaseline.CreateInitialBaseline(
+               md0,
+               EmptyLocalsProvider);
+
+            var diff1 = compilation1.EmitDifference(
+                generation0,
+                ImmutableArray.Create(
+                    SemanticEdit.Create(SemanticEditKind.Update, methodF0, methodF1),
+                    SemanticEdit.Create(SemanticEditKind.Insert, null, methodG1)));
+
+            // Verify delta metadata contains expected rows.
+            using var md1 = diff1.GetMetadata();
+            var reader1 = md1.Reader;
+            var readers = new[] { reader0, reader1 };
+
+            EncValidation.VerifyModuleMvid(1, reader0, reader1);
+
+            CheckNames(readers, reader1.GetTypeDefNames());
+            CheckNames(readers, reader1.GetMethodDefNames(), "F", "G");
+            CheckNames(readers, reader1.GetMemberRefNames(), /*DescriptionAttribute*/".ctor");
+            CheckNames(readers, reader1.GetParameterDefNames(), "input", "a", "input");
+
+            CheckAttributes(reader1,
+                 new CustomAttributeRow(Handle(1, TableIndex.Param), Handle(5, TableIndex.MemberRef)));
+
+            CheckEncLog(reader1,
+                Row(2, TableIndex.AssemblyRef, EditAndContinueOperation.Default),
+                Row(5, TableIndex.MemberRef, EditAndContinueOperation.Default),
+                Row(6, TableIndex.TypeRef, EditAndContinueOperation.Default),
+                Row(7, TableIndex.TypeRef, EditAndContinueOperation.Default),
+                Row(2, TableIndex.StandAloneSig, EditAndContinueOperation.Default),
+                Row(2, TableIndex.MethodDef, EditAndContinueOperation.Default),
+                Row(2, TableIndex.TypeDef, EditAndContinueOperation.AddMethod),
+                Row(4, TableIndex.MethodDef, EditAndContinueOperation.Default),             // New method, G
+                Row(1, TableIndex.Param, EditAndContinueOperation.Default),                 // Update existing param
+                Row(2, TableIndex.Param, EditAndContinueOperation.Default),                 // Update existing param
+                Row(4, TableIndex.MethodDef, EditAndContinueOperation.AddParameter),        // New param on method, G
+                Row(3, TableIndex.Param, EditAndContinueOperation.Default),                 // Support for the above
+                Row(4, TableIndex.CustomAttribute, EditAndContinueOperation.Default));
+
+            CheckEncMap(reader1,
+                Handle(6, TableIndex.TypeRef),
+                Handle(7, TableIndex.TypeRef),
+                Handle(2, TableIndex.MethodDef),
+                Handle(4, TableIndex.MethodDef),
+                Handle(1, TableIndex.Param),
+                Handle(2, TableIndex.Param),
+                Handle(3, TableIndex.Param),
+                Handle(5, TableIndex.MemberRef),
+                Handle(4, TableIndex.CustomAttribute),
+                Handle(2, TableIndex.StandAloneSig),
+                Handle(2, TableIndex.AssemblyRef));
+
+            var diff2 = compilation2.EmitDifference(
+              diff1.NextGeneration,
+              ImmutableArray.Create(
+                  SemanticEdit.Create(SemanticEditKind.Update, methodG1, methodG2)));
+
+            using var md2 = diff2.GetMetadata();
+            var reader2 = md2.Reader;
+            readers = new[] { reader0, reader1, reader2 };
+
+            EncValidation.VerifyModuleMvid(2, reader1, reader2);
+
+            CheckNames(readers, reader2.GetTypeDefNames());
+            CheckNames(readers, reader2.GetMethodDefNames(), "G");
+            CheckNames(readers, reader2.GetMemberRefNames(), /*DescriptionAttribute*/".ctor");
+            CheckNames(readers, reader2.GetParameterDefNames(), "input");
+
+            CheckAttributes(reader2,
+                 new CustomAttributeRow(Handle(3, TableIndex.Param), Handle(6, TableIndex.MemberRef)));
+
+            CheckEncLog(reader2,
+                Row(3, TableIndex.AssemblyRef, EditAndContinueOperation.Default),
+                Row(6, TableIndex.MemberRef, EditAndContinueOperation.Default),
+                Row(8, TableIndex.TypeRef, EditAndContinueOperation.Default),
+                Row(9, TableIndex.TypeRef, EditAndContinueOperation.Default),
+                Row(4, TableIndex.MethodDef, EditAndContinueOperation.Default),
+                Row(3, TableIndex.Param, EditAndContinueOperation.Default),             // Update existing param, from the first delta
+                Row(5, TableIndex.CustomAttribute, EditAndContinueOperation.Default));
+
+            CheckEncMap(reader2,
+                Handle(8, TableIndex.TypeRef),
+                Handle(9, TableIndex.TypeRef),
+                Handle(4, TableIndex.MethodDef),
+                Handle(3, TableIndex.Param),
+                Handle(6, TableIndex.MemberRef),
+                Handle(5, TableIndex.CustomAttribute),
+                Handle(3, TableIndex.AssemblyRef));
+        }
+
+        [Fact]
+        public void ModifyDelegateInvokeMethod_AddAttributes()
+        {
+            var source0 = @"
+class A : System.Attribute { }
+delegate void D(int x);
+";
+            var source1 = @"
+class A : System.Attribute { }
+delegate void D([A]int x);
+";
+
+            var compilation0 = CreateCompilation(source0, options: TestOptions.DebugDll, targetFramework: TargetFramework.NetStandard20);
+            var compilation1 = compilation0.WithSource(source1);
+
+            var invoke0 = compilation0.GetMember<MethodSymbol>("D.Invoke");
+            var beginInvoke0 = compilation0.GetMember<MethodSymbol>("D.BeginInvoke");
+            var invoke1 = compilation1.GetMember<MethodSymbol>("D.Invoke");
+            var beginInvoke1 = compilation1.GetMember<MethodSymbol>("D.BeginInvoke");
+
+            // Verify full metadata contains expected rows.
+            var bytes0 = compilation0.EmitToArray();
+            using var md0 = ModuleMetadata.CreateFromImage(bytes0);
+            var reader0 = md0.MetadataReader;
+
+            var generation0 = EmitBaseline.CreateInitialBaseline(md0, EmptyLocalsProvider);
+
+            var diff1 = compilation1.EmitDifference(
+                generation0,
+                ImmutableArray.Create(
+                    SemanticEdit.Create(SemanticEditKind.Update, invoke0, invoke1),
+                    SemanticEdit.Create(SemanticEditKind.Update, beginInvoke0, beginInvoke1)));
+
+            // Verify delta metadata contains expected rows.
+            using var md1 = diff1.GetMetadata();
+            var reader1 = md1.Reader;
+            var readers = new[] { reader0, reader1 };
+
+            CheckNames(readers, reader1.GetMethodDefNames(), "Invoke", "BeginInvoke");
+            CheckNames(readers, reader1.GetParameterDefNames(), "x", "x", "callback", "object");
+
+            CheckAttributes(reader1,
+                new CustomAttributeRow(Handle(3, TableIndex.Param), Handle(1, TableIndex.MethodDef)),
+                new CustomAttributeRow(Handle(4, TableIndex.Param), Handle(1, TableIndex.MethodDef)));
+
+            CheckEncLog(reader1,
+                Row(2, TableIndex.AssemblyRef, EditAndContinueOperation.Default),
+                Row(10, TableIndex.TypeRef, EditAndContinueOperation.Default),
+                Row(11, TableIndex.TypeRef, EditAndContinueOperation.Default),
+                Row(12, TableIndex.TypeRef, EditAndContinueOperation.Default),
+                Row(13, TableIndex.TypeRef, EditAndContinueOperation.Default),
+                Row(3, TableIndex.MethodDef, EditAndContinueOperation.Default),
+                Row(4, TableIndex.MethodDef, EditAndContinueOperation.Default),
+                Row(3, TableIndex.Param, EditAndContinueOperation.Default),             // Updating existing parameter defs
+                Row(4, TableIndex.Param, EditAndContinueOperation.Default),
+                Row(5, TableIndex.Param, EditAndContinueOperation.Default),
+                Row(6, TableIndex.Param, EditAndContinueOperation.Default),
+                Row(4, TableIndex.CustomAttribute, EditAndContinueOperation.Default),   // Adding new custom attribute rows
+                Row(5, TableIndex.CustomAttribute, EditAndContinueOperation.Default));
+
+            CheckEncMap(reader1,
+                Handle(10, TableIndex.TypeRef),
+                Handle(11, TableIndex.TypeRef),
+                Handle(12, TableIndex.TypeRef),
+                Handle(13, TableIndex.TypeRef),
+                Handle(3, TableIndex.MethodDef),
+                Handle(4, TableIndex.MethodDef),
+                Handle(3, TableIndex.Param),
+                Handle(4, TableIndex.Param),
+                Handle(5, TableIndex.Param),
+                Handle(6, TableIndex.Param),
+                Handle(4, TableIndex.CustomAttribute),
+                Handle(5, TableIndex.CustomAttribute),
+                Handle(2, TableIndex.AssemblyRef));
+        }
+
         /// <summary>
         /// Add a method that requires entries in the ParameterDefs table.
         /// Specifically, normal parameters or return types with attributes.
@@ -1189,16 +1404,20 @@ class C
             EncValidation.VerifyModuleMvid(2, reader1, reader2);
 
             CheckNames(readers, reader2.GetTypeDefNames());
-            CheckNames(readers, reader2.GetMethodDefNames(), "F");
-            CheckNames(readers, reader2.GetParameterDefNames());
-            CheckNames(readers, diff2.EmitResult.ChangedTypes, "C");
-            CheckNames(readers, diff2.EmitResult.UpdatedMethods, "F");
 
             CheckEncLogDefinitions(reader2,
-                Row(3, TableIndex.MethodDef, EditAndContinueOperation.Default)); // C.F
+                Row(3, TableIndex.MethodDef, EditAndContinueOperation.Default), // C.F2
+                Row(1, TableIndex.Param, EditAndContinueOperation.Default),
+                Row(2, TableIndex.Param, EditAndContinueOperation.Default),
+                Row(2, TableIndex.Constant, EditAndContinueOperation.Default),
+                Row(4, TableIndex.CustomAttribute, EditAndContinueOperation.Default));
 
             CheckEncMapDefinitions(reader2,
-                Handle(3, TableIndex.MethodDef));
+                Handle(3, TableIndex.MethodDef),
+                Handle(1, TableIndex.Param),
+                Handle(2, TableIndex.Param),
+                Handle(2, TableIndex.Constant),
+                Handle(4, TableIndex.CustomAttribute));
         }
 
         [Fact]
@@ -1423,8 +1642,10 @@ .maxstack  4
                 Row(10, TableIndex.MethodDef, EditAndContinueOperation.Default),
                 Row(2, TableIndex.TypeDef, EditAndContinueOperation.AddMethod),
                 Row(11, TableIndex.MethodDef, EditAndContinueOperation.Default),
+                Row(3, TableIndex.Param, EditAndContinueOperation.Default),
                 Row(11, TableIndex.MethodDef, EditAndContinueOperation.AddParameter),
                 Row(4, TableIndex.Param, EditAndContinueOperation.Default),
+                Row(6, TableIndex.CustomAttribute, EditAndContinueOperation.Default),
                 Row(11, TableIndex.CustomAttribute, EditAndContinueOperation.Default),
                 Row(12, TableIndex.CustomAttribute, EditAndContinueOperation.Default),
                 Row(13, TableIndex.CustomAttribute, EditAndContinueOperation.Default),
@@ -1451,10 +1672,12 @@ .maxstack  4
                 Handle(9, TableIndex.MethodDef),
                 Handle(10, TableIndex.MethodDef),
                 Handle(11, TableIndex.MethodDef),
+                Handle(3, TableIndex.Param),
                 Handle(4, TableIndex.Param),
                 Handle(7, TableIndex.MemberRef),
                 Handle(8, TableIndex.MemberRef),
                 Handle(9, TableIndex.MemberRef),
+                Handle(6, TableIndex.CustomAttribute),
                 Handle(11, TableIndex.CustomAttribute),
                 Handle(12, TableIndex.CustomAttribute),
                 Handle(13, TableIndex.CustomAttribute),
@@ -2158,11 +2381,15 @@ class C
             CheckEncLogDefinitions(reader3,
                 Row(4, TableIndex.TypeDef, EditAndContinueOperation.Default),
                 Row(5, TableIndex.MethodDef, EditAndContinueOperation.Default),
+                Row(4, TableIndex.Param, EditAndContinueOperation.Default),
+                Row(5, TableIndex.Param, EditAndContinueOperation.Default),
                 Row(4, TableIndex.CustomAttribute, EditAndContinueOperation.Default));
 
             CheckEncMapDefinitions(reader3,
                 Handle(4, TableIndex.TypeDef),
                 Handle(5, TableIndex.MethodDef),
+                Handle(4, TableIndex.Param),
+                Handle(5, TableIndex.Param),
                 Handle(4, TableIndex.CustomAttribute));
         }
 
@@ -2874,6 +3101,12 @@ interface J { }
                 Row(13, TableIndex.MethodDef, EditAndContinueOperation.Default),
                 Row(1, TableIndex.Property, EditAndContinueOperation.Default),
                 Row(2, TableIndex.Property, EditAndContinueOperation.Default),
+                Row(3, TableIndex.Param, EditAndContinueOperation.Default),
+                Row(4, TableIndex.Param, EditAndContinueOperation.Default),
+                Row(5, TableIndex.Param, EditAndContinueOperation.Default),
+                Row(6, TableIndex.Param, EditAndContinueOperation.Default),
+                Row(7, TableIndex.Param, EditAndContinueOperation.Default),
+                Row(8, TableIndex.Param, EditAndContinueOperation.Default),
                 Row(11, TableIndex.MethodSemantics, EditAndContinueOperation.Default),
                 Row(12, TableIndex.MethodSemantics, EditAndContinueOperation.Default),
                 Row(13, TableIndex.MethodSemantics, EditAndContinueOperation.Default),
@@ -11443,13 +11676,15 @@ protected virtual bool PrintMembers(System.Text.StringBuilder builder)
                 Row(22, TableIndex.TypeRef, EditAndContinueOperation.Default),
                 Row(4, TableIndex.TypeSpec, EditAndContinueOperation.Default),
                 Row(3, TableIndex.StandAloneSig, EditAndContinueOperation.Default),
-                Row(10, TableIndex.MethodDef, EditAndContinueOperation.Default)); // R.PrintMembers
+                Row(10, TableIndex.MethodDef, EditAndContinueOperation.Default), // R.PrintMembers
+                Row(3, TableIndex.Param, EditAndContinueOperation.Default));
 
             CheckEncMap(reader1,
                 Handle(20, TableIndex.TypeRef),
                 Handle(21, TableIndex.TypeRef),
                 Handle(22, TableIndex.TypeRef),
                 Handle(10, TableIndex.MethodDef),
+                Handle(3, TableIndex.Param),
                 Handle(3, TableIndex.StandAloneSig),
                 Handle(4, TableIndex.TypeSpec),
                 Handle(2, TableIndex.AssemblyRef));
@@ -11559,13 +11794,15 @@ public void TopLevelStatement_Update()
                 Row(8, TableIndex.TypeRef, EditAndContinueOperation.Default),
                 Row(9, TableIndex.TypeRef, EditAndContinueOperation.Default),
                 Row(10, TableIndex.TypeRef, EditAndContinueOperation.Default),
-                Row(1, TableIndex.MethodDef, EditAndContinueOperation.Default)); // Synthesized Main method
+                Row(1, TableIndex.MethodDef, EditAndContinueOperation.Default), // Synthesized Main method
+                Row(1, TableIndex.Param, EditAndContinueOperation.Default));
 
             CheckEncMap(reader1,
                 Handle(8, TableIndex.TypeRef),
                 Handle(9, TableIndex.TypeRef),
                 Handle(10, TableIndex.TypeRef),
                 Handle(1, TableIndex.MethodDef),
+                Handle(1, TableIndex.Param),
                 Handle(6, TableIndex.MemberRef),
                 Handle(7, TableIndex.MemberRef),
                 Handle(2, TableIndex.AssemblyRef));
diff --git a/src/Compilers/Core/Portable/Emit/EditAndContinue/DeltaMetadataWriter.cs b/src/Compilers/Core/Portable/Emit/EditAndContinue/DeltaMetadataWriter.cs
index c25c01dfb5df4..fbaf49e743f14 100644
--- a/src/Compilers/Core/Portable/Emit/EditAndContinue/DeltaMetadataWriter.cs
+++ b/src/Compilers/Core/Portable/Emit/EditAndContinue/DeltaMetadataWriter.cs
@@ -35,8 +35,8 @@ internal sealed class DeltaMetadataWriter : MetadataWriter
         private readonly DefinitionIndex<IFieldDefinition> _fieldDefs;
         private readonly DefinitionIndex<IMethodDefinition> _methodDefs;
         private readonly DefinitionIndex<IPropertyDefinition> _propertyDefs;
-        private readonly ParameterDefinitionIndex _parameterDefs;
-        private readonly List<KeyValuePair<IMethodDefinition, IParameterDefinition>> _parameterDefList;
+        private readonly DefinitionIndex<IParameterDefinition> _parameterDefs;
+        private readonly Dictionary<IParameterDefinition, IMethodDefinition> _parameterDefList;
         private readonly GenericParameterIndex _genericParameters;
         private readonly EventOrPropertyMapIndex _eventMap;
         private readonly EventOrPropertyMapIndex _propertyMap;
@@ -46,14 +46,13 @@ internal sealed class DeltaMetadataWriter : MetadataWriter
         // correctly map the attributes to row numbers of existing attributes for that target
         private readonly Dictionary<EntityHandle, int> _customAttributeParentCounts;
 
-        // For the EncMap table we need to keep a list of exactly which rows in the CustomAttributes table are updated
-        // since we spread these out amongst existing rows, it's not just a contiguous set
-        private readonly List<int> _customAttributeEncMapRows;
-
         // Keep track of which CustomAttributes rows are added in this and previous deltas, over what is in the
         // original metadata
         private readonly Dictionary<EntityHandle, ImmutableArray<int>> _customAttributesAdded;
 
+        private readonly Dictionary<IParameterDefinition, int> _existingParameterDefs;
+        private readonly Dictionary<MethodDefinitionHandle, int> _firstParamRowMap;
+
         private readonly HeapOrReferenceIndex<AssemblyIdentity> _assemblyRefIndex;
         private readonly HeapOrReferenceIndex<string> _moduleRefIndex;
         private readonly InstanceAndStructuralReferenceIndex<ITypeMemberReference> _memberRefIndex;
@@ -99,17 +98,19 @@ public DeltaMetadataWriter(
             _fieldDefs = new DefinitionIndex<IFieldDefinition>(this.TryGetExistingFieldDefIndex, sizes[(int)TableIndex.Field]);
             _methodDefs = new DefinitionIndex<IMethodDefinition>(this.TryGetExistingMethodDefIndex, sizes[(int)TableIndex.MethodDef]);
             _propertyDefs = new DefinitionIndex<IPropertyDefinition>(this.TryGetExistingPropertyDefIndex, sizes[(int)TableIndex.Property]);
-            _parameterDefs = new ParameterDefinitionIndex(sizes[(int)TableIndex.Param]);
-            _parameterDefList = new List<KeyValuePair<IMethodDefinition, IParameterDefinition>>();
+            _parameterDefs = new DefinitionIndex<IParameterDefinition>(this.TryGetExistingParameterDefIndex, sizes[(int)TableIndex.Param]);
+            _parameterDefList = new Dictionary<IParameterDefinition, IMethodDefinition>(Cci.SymbolEquivalentEqualityComparer.Instance);
             _genericParameters = new GenericParameterIndex(sizes[(int)TableIndex.GenericParam]);
             _eventMap = new EventOrPropertyMapIndex(this.TryGetExistingEventMapIndex, sizes[(int)TableIndex.EventMap]);
             _propertyMap = new EventOrPropertyMapIndex(this.TryGetExistingPropertyMapIndex, sizes[(int)TableIndex.PropertyMap]);
             _methodImpls = new MethodImplIndex(this, sizes[(int)TableIndex.MethodImpl]);
 
             _customAttributeParentCounts = new Dictionary<EntityHandle, int>();
-            _customAttributeEncMapRows = new List<int>();
             _customAttributesAdded = new Dictionary<EntityHandle, ImmutableArray<int>>();
 
+            _firstParamRowMap = new Dictionary<MethodDefinitionHandle, int>();
+            _existingParameterDefs = new Dictionary<IParameterDefinition, int>(ReferenceEqualityComparer.Instance);
+
             _assemblyRefIndex = new HeapOrReferenceIndex<AssemblyIdentity>(this, lastRowId: sizes[(int)TableIndex.AssemblyRef]);
             _moduleRefIndex = new HeapOrReferenceIndex<string>(this, lastRowId: sizes[(int)TableIndex.ModuleRef]);
             _memberRefIndex = new InstanceAndStructuralReferenceIndex<ITypeMemberReference>(this, new MemberRefComparer(this), lastRowId: sizes[(int)TableIndex.MemberRef]);
@@ -200,6 +201,7 @@ internal EmitBaseline GetDelta(Compilation compilation, Guid encId, MetadataSize
                 eventsAdded: AddRange(_previousGeneration.EventsAdded, _eventDefs.GetAdded(), comparer: SymbolEquivalentEqualityComparer.Instance),
                 fieldsAdded: AddRange(_previousGeneration.FieldsAdded, _fieldDefs.GetAdded(), comparer: SymbolEquivalentEqualityComparer.Instance),
                 methodsAdded: AddRange(_previousGeneration.MethodsAdded, _methodDefs.GetAdded(), comparer: SymbolEquivalentEqualityComparer.Instance),
+                firstParamRowMap: AddRange(_previousGeneration.FirstParamRowMap, _firstParamRowMap),
                 propertiesAdded: AddRange(_previousGeneration.PropertiesAdded, _propertyDefs.GetAdded(), comparer: SymbolEquivalentEqualityComparer.Instance),
                 eventMapAdded: AddRange(_previousGeneration.EventMapAdded, _eventMap.GetAdded()),
                 propertyMapAdded: AddRange(_previousGeneration.PropertyMapAdded, _propertyMap.GetAdded()),
@@ -537,14 +539,38 @@ protected override void CreateIndicesForNonTypeMembers(ITypeDefinition typeDef)
 
             foreach (var methodDef in typeDef.GetMethods(this.Context))
             {
-                if (this.AddDefIfNecessary(_methodDefs, methodDef))
+                this.AddDefIfNecessary(_methodDefs, methodDef);
+                var methodChange = _changes.GetChange(methodDef);
+
+                if (methodChange == SymbolChange.Added)
                 {
+                    _firstParamRowMap.Add(GetMethodDefinitionHandle(methodDef), _parameterDefs.NextRowId);
                     foreach (var paramDef in this.GetParametersToEmit(methodDef))
                     {
                         _parameterDefs.Add(paramDef);
-                        _parameterDefList.Add(KeyValuePairUtil.Create(methodDef, paramDef));
+                        _parameterDefList.Add(paramDef, methodDef);
+                    }
+                }
+                else if (methodChange == SymbolChange.Updated)
+                {
+                    // If we're re-emitting parameters for an existing method we need to find their original row numbers
+                    // and reuse them so the EnCLog, EnCMap and CustomAttributes tables refer to the right rows
+
+                    // Unfortunately we have to check the original metadata and deltas separately as nothing tracks the aggregate data
+                    // in a way that we can use
+                    var handle = GetMethodDefinitionHandle(methodDef);
+                    if (_previousGeneration.OriginalMetadata.MetadataReader.GetTableRowCount(TableIndex.MethodDef) >= MetadataTokens.GetRowNumber(handle))
+                    {
+                        EmitParametersFromOriginalMetadata(methodDef, handle);
+                    }
+                    else
+                    {
+                        EmitParametersFromDelta(methodDef, handle);
                     }
+                }
 
+                if (methodChange == SymbolChange.Added)
+                {
                     if (methodDef.GenericParameterCount > 0)
                     {
                         foreach (var typeParameter in methodDef.GenericParameters)
@@ -605,6 +631,36 @@ protected override void CreateIndicesForNonTypeMembers(ITypeDefinition typeDef)
             implementingMethods.Free();
         }
 
+        private void EmitParametersFromOriginalMetadata(IMethodDefinition methodDef, MethodDefinitionHandle handle)
+        {
+            var def = _previousGeneration.OriginalMetadata.MetadataReader.GetMethodDefinition(handle);
+
+            var parameters = def.GetParameters();
+            var paramDefinitions = this.GetParametersToEmit(methodDef);
+            int i = 0;
+            foreach (var param in parameters)
+            {
+                var paramDef = paramDefinitions[i];
+                _parameterDefs.AddUpdated(paramDef);
+                _existingParameterDefs.Add(paramDef, MetadataTokens.GetRowNumber(param));
+                _parameterDefList.Add(paramDef, methodDef);
+                i++;
+            }
+        }
+
+        private void EmitParametersFromDelta(IMethodDefinition methodDef, MethodDefinitionHandle handle)
+        {
+            var ok = _previousGeneration.FirstParamRowMap.TryGetValue(handle, out var firstRowId);
+            Debug.Assert(ok);
+
+            foreach (var paramDef in GetParametersToEmit(methodDef))
+            {
+                _parameterDefs.AddUpdated(paramDef);
+                _existingParameterDefs.Add(paramDef, firstRowId++);
+                _parameterDefList.Add(paramDef, methodDef);
+            }
+        }
+
         private bool AddDefIfNecessary<T>(DefinitionIndex<T> defIndex, T def)
             where T : class, IDefinition
         {
@@ -737,7 +793,16 @@ protected override int AddCustomAttributesToTable(EntityHandle parentHandle, IEn
             return numAttributesEmitted;
         }
 
-        protected override void PopulateEncLogTableRows(ImmutableArray<int> rowCounts)
+        public override void PopulateEncTables(ImmutableArray<int> typeSystemRowCounts)
+        {
+            Debug.Assert(typeSystemRowCounts[(int)TableIndex.EncLog] == 0);
+            Debug.Assert(typeSystemRowCounts[(int)TableIndex.EncMap] == 0);
+
+            PopulateEncLogTableRows(typeSystemRowCounts, out var customAttributeEncMapRows, out var paramEncMapRows);
+            PopulateEncMapTableRows(typeSystemRowCounts, customAttributeEncMapRows, paramEncMapRows);
+        }
+
+        private void PopulateEncLogTableRows(ImmutableArray<int> rowCounts, out List<int> customAttributeEncMapRows, out List<int> paramEncMapRows)
         {
             // The EncLog table is a log of all the operations needed
             // to update the previous metadata. That means all
@@ -762,10 +827,10 @@ protected override void PopulateEncLogTableRows(ImmutableArray<int> rowCounts)
             PopulateEncLogTableFieldsOrMethods(_methodDefs, TableIndex.MethodDef, EditAndContinueOperation.AddMethod);
             PopulateEncLogTableEventsOrProperties(_propertyDefs, TableIndex.Property, EditAndContinueOperation.AddProperty, _propertyMap, TableIndex.PropertyMap);
 
-            PopulateEncLogTableParameters();
+            PopulateEncLogTableParameters(out paramEncMapRows);
 
             PopulateEncLogTableRows(TableIndex.Constant, previousSizes, deltaSizes);
-            PopulateEncLogTableCustomAttributes();
+            PopulateEncLogTableCustomAttributes(out customAttributeEncMapRows);
             PopulateEncLogTableRows(TableIndex.DeclSecurity, previousSizes, deltaSizes);
             PopulateEncLogTableRows(TableIndex.ClassLayout, previousSizes, deltaSizes);
             PopulateEncLogTableRows(TableIndex.FieldLayout, previousSizes, deltaSizes);
@@ -826,20 +891,38 @@ private void PopulateEncLogTableFieldsOrMethods<T>(
             }
         }
 
-        private void PopulateEncLogTableParameters()
+        private void PopulateEncLogTableParameters(out List<int> paramEncMapRows)
         {
+            paramEncMapRows = new List<int>();
+
             var parameterFirstId = _parameterDefs.FirstRowId;
-            for (int i = 0; i < _parameterDefList.Count; i++)
+            int i = 0;
+            foreach (var paramDef in GetParameterDefs())
             {
-                var methodDef = _parameterDefList[i].Key;
+                var methodDef = _parameterDefList[paramDef];
 
-                metadata.AddEncLogEntry(
-                    entity: MetadataTokens.MethodDefinitionHandle(_methodDefs.GetRowId(methodDef)),
-                    code: EditAndContinueOperation.AddParameter);
+                if (_methodDefs.IsAddedNotChanged(methodDef))
+                {
+                    // For parameters on new methods we emit AddParameter rows for the method too
+                    paramEncMapRows.Add(parameterFirstId + i);
+                    metadata.AddEncLogEntry(
+                        entity: MetadataTokens.MethodDefinitionHandle(_methodDefs.GetRowId(methodDef)),
+                        code: EditAndContinueOperation.AddParameter);
 
-                metadata.AddEncLogEntry(
-                    entity: MetadataTokens.ParameterHandle(parameterFirstId + i),
-                    code: EditAndContinueOperation.Default);
+                    metadata.AddEncLogEntry(
+                        entity: MetadataTokens.ParameterHandle(parameterFirstId + i),
+                        code: EditAndContinueOperation.Default);
+                    i++;
+                }
+                else
+                {
+                    // For previously emitted parameters we just update the Param row
+                    var param = GetParameterHandle(paramDef);
+                    paramEncMapRows.Add(MetadataTokens.GetRowNumber(param));
+                    metadata.AddEncLogEntry(
+                        entity: param,
+                        code: EditAndContinueOperation.Default);
+                }
             }
         }
 
@@ -851,8 +934,10 @@ private void PopulateEncLogTableParameters()
         /// compilation or subsequent deltas, and only add more if we need to. The EncLog table is the thing that tells
         /// the runtime which row a CustomAttributes row is (ie, new or existing)
         /// </summary>
-        private void PopulateEncLogTableCustomAttributes()
+        private void PopulateEncLogTableCustomAttributes(out List<int> customAttributeEncMapRows)
         {
+            customAttributeEncMapRows = new List<int>();
+
             // List of attributes that need to be emitted to delete a previously emitted attribute
             var deletedAttributeRows = new List<(int parentRowId, HandleKind kind)>();
             var customAttributesAdded = new Dictionary<EntityHandle, ArrayBuilder<int>>();
@@ -878,7 +963,7 @@ private void PopulateEncLogTableCustomAttributes()
                 foreach (var attributeHandle in existingCustomAttributes)
                 {
                     int rowId = MetadataTokens.GetRowNumber(attributeHandle);
-                    AddLogEntryOrDelete(rowId, parent, add: index < count);
+                    AddLogEntryOrDelete(rowId, parent, add: index < count, customAttributeEncMapRows);
                     index++;
                 }
 
@@ -889,7 +974,7 @@ private void PopulateEncLogTableCustomAttributes()
                     foreach (var rowId in rowIds)
                     {
                         TrackCustomAttributeAdded(rowId, parent);
-                        AddLogEntryOrDelete(rowId, parent, add: index < count);
+                        AddLogEntryOrDelete(rowId, parent, add: index < count, customAttributeEncMapRows);
                         index++;
                     }
                 }
@@ -899,7 +984,7 @@ private void PopulateEncLogTableCustomAttributes()
                 {
                     lastRowId++;
                     TrackCustomAttributeAdded(lastRowId, parent);
-                    AddEncLogEntry(lastRowId);
+                    AddEncLogEntry(lastRowId, customAttributeEncMapRows);
                 }
             }
 
@@ -919,23 +1004,23 @@ private void PopulateEncLogTableCustomAttributes()
                 }
                 metadata.AddCustomAttribute(MetadataTokens.Handle(tableIndex, 0), MetadataTokens.EntityHandle(TableIndex.MemberRef, 0), value: default);
 
-                AddEncLogEntry(row.parentRowId);
+                AddEncLogEntry(row.parentRowId, customAttributeEncMapRows);
             }
 
-            void AddEncLogEntry(int rowId)
+            void AddEncLogEntry(int rowId, List<int> customAttributeEncMapRows)
             {
-                _customAttributeEncMapRows.Add(rowId);
+                customAttributeEncMapRows.Add(rowId);
                 metadata.AddEncLogEntry(
                     entity: MetadataTokens.CustomAttributeHandle(rowId),
                     code: EditAndContinueOperation.Default);
             }
 
-            void AddLogEntryOrDelete(int rowId, EntityHandle parent, bool add)
+            void AddLogEntryOrDelete(int rowId, EntityHandle parent, bool add, List<int> customAttributeEncMapRows)
             {
                 if (add)
                 {
                     // Update this row
-                    AddEncLogEntry(rowId);
+                    AddEncLogEntry(rowId, customAttributeEncMapRows);
                 }
                 else
                 {
@@ -981,7 +1066,7 @@ private void PopulateEncLogTableRows(TableIndex tableIndex, int firstRowId, int
             }
         }
 
-        protected override void PopulateEncMapTableRows(ImmutableArray<int> rowCounts)
+        private void PopulateEncMapTableRows(ImmutableArray<int> rowCounts, List<int> customAttributeEncMapRows, List<int> paramEncMapRows)
         {
             // The EncMap table maps from offset in each table in the delta
             // metadata to token. As such, the EncMap is a concatenated
@@ -1005,9 +1090,9 @@ protected override void PopulateEncMapTableRows(ImmutableArray<int> rowCounts)
             AddDefinitionTokens(tokens, _methodDefs, TableIndex.MethodDef);
             AddDefinitionTokens(tokens, _propertyDefs, TableIndex.Property);
 
-            AddReferencedTokens(tokens, TableIndex.Param, previousSizes, deltaSizes);
+            AddRowNumberTokens(tokens, paramEncMapRows, TableIndex.Param);
             AddReferencedTokens(tokens, TableIndex.Constant, previousSizes, deltaSizes);
-            AddRowNumberTokens(tokens, _customAttributeEncMapRows, TableIndex.CustomAttribute);
+            AddRowNumberTokens(tokens, customAttributeEncMapRows, TableIndex.CustomAttribute);
             AddReferencedTokens(tokens, TableIndex.DeclSecurity, previousSizes, deltaSizes);
             AddReferencedTokens(tokens, TableIndex.ClassLayout, previousSizes, deltaSizes);
             AddReferencedTokens(tokens, TableIndex.FieldLayout, previousSizes, deltaSizes);
@@ -1410,6 +1495,11 @@ private bool TryGetExistingPropertyDefIndex(IPropertyDefinition item, out int in
             return false;
         }
 
+        private bool TryGetExistingParameterDefIndex(IParameterDefinition item, out int index)
+        {
+            return _existingParameterDefs.TryGetValue(item, out index);
+        }
+
         private bool TryGetExistingEventMapIndex(int item, out int index)
         {
             if (_previousGeneration.EventMapAdded.TryGetValue(item, out index))
@@ -1458,28 +1548,6 @@ private bool TryGetExistingMethodImplIndex(MethodImplKey item, out int index)
             return false;
         }
 
-        private sealed class ParameterDefinitionIndex : DefinitionIndexBase<IParameterDefinition>
-        {
-            public ParameterDefinitionIndex(int lastRowId)
-                : base(lastRowId, ReferenceEqualityComparer.Instance)
-            {
-            }
-
-            public override bool TryGetRowId(IParameterDefinition item, out int index)
-            {
-                return this.added.TryGetValue(item, out index);
-            }
-
-            public void Add(IParameterDefinition item)
-            {
-                Debug.Assert(!this.IsFrozen);
-
-                int index = this.NextRowId;
-                this.added.Add(item, index);
-                this.rows.Add(item);
-            }
-        }
-
         private sealed class GenericParameterIndex : DefinitionIndexBase<IGenericParameter>
         {
             public GenericParameterIndex(int lastRowId)
diff --git a/src/Compilers/Core/Portable/Emit/EditAndContinue/EmitBaseline.cs b/src/Compilers/Core/Portable/Emit/EditAndContinue/EmitBaseline.cs
index ac37c774a1c30..fa7809a16fb33 100644
--- a/src/Compilers/Core/Portable/Emit/EditAndContinue/EmitBaseline.cs
+++ b/src/Compilers/Core/Portable/Emit/EditAndContinue/EmitBaseline.cs
@@ -209,6 +209,7 @@ public static EmitBaseline CreateInitialBaseline(
                 eventsAdded: new Dictionary<Cci.IEventDefinition, int>(),
                 fieldsAdded: new Dictionary<Cci.IFieldDefinition, int>(),
                 methodsAdded: new Dictionary<Cci.IMethodDefinition, int>(),
+                firstParamRowMap: new Dictionary<MethodDefinitionHandle, int>(),
                 propertiesAdded: new Dictionary<Cci.IPropertyDefinition, int>(),
                 eventMapAdded: new Dictionary<int, int>(),
                 propertyMapAdded: new Dictionary<int, int>(),
@@ -265,6 +266,7 @@ public static EmitBaseline CreateInitialBaseline(
         internal readonly IReadOnlyDictionary<Cci.IEventDefinition, int> EventsAdded;
         internal readonly IReadOnlyDictionary<Cci.IFieldDefinition, int> FieldsAdded;
         internal readonly IReadOnlyDictionary<Cci.IMethodDefinition, int> MethodsAdded;
+        internal readonly IReadOnlyDictionary<MethodDefinitionHandle, int> FirstParamRowMap;
         internal readonly IReadOnlyDictionary<Cci.IPropertyDefinition, int> PropertiesAdded;
         internal readonly IReadOnlyDictionary<int, int> EventMapAdded;
         internal readonly IReadOnlyDictionary<int, int> PropertyMapAdded;
@@ -322,6 +324,7 @@ private EmitBaseline(
             IReadOnlyDictionary<Cci.IEventDefinition, int> eventsAdded,
             IReadOnlyDictionary<Cci.IFieldDefinition, int> fieldsAdded,
             IReadOnlyDictionary<Cci.IMethodDefinition, int> methodsAdded,
+            IReadOnlyDictionary<MethodDefinitionHandle, int> firstParamRowMap,
             IReadOnlyDictionary<Cci.IPropertyDefinition, int> propertiesAdded,
             IReadOnlyDictionary<int, int> eventMapAdded,
             IReadOnlyDictionary<int, int> propertyMapAdded,
@@ -384,6 +387,7 @@ private EmitBaseline(
             EventsAdded = eventsAdded;
             FieldsAdded = fieldsAdded;
             MethodsAdded = methodsAdded;
+            FirstParamRowMap = firstParamRowMap;
             PropertiesAdded = propertiesAdded;
             EventMapAdded = eventMapAdded;
             PropertyMapAdded = propertyMapAdded;
@@ -416,6 +420,7 @@ internal EmitBaseline With(
             IReadOnlyDictionary<Cci.IEventDefinition, int> eventsAdded,
             IReadOnlyDictionary<Cci.IFieldDefinition, int> fieldsAdded,
             IReadOnlyDictionary<Cci.IMethodDefinition, int> methodsAdded,
+            IReadOnlyDictionary<MethodDefinitionHandle, int> firstParamRowMap,
             IReadOnlyDictionary<Cci.IPropertyDefinition, int> propertiesAdded,
             IReadOnlyDictionary<int, int> eventMapAdded,
             IReadOnlyDictionary<int, int> propertyMapAdded,
@@ -449,6 +454,7 @@ internal EmitBaseline With(
                 eventsAdded,
                 fieldsAdded,
                 methodsAdded,
+                firstParamRowMap,
                 propertiesAdded,
                 eventMapAdded,
                 propertyMapAdded,
diff --git a/src/Compilers/Core/Portable/Emit/EditAndContinue/SymbolMatcher.cs b/src/Compilers/Core/Portable/Emit/EditAndContinue/SymbolMatcher.cs
index 5da29a197116f..15ce2d0d8e0d1 100644
--- a/src/Compilers/Core/Portable/Emit/EditAndContinue/SymbolMatcher.cs
+++ b/src/Compilers/Core/Portable/Emit/EditAndContinue/SymbolMatcher.cs
@@ -49,6 +49,7 @@ public EmitBaseline MapBaselineToCompilation(
                 eventsAdded,
                 fieldsAdded,
                 methodsAdded,
+                firstParamRowMap: baseline.FirstParamRowMap,
                 propertiesAdded,
                 eventMapAdded: baseline.EventMapAdded,
                 propertyMapAdded: baseline.PropertyMapAdded,
diff --git a/src/Compilers/Core/Portable/PEWriter/FullMetadataWriter.cs b/src/Compilers/Core/Portable/PEWriter/FullMetadataWriter.cs
index 9aeb067d37667..b66d8e1dee847 100644
--- a/src/Compilers/Core/Portable/PEWriter/FullMetadataWriter.cs
+++ b/src/Compilers/Core/Portable/PEWriter/FullMetadataWriter.cs
@@ -330,14 +330,6 @@ internal FullReferenceIndexer(MetadataWriter metadataWriter)
             }
         }
 
-        protected override void PopulateEncLogTableRows(ImmutableArray<int> rowCounts)
-        {
-        }
-
-        protected override void PopulateEncMapTableRows(ImmutableArray<int> rowCounts)
-        {
-        }
-
         protected override void PopulateEventMapTableRows()
         {
             ITypeDefinition? lastParent = null;
diff --git a/src/Compilers/Core/Portable/PEWriter/MetadataWriter.cs b/src/Compilers/Core/Portable/PEWriter/MetadataWriter.cs
index fbaa34f834fc5..bbb0c97aaeb42 100644
--- a/src/Compilers/Core/Portable/PEWriter/MetadataWriter.cs
+++ b/src/Compilers/Core/Portable/PEWriter/MetadataWriter.cs
@@ -404,16 +404,6 @@ private bool IsMinimalDelta
         /// </summary>
         protected abstract void PopulatePropertyMapTableRows();
 
-        /// <summary>
-        /// Populate EncLog table.
-        /// </summary>
-        protected abstract void PopulateEncLogTableRows(ImmutableArray<int> rowCounts);
-
-        /// <summary>
-        /// Populate EncMap table.
-        /// </summary>
-        protected abstract void PopulateEncMapTableRows(ImmutableArray<int> rowCounts);
-
         protected abstract void ReportReferencesToAddedSymbols();
 
         // If true, it is allowed to have methods not have bodies (for emitting metadata-only
@@ -1120,8 +1110,7 @@ private BlobHandle GetMemberReferenceSignatureHandle(ITypeMemberReference member
 
         internal BlobHandle GetMethodSignatureHandle(IMethodReference methodReference)
         {
-            ImmutableArray<byte> signatureBlob;
-            return GetMethodSignatureHandleAndBlob(methodReference, out signatureBlob);
+            return GetMethodSignatureHandleAndBlob(methodReference, out _);
         }
 
         internal byte[] GetMethodSignature(IMethodReference methodReference)
@@ -1729,6 +1718,8 @@ public void WriteMetadataAndIL(PdbWriter nativePdbWriterOpt, Stream metadataStre
                 out Blob mvidStringFixup);
 
             var typeSystemRowCounts = metadata.GetRowCounts();
+            Debug.Assert(typeSystemRowCounts[(int)TableIndex.EncLog] == 0);
+            Debug.Assert(typeSystemRowCounts[(int)TableIndex.EncMap] == 0);
             PopulateEncTables(typeSystemRowCounts);
 
             Debug.Assert(mappedFieldDataBuilder.Count == 0);
@@ -1844,13 +1835,8 @@ public void BuildMetadataAndIL(
             PopulateTypeSystemTables(methodBodyOffsets, mappedFieldDataBuilder, managedResourceDataBuilder, dynamicAnalysisDataOpt, out mvidFixup);
         }
 
-        public void PopulateEncTables(ImmutableArray<int> typeSystemRowCounts)
+        public virtual void PopulateEncTables(ImmutableArray<int> typeSystemRowCounts)
         {
-            Debug.Assert(typeSystemRowCounts[(int)TableIndex.EncLog] == 0);
-            Debug.Assert(typeSystemRowCounts[(int)TableIndex.EncMap] == 0);
-
-            PopulateEncLogTableRows(typeSystemRowCounts);
-            PopulateEncMapTableRows(typeSystemRowCounts);
         }
 
         public MetadataRootBuilder GetRootBuilder()
diff --git a/src/Compilers/VisualBasic/Test/Emit/Emit/EditAndContinue/EditAndContinueClosureTests.vb b/src/Compilers/VisualBasic/Test/Emit/Emit/EditAndContinue/EditAndContinueClosureTests.vb
index d1a70a3446e3e..b174ddc922ccb 100644
--- a/src/Compilers/VisualBasic/Test/Emit/Emit/EditAndContinue/EditAndContinueClosureTests.vb
+++ b/src/Compilers/VisualBasic/Test/Emit/Emit/EditAndContinue/EditAndContinueClosureTests.vb
@@ -66,6 +66,7 @@ End Class
                     Row(7, TableIndex.MethodDef, EditAndContinueOperation.Default),
                     Row(4, TableIndex.TypeDef, EditAndContinueOperation.AddMethod),
                     Row(8, TableIndex.MethodDef, EditAndContinueOperation.Default),
+                    Row(6, TableIndex.Param, EditAndContinueOperation.Default),
                     Row(4, TableIndex.CustomAttribute, EditAndContinueOperation.Default),
                     Row(1, TableIndex.NestedClass, EditAndContinueOperation.Default))
             End Using
@@ -222,6 +223,7 @@ End Class
                 Row(4, TableIndex.StandAloneSig, EditAndContinueOperation.Default),
                 Row(2, TableIndex.MethodDef, EditAndContinueOperation.Default),
                 Row(3, TableIndex.MethodDef, EditAndContinueOperation.Default),
+                Row(1, TableIndex.Param, EditAndContinueOperation.Default),
                 Row(4, TableIndex.CustomAttribute, EditAndContinueOperation.Default))
         End Sub
 
@@ -274,7 +276,8 @@ End Class
                 Row(3, TableIndex.StandAloneSig, EditAndContinueOperation.Default),
                 Row(4, TableIndex.StandAloneSig, EditAndContinueOperation.Default),
                 Row(2, TableIndex.MethodDef, EditAndContinueOperation.Default),
-                Row(4, TableIndex.MethodDef, EditAndContinueOperation.Default))
+                Row(4, TableIndex.MethodDef, EditAndContinueOperation.Default),
+                Row(1, TableIndex.Param, EditAndContinueOperation.Default))
         End Sub
 
         <Fact>
@@ -358,7 +361,9 @@ End Class
                 Row(7, TableIndex.MethodDef, EditAndContinueOperation.Default),
                 Row(8, TableIndex.MethodDef, EditAndContinueOperation.Default),
                 Row(9, TableIndex.MethodDef, EditAndContinueOperation.Default),
-                Row(10, TableIndex.MethodDef, EditAndContinueOperation.Default))
+                Row(10, TableIndex.MethodDef, EditAndContinueOperation.Default),
+                Row(2, TableIndex.Param, EditAndContinueOperation.Default),
+                Row(3, TableIndex.Param, EditAndContinueOperation.Default))
         End Sub
 
         <Fact>
@@ -435,6 +440,7 @@ End Module
                 Row(7, TableIndex.StandAloneSig, EditAndContinueOperation.Default),
                 Row(6, TableIndex.MethodDef, EditAndContinueOperation.Default),
                 Row(11, TableIndex.MethodDef, EditAndContinueOperation.Default),
+                Row(6, TableIndex.Param, EditAndContinueOperation.Default),
                 Row(9, TableIndex.CustomAttribute, EditAndContinueOperation.Default),
                 Row(10, TableIndex.CustomAttribute, EditAndContinueOperation.Default),
                 Row(14, TableIndex.CustomAttribute, EditAndContinueOperation.Default))
@@ -705,7 +711,17 @@ End Class
                 Row(31, TableIndex.MethodDef, EditAndContinueOperation.Default),
                 Row(32, TableIndex.MethodDef, EditAndContinueOperation.Default),
                 Row(33, TableIndex.MethodDef, EditAndContinueOperation.Default),
-                Row(34, TableIndex.MethodDef, EditAndContinueOperation.Default))
+                Row(34, TableIndex.MethodDef, EditAndContinueOperation.Default),
+                Row(14, TableIndex.Param, EditAndContinueOperation.Default),
+                Row(15, TableIndex.Param, EditAndContinueOperation.Default),
+                Row(16, TableIndex.Param, EditAndContinueOperation.Default),
+                Row(17, TableIndex.Param, EditAndContinueOperation.Default),
+                Row(18, TableIndex.Param, EditAndContinueOperation.Default),
+                Row(19, TableIndex.Param, EditAndContinueOperation.Default),
+                Row(20, TableIndex.Param, EditAndContinueOperation.Default),
+                Row(21, TableIndex.Param, EditAndContinueOperation.Default),
+                Row(22, TableIndex.Param, EditAndContinueOperation.Default),
+                Row(23, TableIndex.Param, EditAndContinueOperation.Default))
         End Sub
 
         <Fact>
@@ -761,7 +777,8 @@ End Class
             CheckEncLogDefinitions(reader1,
                 Row(4, TableIndex.StandAloneSig, EditAndContinueOperation.Default),
                 Row(9, TableIndex.MethodDef, EditAndContinueOperation.Default),
-                Row(12, TableIndex.MethodDef, EditAndContinueOperation.Default))
+                Row(12, TableIndex.MethodDef, EditAndContinueOperation.Default),
+                Row(5, TableIndex.Param, EditAndContinueOperation.Default))
         End Sub
 
         ''' <summary>
@@ -824,7 +841,8 @@ End Class
             CheckEncLogDefinitions(reader1,
                 Row(4, TableIndex.StandAloneSig, EditAndContinueOperation.Default),
                 Row(9, TableIndex.MethodDef, EditAndContinueOperation.Default),
-                Row(12, TableIndex.MethodDef, EditAndContinueOperation.Default))
+                Row(12, TableIndex.MethodDef, EditAndContinueOperation.Default),
+                Row(5, TableIndex.Param, EditAndContinueOperation.Default))
         End Sub
 
         ''' <summary>
@@ -891,7 +909,11 @@ End Class
                 Row(9, TableIndex.MethodDef, EditAndContinueOperation.Default),
                 Row(12, TableIndex.MethodDef, EditAndContinueOperation.Default),
                 Row(13, TableIndex.MethodDef, EditAndContinueOperation.Default),
-                Row(14, TableIndex.MethodDef, EditAndContinueOperation.Default))
+                Row(14, TableIndex.MethodDef, EditAndContinueOperation.Default),
+                Row(5, TableIndex.Param, EditAndContinueOperation.Default),
+                Row(6, TableIndex.Param, EditAndContinueOperation.Default),
+                Row(7, TableIndex.Param, EditAndContinueOperation.Default),
+                Row(8, TableIndex.Param, EditAndContinueOperation.Default))
         End Sub
 
         ' TODO: AggregateClauseCrossMatch
diff --git a/src/Compilers/VisualBasic/Test/Emit/Emit/EditAndContinue/EditAndContinueStateMachineTests.vb b/src/Compilers/VisualBasic/Test/Emit/Emit/EditAndContinue/EditAndContinueStateMachineTests.vb
index fc4bd161e65f5..763c58165d500 100644
--- a/src/Compilers/VisualBasic/Test/Emit/Emit/EditAndContinue/EditAndContinueStateMachineTests.vb
+++ b/src/Compilers/VisualBasic/Test/Emit/Emit/EditAndContinue/EditAndContinueStateMachineTests.vb
@@ -1041,6 +1041,9 @@ End Class
                         Row(6, TableIndex.MethodDef, EditAndContinueOperation.Default),
                         Row(9, TableIndex.MethodDef, EditAndContinueOperation.Default),
                         Row(12, TableIndex.MethodDef, EditAndContinueOperation.Default),
+                        Row(1, TableIndex.Param, EditAndContinueOperation.Default),
+                        Row(2, TableIndex.Param, EditAndContinueOperation.Default),
+                        Row(3, TableIndex.Param, EditAndContinueOperation.Default),
                         Row(4, TableIndex.CustomAttribute, EditAndContinueOperation.Default),
                         Row(5, TableIndex.CustomAttribute, EditAndContinueOperation.Default),
                         Row(6, TableIndex.CustomAttribute, EditAndContinueOperation.Default),
@@ -1503,6 +1506,7 @@ End Class
                         Row(2, TableIndex.MethodDef, EditAndContinueOperation.Default),
                         Row(4, TableIndex.MethodDef, EditAndContinueOperation.Default),
                         Row(5, TableIndex.MethodDef, EditAndContinueOperation.Default),
+                        Row(1, TableIndex.Param, EditAndContinueOperation.Default),
                         Row(4, TableIndex.CustomAttribute, EditAndContinueOperation.Default),
                         Row(6, TableIndex.CustomAttribute, EditAndContinueOperation.Default),
                         Row(7, TableIndex.CustomAttribute, EditAndContinueOperation.Default))
@@ -1617,6 +1621,7 @@ End Class
                         Row(2, TableIndex.MethodDef, EditAndContinueOperation.Default),
                         Row(4, TableIndex.MethodDef, EditAndContinueOperation.Default),
                         Row(5, TableIndex.MethodDef, EditAndContinueOperation.Default),
+                        Row(1, TableIndex.Param, EditAndContinueOperation.Default),
                         Row(4, TableIndex.CustomAttribute, EditAndContinueOperation.Default),
                         Row(6, TableIndex.CustomAttribute, EditAndContinueOperation.Default),
                         Row(7, TableIndex.CustomAttribute, EditAndContinueOperation.Default))
@@ -1734,6 +1739,7 @@ End Class
                         Row(2, TableIndex.MethodDef, EditAndContinueOperation.Default),
                         Row(4, TableIndex.MethodDef, EditAndContinueOperation.Default),
                         Row(5, TableIndex.MethodDef, EditAndContinueOperation.Default),
+                        Row(1, TableIndex.Param, EditAndContinueOperation.Default),
                         Row(4, TableIndex.CustomAttribute, EditAndContinueOperation.Default),
                         Row(6, TableIndex.CustomAttribute, EditAndContinueOperation.Default),
                         Row(7, TableIndex.CustomAttribute, EditAndContinueOperation.Default))
@@ -2137,6 +2143,7 @@ End Class
                         Row(4, TableIndex.StandAloneSig, EditAndContinueOperation.Default),
                         Row(2, TableIndex.MethodDef, EditAndContinueOperation.Default),
                         Row(4, TableIndex.MethodDef, EditAndContinueOperation.Default),
+                        Row(1, TableIndex.Param, EditAndContinueOperation.Default),
                         Row(4, TableIndex.CustomAttribute, EditAndContinueOperation.Default),
                         Row(5, TableIndex.CustomAttribute, EditAndContinueOperation.Default),
                         Row(7, TableIndex.CustomAttribute, EditAndContinueOperation.Default))
@@ -2299,6 +2306,7 @@ End Class
                         Row(7, TableIndex.Field, EditAndContinueOperation.Default),
                         Row(2, TableIndex.MethodDef, EditAndContinueOperation.Default),
                         Row(4, TableIndex.MethodDef, EditAndContinueOperation.Default),
+                        Row(1, TableIndex.Param, EditAndContinueOperation.Default),
                         Row(4, TableIndex.CustomAttribute, EditAndContinueOperation.Default),
                         Row(5, TableIndex.CustomAttribute, EditAndContinueOperation.Default),
                         Row(7, TableIndex.CustomAttribute, EditAndContinueOperation.Default))
@@ -2464,6 +2472,7 @@ End Class
                         Row(7, TableIndex.Field, EditAndContinueOperation.Default),
                         Row(2, TableIndex.MethodDef, EditAndContinueOperation.Default),
                         Row(4, TableIndex.MethodDef, EditAndContinueOperation.Default),
+                        Row(1, TableIndex.Param, EditAndContinueOperation.Default),
                         Row(4, TableIndex.CustomAttribute, EditAndContinueOperation.Default),
                         Row(5, TableIndex.CustomAttribute, EditAndContinueOperation.Default),
                         Row(7, TableIndex.CustomAttribute, EditAndContinueOperation.Default))
diff --git a/src/EditorFeatures/CSharpTest/EditAndContinue/TopLevelEditingTests.cs b/src/EditorFeatures/CSharpTest/EditAndContinue/TopLevelEditingTests.cs
index 20e6cbce4bf11..f1a2f3229de37 100644
--- a/src/EditorFeatures/CSharpTest/EditAndContinue/TopLevelEditingTests.cs
+++ b/src/EditorFeatures/CSharpTest/EditAndContinue/TopLevelEditingTests.cs
@@ -8292,6 +8292,43 @@ public void Operator_ReadOnlyRef_Parameter_Update()
 
         #region Constructor, Destructor
 
+        [Fact]
+        public void Constructor_Parameter_AddAttribute()
+        {
+            var src1 = @"
+class C
+{
+    private int x = 1;
+
+    public C(int a)
+    {
+    }
+}";
+            var src2 = @"
+class C
+{
+    private int x = 2;
+
+    public C([System.Obsolete]int a)
+    {
+    }
+}";
+
+            var edits = GetTopEdits(src1, src2);
+
+            edits.VerifyEdits(
+                "Update [x = 1]@30 -> [x = 2]@30",
+                "Update [int a]@53 -> [[System.Obsolete]int a]@53");
+
+            edits.VerifySemantics(
+                ActiveStatementsDescription.Empty,
+                new[]
+                {
+                    SemanticEdit(SemanticEditKind.Update, c => c.GetMember("C..ctor"))
+                },
+                capabilities: EditAndContinueTestHelpers.Net6RuntimeCapabilities);
+        }
+
         [Fact]
         [WorkItem(2068, "https://github.com/dotnet/roslyn/issues/2068")]
         public void Constructor_ExternModifier_Add()