diff --git a/src/Compilers/CSharp/Portable/Symbols/MethodSymbol.cs b/src/Compilers/CSharp/Portable/Symbols/MethodSymbol.cs
index 6f900ad67a150..3a3156c984eda 100644
--- a/src/Compilers/CSharp/Portable/Symbols/MethodSymbol.cs
+++ b/src/Compilers/CSharp/Portable/Symbols/MethodSymbol.cs
@@ -302,20 +302,18 @@ internal virtual bool IsExplicitInterfaceImplementation
             get { return ExplicitInterfaceImplementations.Any(); }
         }
 
-        // PROTOTYPE: this should become public API
         /// <summary>
         /// Indicates whether the method is declared readonly, i.e.
-        /// whether 'this' is readonly in the scope of the method.
+        /// whether the 'this' receiver parameter is 'ref readonly'.
         /// See also <see cref="IsEffectivelyReadOnly"/>
         /// </summary>
         internal abstract bool IsDeclaredReadOnly { get; }
 
-        // PROTOTYPE: this should become public API
         /// <summary>
         /// Indicates whether the method is effectively readonly,
         /// by either the method or the containing type being marked readonly.
         /// </summary>
-        internal bool IsEffectivelyReadOnly => (IsDeclaredReadOnly || ContainingType?.IsReadOnly == true) && IsValidReadOnlyTarget;
+        internal virtual bool IsEffectivelyReadOnly => (IsDeclaredReadOnly || ContainingType?.IsReadOnly == true) && IsValidReadOnlyTarget;
 
         protected bool IsValidReadOnlyTarget => !IsStatic && ContainingType.IsStructType() && MethodKind != MethodKind.Constructor;
 
@@ -1063,6 +1061,14 @@ IMethodSymbol IMethodSymbol.ConstructedFrom
             }
         }
 
+        bool IMethodSymbol.IsReadOnly
+        {
+            get
+            {
+                return this.IsEffectivelyReadOnly;
+            }
+        }
+
         IMethodSymbol IMethodSymbol.OriginalDefinition
         {
             get
diff --git a/src/Compilers/CSharp/Portable/Symbols/ReducedExtensionMethodSymbol.cs b/src/Compilers/CSharp/Portable/Symbols/ReducedExtensionMethodSymbol.cs
index b53733ef2482e..a734e643634d8 100644
--- a/src/Compilers/CSharp/Portable/Symbols/ReducedExtensionMethodSymbol.cs
+++ b/src/Compilers/CSharp/Portable/Symbols/ReducedExtensionMethodSymbol.cs
@@ -373,9 +373,10 @@ internal override bool IsExplicitInterfaceImplementation
             get { return false; }
         }
 
-        // extension methods are static and therefore not readonly
         internal override bool IsDeclaredReadOnly => false;
 
+        internal override bool IsEffectivelyReadOnly => _reducedFrom.Parameters[0].RefKind == RefKind.In;
+
         public override ImmutableArray<MethodSymbol> ExplicitInterfaceImplementations
         {
             get { return ImmutableArray<MethodSymbol>.Empty; }
diff --git a/src/Compilers/CSharp/Test/Semantic/Semantics/ReadOnlyStructsTests.cs b/src/Compilers/CSharp/Test/Semantic/Semantics/ReadOnlyStructsTests.cs
index 6c1c461694ec6..a76581d75a53e 100644
--- a/src/Compilers/CSharp/Test/Semantic/Semantics/ReadOnlyStructsTests.cs
+++ b/src/Compilers/CSharp/Test/Semantic/Semantics/ReadOnlyStructsTests.cs
@@ -1000,6 +1000,179 @@ public readonly int M()
             Assert.True(method.IsEffectivelyReadOnly);
         }
 
+        [Fact]
+        public void ReadOnlyMembers_SemanticModel()
+        {
+            var csharp = @"
+using System;
+
+public struct S1
+{
+    public void M1() {}
+    public readonly void M2() {}
+
+    public int P1 { get; set; }
+    public readonly int P2 => 42;
+    public int P3 { readonly get => 123; }
+    public int P4 { readonly set {} }
+    public static int P5 { get; set; }
+    public readonly event Action<EventArgs> E { add {} remove {} }
+}
+
+public readonly struct S2
+{
+    public void M1() {}
+    public static void M2() {}
+
+    public int P1 { get; }
+    public int P2 => 42;
+    public int P3 { set {} }
+    public static int P4 { get; set; }
+    public event Action<EventArgs> E { add {} remove {} }
+}
+";
+            Compilation comp = CreateCompilation(csharp);
+            var s1 = (INamedTypeSymbol)comp.GetSymbolsWithName("S1").Single();
+
+            Assert.False(getMethod(s1, "M1").IsReadOnly);
+
+            Assert.True(getMethod(s1, "M2").IsReadOnly);
+
+            Assert.True(getProperty(s1, "P1").GetMethod.IsReadOnly);
+            Assert.False(getProperty(s1, "P1").SetMethod.IsReadOnly);
+
+            Assert.True(getProperty(s1, "P2").GetMethod.IsReadOnly);
+
+            Assert.True(getProperty(s1, "P3").GetMethod.IsReadOnly);
+
+            Assert.True(getProperty(s1, "P4").SetMethod.IsReadOnly);
+
+            Assert.False(getProperty(s1, "P5").GetMethod.IsReadOnly);
+            Assert.False(getProperty(s1, "P5").SetMethod.IsReadOnly);
+
+            Assert.True(getEvent(s1, "E").AddMethod.IsReadOnly);
+            Assert.True(getEvent(s1, "E").RemoveMethod.IsReadOnly);
+
+            var s2 = comp.GetMember<NamedTypeSymbol>("S2");
+            Assert.True(getMethod(s2, "M1").IsReadOnly);
+            Assert.False(getMethod(s2, "M2").IsReadOnly);
+
+            Assert.True(getProperty(s2, "P1").GetMethod.IsReadOnly);
+
+            Assert.True(getProperty(s2, "P2").GetMethod.IsReadOnly);
+
+            Assert.True(getProperty(s2, "P3").SetMethod.IsReadOnly);
+
+            Assert.False(getProperty(s2, "P4").GetMethod.IsReadOnly);
+            Assert.False(getProperty(s2, "P4").SetMethod.IsReadOnly);
+
+            Assert.True(getEvent(s2, "E").AddMethod.IsReadOnly);
+            Assert.True(getEvent(s2, "E").RemoveMethod.IsReadOnly);
+
+            static IMethodSymbol getMethod(INamedTypeSymbol symbol, string name) => (IMethodSymbol)symbol.GetMembers(name).Single();
+            static IPropertySymbol getProperty(INamedTypeSymbol symbol, string name) => (IPropertySymbol)symbol.GetMembers(name).Single();
+            static IEventSymbol getEvent(INamedTypeSymbol symbol, string name) => (IEventSymbol)symbol.GetMembers(name).Single();
+        }
+
+        [Fact]
+        public void ReadOnlyMembers_ExtensionMethods_SemanticModel()
+        {
+            var csharp = @"
+public struct S1 {}
+public readonly struct S2 {}
+
+public static class C
+{
+    static void M1(this S1 s1) {}
+    static void M2(this ref S1 s1) {}
+    static void M3(this in S1 s1) {}
+    static void M4(this S2 s2) {}
+    static void M5(this ref S2 s2) {}
+    static void M6(this in S2 s2) {}
+
+    static void Test()
+    {
+        var s1 = new S1();
+        s1.M1();
+        s1.M2();
+        s1.M3();
+
+        var s2 = new S2();
+        s2.M4();
+        s2.M5();
+        s2.M6();
+    }
+}
+";
+            Compilation comp = CreateCompilation(csharp);
+
+            var c = comp.GetMember<MethodSymbol>("C.Test");
+            var testMethodSyntax = (MethodDeclarationSyntax)c.DeclaringSyntaxReferences.Single().GetSyntax();
+
+            var semanticModel = comp.GetSemanticModel(testMethodSyntax.SyntaxTree);
+            var statements = testMethodSyntax.Body.Statements;
+
+            testStatement(statements[1], false);
+            testStatement(statements[2], false);
+            testStatement(statements[3], true);
+
+            testStatement(statements[5], false);
+            testStatement(statements[6], false);
+            testStatement(statements[7], true);
+
+            void testStatement(StatementSyntax statementSyntax, bool isEffectivelyReadOnly)
+            {
+                var expressionStatement = (ExpressionStatementSyntax)statementSyntax;
+                var invocationExpression = (InvocationExpressionSyntax)expressionStatement.Expression;
+
+                var symbol = (MethodSymbol)semanticModel.GetSymbolInfo(invocationExpression.Expression).Symbol;
+                var reducedFrom = symbol.ReducedFrom;
+
+                Assert.Equal(isEffectivelyReadOnly, symbol.IsEffectivelyReadOnly);
+                Assert.Equal(isEffectivelyReadOnly, ((IMethodSymbol)symbol).IsReadOnly);
+
+                Assert.False(symbol.IsDeclaredReadOnly);
+                Assert.False(reducedFrom.IsDeclaredReadOnly);
+                Assert.False(reducedFrom.IsEffectivelyReadOnly);
+                Assert.False(((IMethodSymbol)reducedFrom).IsReadOnly);
+            }
+        }
+
+        [Fact]
+        public void ReadOnlyMembers_RefReturningProperty()
+        {
+            var csharp = @"
+public struct S1
+{
+    private static int i = 0;
+
+    public ref int P1 => ref i;
+    public readonly ref int P2 => ref i;
+    public ref readonly int P3 => ref i;
+    public readonly ref readonly int P4 => ref i;
+}
+";
+            var comp = CreateCompilation(csharp);
+
+            var s1 = comp.GetMember<NamedTypeSymbol>("S1");
+
+            check(s1.GetProperty("P1"), true, false, false);
+            check(s1.GetProperty("P2"), true, false, true);
+            check(s1.GetProperty("P3"), false, true, false);
+            check(s1.GetProperty("P4"), false, true, true);
+
+            static void check(PropertySymbol property, bool returnsByRef, bool returnsByRefReadonly, bool isReadOnly)
+            {
+                Assert.Equal(returnsByRef, property.ReturnsByRef);
+                Assert.Equal(returnsByRefReadonly, property.ReturnsByRefReadonly);
+
+                Assert.True(property.IsReadOnly);
+                Assert.Equal(isReadOnly, property.GetMethod.IsDeclaredReadOnly);
+                Assert.Equal(isReadOnly, property.GetMethod.IsEffectivelyReadOnly);
+                Assert.Equal(isReadOnly, ((IMethodSymbol)property.GetMethod).IsReadOnly);
+            }
+        }
+
         [Fact]
         public void ReadOnlyClass()
         {
diff --git a/src/Compilers/Core/Portable/PublicAPI.Unshipped.txt b/src/Compilers/Core/Portable/PublicAPI.Unshipped.txt
index 5ac7d7cdc85a5..4f003152c24f7 100644
--- a/src/Compilers/Core/Portable/PublicAPI.Unshipped.txt
+++ b/src/Compilers/Core/Portable/PublicAPI.Unshipped.txt
@@ -1,5 +1,6 @@
 *REMOVED*Microsoft.CodeAnalysis.Operations.IEventAssignmentOperation.EventReference.get -> Microsoft.CodeAnalysis.Operations.IEventReferenceOperation
 *REMOVED*Microsoft.CodeAnalysis.SpecialType.Count = 43 -> Microsoft.CodeAnalysis.SpecialType
+Microsoft.CodeAnalysis.IMethodSymbol.IsReadOnly.get -> bool
 Microsoft.CodeAnalysis.IFieldSymbol.IsFixedSizeBuffer.get -> bool
 Microsoft.CodeAnalysis.ITypeSymbol.IsRefLikeType.get -> bool
 Microsoft.CodeAnalysis.ITypeSymbol.IsUnmanagedType.get -> bool
diff --git a/src/Compilers/Core/Portable/Symbols/IMethodSymbol.cs b/src/Compilers/Core/Portable/Symbols/IMethodSymbol.cs
index 85043194a20cb..fb28b65a1b379 100644
--- a/src/Compilers/Core/Portable/Symbols/IMethodSymbol.cs
+++ b/src/Compilers/Core/Portable/Symbols/IMethodSymbol.cs
@@ -117,6 +117,14 @@ public interface IMethodSymbol : ISymbol
         /// </summary>
         IMethodSymbol ConstructedFrom { get; }
 
+        /// <summary>
+        /// Indicates whether the method is readonly, i.e.
+        /// i.e. whether the 'this' receiver parameter is 'ref readonly'.
+        /// Returns true for readonly instance methods and accessors
+        /// and for reduced extension methods with a 'this in' parameter.
+        /// </summary>
+        bool IsReadOnly { get; }
+
         /// <summary>
         /// Get the original definition of this symbol. If this symbol is derived from another
         /// symbol by (say) type substitution, this gets the original symbol, as it was defined in
diff --git a/src/Compilers/VisualBasic/Portable/Symbols/MethodSymbol.vb b/src/Compilers/VisualBasic/Portable/Symbols/MethodSymbol.vb
index d27c72c44f527..25b868e5491f7 100644
--- a/src/Compilers/VisualBasic/Portable/Symbols/MethodSymbol.vb
+++ b/src/Compilers/VisualBasic/Portable/Symbols/MethodSymbol.vb
@@ -100,6 +100,15 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.Symbols
             End Get
         End Property
 
+        ''' <summary>
+        ''' Always returns false because the 'readonly members' feature is not available in VB.
+        ''' </summary>
+        Private ReadOnly Property IMethodSymbol_IsReadOnly As Boolean Implements IMethodSymbol.IsReadOnly
+            Get
+                Return False
+            End Get
+        End Property
+
         ''' <summary>
         ''' Returns true if this method has no return type; i.e., is a Sub instead of a Function.
         ''' </summary>
diff --git a/src/Features/Core/Portable/MetadataAsSource/AbstractMetadataAsSourceService.WrappedMethodSymbol.cs b/src/Features/Core/Portable/MetadataAsSource/AbstractMetadataAsSourceService.WrappedMethodSymbol.cs
index 13e54207aff59..85fe2f63c5007 100644
--- a/src/Features/Core/Portable/MetadataAsSource/AbstractMetadataAsSourceService.WrappedMethodSymbol.cs
+++ b/src/Features/Core/Portable/MetadataAsSource/AbstractMetadataAsSourceService.WrappedMethodSymbol.cs
@@ -25,6 +25,8 @@ public WrappedMethodSymbol(IMethodSymbol methodSymbol, bool canImplementImplicit
 
             public IMethodSymbol ConstructedFrom => _symbol.ConstructedFrom;
 
+            public bool IsReadOnly => _symbol.IsReadOnly;
+
             public ImmutableArray<IMethodSymbol> ExplicitInterfaceImplementations
             {
                 get
diff --git a/src/Workspaces/Core/Portable/CodeGeneration/Symbols/CodeGenerationAbstractMethodSymbol.cs b/src/Workspaces/Core/Portable/CodeGeneration/Symbols/CodeGenerationAbstractMethodSymbol.cs
index c3936eece6ae6..c440086bb2949 100644
--- a/src/Workspaces/Core/Portable/CodeGeneration/Symbols/CodeGenerationAbstractMethodSymbol.cs
+++ b/src/Workspaces/Core/Portable/CodeGeneration/Symbols/CodeGenerationAbstractMethodSymbol.cs
@@ -1,6 +1,5 @@
 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
-using System.Collections.Generic;
 using System.Collections.Immutable;
 using Microsoft.CodeAnalysis.Editing;
 
@@ -39,6 +38,7 @@ protected CodeGenerationAbstractMethodSymbol(
         public abstract ImmutableArray<ITypeParameterSymbol> TypeParameters { get; }
         public abstract ImmutableArray<IParameterSymbol> Parameters { get; }
         public abstract IMethodSymbol ConstructedFrom { get; }
+        public abstract bool IsReadOnly { get; }
         public abstract IMethodSymbol OverriddenMethod { get; }
         public abstract IMethodSymbol ReducedFrom { get; }
         public abstract ITypeSymbol GetTypeInferredDuringReduction(ITypeParameterSymbol reducedFromTypeParameter);
diff --git a/src/Workspaces/Core/Portable/CodeGeneration/Symbols/CodeGenerationConstructedMethodSymbol.cs b/src/Workspaces/Core/Portable/CodeGeneration/Symbols/CodeGenerationConstructedMethodSymbol.cs
index 28e10e143a3d9..8c3504d2d68a6 100644
--- a/src/Workspaces/Core/Portable/CodeGeneration/Symbols/CodeGenerationConstructedMethodSymbol.cs
+++ b/src/Workspaces/Core/Portable/CodeGeneration/Symbols/CodeGenerationConstructedMethodSymbol.cs
@@ -70,6 +70,8 @@ public override ImmutableArray<IParameterSymbol> Parameters
 
         public override IMethodSymbol ConstructedFrom => _constructedFrom;
 
+        public override bool IsReadOnly => _constructedFrom.IsReadOnly;
+
         public override IMethodSymbol OverriddenMethod =>
                 // TODO(cyrusn): Construct this.
                 _constructedFrom.OverriddenMethod;
diff --git a/src/Workspaces/Core/Portable/CodeGeneration/Symbols/CodeGenerationMethodSymbol.cs b/src/Workspaces/Core/Portable/CodeGeneration/Symbols/CodeGenerationMethodSymbol.cs
index 06fc76867d504..3ac4c6d2ba05c 100644
--- a/src/Workspaces/Core/Portable/CodeGeneration/Symbols/CodeGenerationMethodSymbol.cs
+++ b/src/Workspaces/Core/Portable/CodeGeneration/Symbols/CodeGenerationMethodSymbol.cs
@@ -3,6 +3,7 @@
 using System;
 using System.Collections.Immutable;
 using Microsoft.CodeAnalysis.Editing;
+using Roslyn.Utilities;
 
 namespace Microsoft.CodeAnalysis.CodeGeneration
 {
@@ -85,6 +86,8 @@ public override ImmutableArray<ITypeSymbol> TypeArguments
 
         public override IMethodSymbol ConstructedFrom => this;
 
+        public override bool IsReadOnly => throw ExceptionUtilities.Unreachable;
+
         public override IMethodSymbol OverriddenMethod => null;
 
         public override IMethodSymbol ReducedFrom => null;