Enable a default implementation of an interface event to be provided as part of its declaration.

docs/features/DefaultInterfaceImplementation.md

@@ -29,5 +29,7 @@ class Test1 : I1
 
 - Supplying an implementation along with declaration of a property or an indexer and recognizing that implementation as default implementation for them when a type implements the interface. 
 
+- Supplying an implementation along with declaration of an event and recognizing that implementation as default implementation for the event when a type implements the interface. 
+
 
 **Open issues and work items** are tracked in https://github.com/dotnet/roslyn/issues/17952.

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -513,9 +513,6 @@
   <data name="ERR_InterfaceEventInitializer" xml:space="preserve">
     <value>'{0}': event in interface cannot have initializer</value>
   </data>
-  <data name="ERR_EventPropertyInInterface" xml:space="preserve">
-    <value>An event in an interface cannot have add or remove accessors</value>
-  </data>
   <data name="ERR_BadEventUsage" xml:space="preserve">
     <value>The event '{0}' can only appear on the left hand side of += or -= (except when used from within the type '{1}')</value>
   </data>

src/Compilers/CSharp/Portable/Errors/ErrorCode.cs

@@ -64,7 +64,7 @@ internal enum ErrorCode
         ERR_EventNotDelegate = 66,
         WRN_UnreferencedEvent = 67,
         ERR_InterfaceEventInitializer = 68,
-        ERR_EventPropertyInInterface = 69,
+        //ERR_EventPropertyInInterface = 69,
         ERR_BadEventUsage = 70,
         ERR_ExplicitEventFieldImpl = 71,
         ERR_CantOverrideNonEvent = 72,

src/Compilers/CSharp/Portable/Symbols/Source/SourceCustomEventAccessorSymbol.cs

@@ -62,26 +62,28 @@ internal SourceCustomEventAccessorSymbol(
                 isExtensionMethod: false,
                 isMetadataVirtualIgnoringModifiers: explicitInterfaceImplementations.Any());
 
-            if (@event.ContainingType.IsInterface)
+            var bodyOpt = syntax.Body;
+            if (bodyOpt != null)
             {
-                diagnostics.Add(ErrorCode.ERR_EventPropertyInInterface, this.Location);
-            }
-            else
-            {
-                var bodyOpt = syntax.Body;
-                if (bodyOpt != null)
+                if (IsExtern && !IsAbstract)
                 {
-                    if (IsExtern && !IsAbstract)
-                    {
-                        diagnostics.Add(ErrorCode.ERR_ExternHasBody, this.Location, this);
-                    }
-                    else if (IsAbstract && !IsExtern)
+                    diagnostics.Add(ErrorCode.ERR_ExternHasBody, this.Location, this);
+                }
+                else if (IsAbstract && !IsExtern)
+                {
+                    diagnostics.Add(ErrorCode.ERR_AbstractHasBody, this.Location, this);
+                }
+                else if (@event.ContainingType.IsInterface)
+                {
+                    Binder.CheckFeatureAvailability(syntax, MessageID.IDS_DefaultInterfaceImplementation, diagnostics, this.Location);
+
+                    if (!ContainingAssembly.RuntimeSupportsDefaultInterfaceImplementation)
                     {
-                        diagnostics.Add(ErrorCode.ERR_AbstractHasBody, this.Location, this);
+                        diagnostics.Add(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, this.Location);
                     }
-                    // Do not report error for IsAbstract && IsExtern. Dev10 reports CS0180 only
-                    // in that case ("member cannot be both extern and abstract").
                 }
+                // Do not report error for IsAbstract && IsExtern. Dev10 reports CS0180 only
+                // in that case ("member cannot be both extern and abstract").
             }
 
             _name = GetOverriddenAccessorName(@event, isAdder) ?? _name;

src/Compilers/CSharp/Portable/Symbols/Source/SourceCustomEventSymbol.cs

@@ -23,7 +23,9 @@ internal sealed class SourceCustomEventSymbol : SourceEventSymbol
         private readonly ImmutableArray<EventSymbol> _explicitInterfaceImplementations;
 
         internal SourceCustomEventSymbol(SourceMemberContainerTypeSymbol containingType, Binder binder, EventDeclarationSyntax syntax, DiagnosticBag diagnostics) :
-            base(containingType, syntax, syntax.Modifiers, syntax.ExplicitInterfaceSpecifier, syntax.Identifier, diagnostics)
+            base(containingType, syntax, syntax.Modifiers, isFieldLike:false,
+                 interfaceSpecifierSyntaxOpt:syntax.ExplicitInterfaceSpecifier,
+                 nameTokenSyntax:syntax.Identifier, diagnostics:diagnostics)
         {
             ExplicitInterfaceSpecifierSyntax interfaceSpecifier = syntax.ExplicitInterfaceSpecifier;
             SyntaxToken nameToken = syntax.Identifier;
@@ -113,21 +115,9 @@ internal SourceCustomEventSymbol(SourceMemberContainerTypeSymbol containingType,
             _addMethod = CreateAccessorSymbol(addSyntax, explicitlyImplementedEvent, aliasQualifierOpt, diagnostics);
             _removeMethod = CreateAccessorSymbol(removeSyntax, explicitlyImplementedEvent, aliasQualifierOpt, diagnostics);
 
-            if (containingType.IsInterfaceType())
+            if (addSyntax == null || removeSyntax == null)
             {
-                if (addSyntax == null && removeSyntax == null) //NOTE: AND - different error code produced if one is present
-                {
-                    // CONSIDER: we're matching dev10, but it would probably be more helpful to give
-                    // an error like ERR_EventPropertyInInterface.
-                    diagnostics.Add(ErrorCode.ERR_EventNeedsBothAccessors, this.Locations[0], this);
-                }
-            }
-            else
-            {
-                if (addSyntax == null || removeSyntax == null)
-                {
-                    diagnostics.Add(ErrorCode.ERR_EventNeedsBothAccessors, this.Locations[0], this);
-                }
+                diagnostics.Add(ErrorCode.ERR_EventNeedsBothAccessors, this.Locations[0], this);
             }
 
             _explicitInterfaceImplementations =

src/Compilers/CSharp/Portable/Symbols/Source/SourceEventSymbol.cs

@@ -37,6 +37,7 @@ internal SourceEventSymbol(
             SourceMemberContainerTypeSymbol containingType,
             CSharpSyntaxNode syntax,
             SyntaxTokenList modifiers,
+            bool isFieldLike,
             ExplicitInterfaceSpecifierSyntax interfaceSpecifierSyntaxOpt,
             SyntaxToken nameTokenSyntax,
             DiagnosticBag diagnostics)
@@ -49,7 +50,7 @@ internal SourceEventSymbol(
 
             var isExplicitInterfaceImplementation = interfaceSpecifierSyntaxOpt != null;
             bool modifierErrors;
-            _modifiers = MakeModifiers(modifiers, isExplicitInterfaceImplementation, _location, diagnostics, out modifierErrors);
+            _modifiers = MakeModifiers(modifiers, isExplicitInterfaceImplementation, isFieldLike, _location, diagnostics, out modifierErrors);
             this.CheckAccessibility(_location, diagnostics);
         }
 
@@ -398,7 +399,9 @@ private void CheckAccessibility(Location location, DiagnosticBag diagnostics)
             }
         }
 
-        private DeclarationModifiers MakeModifiers(SyntaxTokenList modifiers, bool explicitInterfaceImplementation, Location location, DiagnosticBag diagnostics, out bool modifierErrors)
+        private DeclarationModifiers MakeModifiers(SyntaxTokenList modifiers, bool explicitInterfaceImplementation,
+                                                   bool isFieldLike, Location location, 
+                                                   DiagnosticBag diagnostics, out bool modifierErrors)
         {
             bool isInterface = this.ContainingType.IsInterface;
             var defaultAccess = isInterface ? DeclarationModifiers.Public : DeclarationModifiers.Private;
@@ -434,7 +437,8 @@ private DeclarationModifiers MakeModifiers(SyntaxTokenList modifiers, bool expli
             // Proper errors must have been reported by now.
             if (isInterface)
             {
-                mods = (mods & ~DeclarationModifiers.AccessibilityMask) | DeclarationModifiers.Abstract | DeclarationModifiers.Public;
+                mods = (mods & ~DeclarationModifiers.AccessibilityMask) | DeclarationModifiers.Public |
+                       (isFieldLike ? DeclarationModifiers.Abstract : DeclarationModifiers.Virtual);
             }
 
             return mods;

src/Compilers/CSharp/Portable/Symbols/Source/SourceFieldLikeEventSymbol.cs

@@ -24,7 +24,8 @@ internal sealed class SourceFieldLikeEventSymbol : SourceEventSymbol
         private readonly SynthesizedFieldLikeEventAccessorSymbol _removeMethod;
 
         internal SourceFieldLikeEventSymbol(SourceMemberContainerTypeSymbol containingType, Binder binder, SyntaxTokenList modifiers, VariableDeclaratorSyntax declaratorSyntax, DiagnosticBag diagnostics)
-            : base(containingType, declaratorSyntax, modifiers, null, declaratorSyntax.Identifier, diagnostics)
+            : base(containingType, declaratorSyntax, modifiers, isFieldLike:true, interfaceSpecifierSyntaxOpt:null, 
+                   nameTokenSyntax:declaratorSyntax.Identifier, diagnostics:diagnostics)
         {
             _name = declaratorSyntax.Identifier.ValueText;
 

src/Compilers/CSharp/Portable/Symbols/Source/SourcePropertySymbol.cs

@@ -79,7 +79,7 @@ private SourcePropertySymbol(
             bool hasExpressionBody = arrowExpression != null;
             bool hasInitializer = !isIndexer && propertySyntax.Initializer != null;
 
-            GetAcessorDeclarations(syntax, diagnostics, out bool notRegularProperty, out bool hasAccessorList,
+            GetAcessorDeclarations(syntax, diagnostics, out bool isAutoProperty, out bool hasAccessorList,
                                    out AccessorDeclarationSyntax getSyntax, out AccessorDeclarationSyntax setSyntax);
 
             bool accessorsHaveImplementation;
@@ -101,7 +101,7 @@ private SourcePropertySymbol(
 
             this.CheckModifiers(location, isIndexer, diagnostics);
 
-            notRegularProperty = notRegularProperty && (!containingType.IsInterface && !IsAbstract && !IsExtern && !isIndexer && hasAccessorList);
+            isAutoProperty = isAutoProperty && (!containingType.IsInterface && !IsAbstract && !IsExtern && !isIndexer && hasAccessorList);
 
             if (isIndexer && !isExplicitInterfaceImplementation)
             {
@@ -134,13 +134,13 @@ private SourcePropertySymbol(
 
             if (hasInitializer)
             {
-                CheckInitializer(notRegularProperty, location, diagnostics);
+                CheckInitializer(isAutoProperty, location, diagnostics);
             }
 
-            if (notRegularProperty || hasInitializer)
+            if (isAutoProperty || hasInitializer)
             {
                 var hasGetSyntax = getSyntax != null;
-                _isAutoProperty = notRegularProperty && hasGetSyntax;
+                _isAutoProperty = isAutoProperty && hasGetSyntax;
                 bool isReadOnly = hasGetSyntax && setSyntax == null;
 
                 if (_isAutoProperty || hasInitializer)
@@ -165,7 +165,7 @@ private SourcePropertySymbol(
                                                                           hasInitializer);
                 }
 
-                if (notRegularProperty)
+                if (isAutoProperty)
                 {
                     Binder.CheckFeatureAvailability(
                         syntax,
@@ -261,8 +261,8 @@ private SourcePropertySymbol(
             }
             else
             {
-                _getMethod = CreateAccessorSymbol(getSyntax, explicitlyImplementedProperty, aliasQualifierOpt, notRegularProperty, diagnostics);
-                _setMethod = CreateAccessorSymbol(setSyntax, explicitlyImplementedProperty, aliasQualifierOpt, notRegularProperty, diagnostics);
+                _getMethod = CreateAccessorSymbol(getSyntax, explicitlyImplementedProperty, aliasQualifierOpt, isAutoProperty, diagnostics);
+                _setMethod = CreateAccessorSymbol(setSyntax, explicitlyImplementedProperty, aliasQualifierOpt, isAutoProperty, diagnostics);
 
                 if ((getSyntax == null) || (setSyntax == null))
                 {
@@ -277,7 +277,7 @@ private SourcePropertySymbol(
                             diagnostics.Add(ErrorCode.ERR_RefPropertyMustHaveGetAccessor, location, this);
                         }
                     }
-                    else if (notRegularProperty)
+                    else if (isAutoProperty)
                     {
                         var accessor = _getMethod ?? _setMethod;
                         if (getSyntax == null)
@@ -349,10 +349,10 @@ private SourcePropertySymbol(
         }
 
         private static void GetAcessorDeclarations(BasePropertyDeclarationSyntax syntax, DiagnosticBag diagnostics, 
-                                                   out bool notRegularProperty, out bool hasAccessorList, 
+                                                   out bool isAutoProperty, out bool hasAccessorList, 
                                                    out AccessorDeclarationSyntax getSyntax, out AccessorDeclarationSyntax setSyntax)
         {
-            notRegularProperty = true;
+            isAutoProperty = true;
             hasAccessorList = syntax.AccessorList != null;
             getSyntax = null;
             setSyntax = null;
@@ -397,13 +397,13 @@ private static void GetAcessorDeclarations(BasePropertyDeclarationSyntax syntax,
 
                     if (accessor.Body != null || accessor.ExpressionBody != null)
                     {
-                        notRegularProperty = false;
+                        isAutoProperty = false;
                     }
                 }
             }
             else
             {
-                notRegularProperty = false;
+                isAutoProperty = false;
             }
         }
 

src/Compilers/CSharp/Test/Semantic/Semantics/SemanticErrorTests.cs

@@ -2006,40 +2006,30 @@ public void CS0069ERR_EventPropertyInInterface()
             var text = @"
 interface I
 {
-    event System.Action E1 { add; } // CS0069 on add
-    event System.Action E2 { remove; } // CS0069 on remove
-    event System.Action E3 { add; remove; } // CS0069 on both (i.e. x 2)
+    event System.Action E1 { add; }
+    event System.Action E2 { remove; }
+    event System.Action E3 { add; remove; }
 }
 ";
             CreateCompilationWithMscorlib(text).VerifyDiagnostics(
-                // (4,30): error CS0069: An event in an interface cannot have add or remove accessors
-                //     event System.Action E1 { add; } // CS0069 on add
-                Diagnostic(ErrorCode.ERR_EventPropertyInInterface, "add"),
-                // (5,30): error CS0069: An event in an interface cannot have add or remove accessors
-                //     event System.Action E2 { remove; } // CS0069 on remove
-                Diagnostic(ErrorCode.ERR_EventPropertyInInterface, "remove"),
-                // (6,30): error CS0069: An event in an interface cannot have add or remove accessors
-                //     event System.Action E3 { add; remove; } // CS0069 on both (i.e. x 2)
-                Diagnostic(ErrorCode.ERR_EventPropertyInInterface, "add"),
-                // (6,35): error CS0069: An event in an interface cannot have add or remove accessors
-                //     event System.Action E3 { add; remove; } // CS0069 on both (i.e. x 2)
-                Diagnostic(ErrorCode.ERR_EventPropertyInInterface, "remove"),
-
-                // CONSIDER: dev10 doesn't report these, but we report them in the parser so they're
-                // hard to suppress.
-
                 // (4,33): error CS0073: An add or remove accessor must have a body
-                //     event System.Action E1 { add; } // CS0069 on add
-                Diagnostic(ErrorCode.ERR_AddRemoveMustHaveBody, ";"),
+                //     event System.Action E1 { add; }
+                Diagnostic(ErrorCode.ERR_AddRemoveMustHaveBody, ";").WithLocation(4, 33),
                 // (5,36): error CS0073: An add or remove accessor must have a body
-                //     event System.Action E2 { remove; } // CS0069 on remove
-                Diagnostic(ErrorCode.ERR_AddRemoveMustHaveBody, ";"),
+                //     event System.Action E2 { remove; }
+                Diagnostic(ErrorCode.ERR_AddRemoveMustHaveBody, ";").WithLocation(5, 36),
                 // (6,33): error CS0073: An add or remove accessor must have a body
-                //     event System.Action E3 { add; remove; } // CS0069 on both (i.e. x 2)
-                Diagnostic(ErrorCode.ERR_AddRemoveMustHaveBody, ";"),
+                //     event System.Action E3 { add; remove; }
+                Diagnostic(ErrorCode.ERR_AddRemoveMustHaveBody, ";").WithLocation(6, 33),
                 // (6,41): error CS0073: An add or remove accessor must have a body
-                //     event System.Action E3 { add; remove; } // CS0069 on both (i.e. x 2)
-                Diagnostic(ErrorCode.ERR_AddRemoveMustHaveBody, ";"));
+                //     event System.Action E3 { add; remove; }
+                Diagnostic(ErrorCode.ERR_AddRemoveMustHaveBody, ";").WithLocation(6, 41),
+                // (4,25): error CS0065: 'I.E1': event property must have both add and remove accessors
+                //     event System.Action E1 { add; }
+                Diagnostic(ErrorCode.ERR_EventNeedsBothAccessors, "E1").WithArguments("I.E1").WithLocation(4, 25),
+                // (5,25): error CS0065: 'I.E2': event property must have both add and remove accessors
+                //     event System.Action E2 { remove; }
+                Diagnostic(ErrorCode.ERR_EventNeedsBothAccessors, "E2").WithArguments("I.E2").WithLocation(5, 25));
         }
 
         [Fact]