Merge pull request #1152 from gafter/fix406

Detect and report attempt to use abstract event through base

src/Compilers/CSharp/Portable/Binder/Binder_Statements.cs

@@ -1752,7 +1752,12 @@ private bool CheckEventValueKind(BoundEventAccess boundEvent, BindValueKind valu
                         // NOTE: availability of well-known members is checked in BindEventAssignment because
                         // we don't have the context to determine whether addition or subtraction is being performed.
 
-                        if (ReportUseSiteDiagnostics(eventSymbol, diagnostics, eventSyntax))
+                        if (receiver?.Kind == BoundKind.BaseReference && eventSymbol.IsAbstract)
+                        {
+                            Error(diagnostics, ErrorCode.ERR_AbstractBaseCall, boundEvent.Syntax, eventSymbol);
+                            return false;
+                        }
+                        else if (ReportUseSiteDiagnostics(eventSymbol, diagnostics, eventSyntax))
                         {
                             // NOTE: BindEventAssignment checks use site errors on the specific accessor 
                             // (since we don't know which is being used).

src/Compilers/CSharp/Test/Symbol/Symbols/Source/EventTests.cs

@@ -1884,6 +1884,55 @@ public abstract class A
             Assert.NotNull(eventF.AssociatedField); // Since it has an initializer.
         }
 
+        [Fact, WorkItem(406, "https://github.com/dotnet/roslyn/issues/406")]
+        public void AbstractBaseEvent()
+        {
+            var source =
+@"using System;
+
+namespace ConsoleApplication3
+{
+    public abstract class BaseWithAbstractEvent
+    {
+        public abstract event Action MyEvent;
+    }
+
+    public class SuperWithOverridenEvent : BaseWithAbstractEvent
+    {
+        public override event Action MyEvent
+        {
+            add { base.MyEvent += value; } // error
+            remove { base.MyEvent -= value; } // error
+        }
+
+        public void Foo()
+        {
+            base.MyEvent += Foo; // error
+        }
+    }
+
+    class Program
+    {
+        static void Main()
+        {
+            SuperWithOverridenEvent swoe = new SuperWithOverridenEvent();
+            swoe.MyEvent += Main;
+        }
+    }
+}";
+            CreateCompilationWithMscorlib(source).VerifyDiagnostics(
+                // (14,19): error CS0205: Cannot call an abstract base member: 'BaseWithAbstractEvent.MyEvent'
+                //             add { base.MyEvent += value; } // error
+                Diagnostic(ErrorCode.ERR_AbstractBaseCall, "base.MyEvent").WithArguments("ConsoleApplication3.BaseWithAbstractEvent.MyEvent").WithLocation(14, 19),
+                // (15,22): error CS0205: Cannot call an abstract base member: 'BaseWithAbstractEvent.MyEvent'
+                //             remove { base.MyEvent -= value; } // error
+                Diagnostic(ErrorCode.ERR_AbstractBaseCall, "base.MyEvent").WithArguments("ConsoleApplication3.BaseWithAbstractEvent.MyEvent").WithLocation(15, 22),
+                // (20,13): error CS0205: Cannot call an abstract base member: 'BaseWithAbstractEvent.MyEvent'
+                //             base.MyEvent += Foo; // error
+                Diagnostic(ErrorCode.ERR_AbstractBaseCall, "base.MyEvent").WithArguments("ConsoleApplication3.BaseWithAbstractEvent.MyEvent").WithLocation(20, 13)
+                );
+        }
+
         #endregion
     }
 }