Fix for 7845: add DynamicAttribute to event with dynamic type

src/Compilers/CSharp/Portable/Symbols/Metadata/PE/PEEventSymbol.cs

@@ -86,17 +86,21 @@ private enum Flags : byte
                 }
             }
 
+            TypeSymbol originalEventType = _eventType;
             if ((object)_eventType == null)
             {
                 var metadataDecoder = new MetadataDecoder(moduleSymbol, containingType);
-                _eventType = metadataDecoder.GetTypeOfToken(eventType);
+                originalEventType = metadataDecoder.GetTypeOfToken(eventType);
+
+                const int targetSymbolCustomModifierCount = 0;
+                _eventType = DynamicTypeDecoder.TransformType(originalEventType, targetSymbolCustomModifierCount, handle, moduleSymbol);
             }
 
             // IsWindowsRuntimeEvent checks the signatures, so we just have to check the accessors.
             bool isWindowsRuntimeEvent = IsWindowsRuntimeEvent;
             bool callMethodsDirectly = isWindowsRuntimeEvent
                 ? !DoModifiersMatch(_addMethod, _removeMethod)
-                : !DoSignaturesMatch(moduleSymbol, _eventType, _addMethod, _removeMethod);
+                : !DoSignaturesMatch(moduleSymbol, originalEventType, _addMethod, _removeMethod);
 
             if (callMethodsDirectly)
             {

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

@@ -287,6 +287,17 @@ internal sealed override void DecodeWellKnownAttribute(ref DecodeWellKnownAttrib
             }
         }
 
+        internal override void AddSynthesizedAttributes(ModuleCompilationState compilationState, ref ArrayBuilder<SynthesizedAttributeData> attributes)
+        {
+            base.AddSynthesizedAttributes(compilationState, ref attributes);
+
+            if (this.Type.ContainsDynamic())
+            {
+                var compilation = this.DeclaringCompilation;
+                AddSynthesizedAttribute(ref attributes, compilation.SynthesizeDynamicAttribute(this.Type, customModifiersCount: 0));
+            }
+        }
+
         internal sealed override bool HasSpecialName
         {
             get

src/Compilers/CSharp/Portable/Syntax/SyntaxExtensions.cs

@@ -211,6 +211,7 @@ private static bool IsInContextWhichNeedsDynamicAttribute(CSharpSyntaxNode node)
                 case SyntaxKind.PropertyDeclaration:
                 case SyntaxKind.DelegateDeclaration:
                 case SyntaxKind.EventDeclaration:
+                case SyntaxKind.EventFieldDeclaration:
                 case SyntaxKind.BaseList:
                 case SyntaxKind.SimpleBaseType:
                     return true;

src/Compilers/CSharp/Test/Symbol/Symbols/CustomModifiersTests.cs

@@ -1916,5 +1916,96 @@ Remove Overridden2
 Add Overridden3
 Remove Overridden3");
         }
+
+        [Fact, WorkItem(7845, "https://github.com/dotnet/roslyn/issues/7845")]
+        public void EventFieldWithDynamic()
+        {
+            var ilSource = @"
+.class public auto ansi beforefieldinit CL1
+       extends [mscorlib] System.Object
+{
+    .method public hidebysig specialname rtspecialname
+            instance void  .ctor() cil managed
+    {
+      // Code size       7 (0x7)
+      .maxstack  1
+      IL_0000: ldarg.0
+      IL_0001: call instance void[mscorlib] System.Object::.ctor()
+      IL_0006: ret
+    } // end of method CL1`1::.ctor
+
+    .event class [mscorlib]System.Action`1<object modopt([mscorlib]System.Runtime.CompilerServices.IsConst)[]> Test
+    {
+      .addon instance void CL1::add_Test(class [mscorlib]System.Action`1<object modopt([mscorlib]System.Runtime.CompilerServices.IsConst)[]>)
+      .removeon instance void CL1::remove_Test(class [mscorlib]System.Action`1<object modopt([mscorlib]System.Runtime.CompilerServices.IsConst)[]>)
+    } // end of event CL1::Test
+
+    .method public hidebysig newslot specialname virtual 
+            instance void  add_Test(class [mscorlib]System.Action`1<object modopt([mscorlib]System.Runtime.CompilerServices.IsConst)[]> 'value') cil managed
+    {
+      // Code size       2 (0x2)
+      .maxstack  1
+      IL_0000: ldarg.0
+      IL_0001: throw
+    } // end of method CL1::get_Test
+
+    .method public hidebysig newslot specialname virtual 
+            instance void  remove_Test(class [mscorlib]System.Action`1<object modopt([mscorlib]System.Runtime.CompilerServices.IsConst)[]> 'value') cil managed
+    {
+      // Code size       3 (0x3)
+      .maxstack  1
+      IL_0000: ldarg.0
+      IL_0001: throw
+      IL_0002:  ret
+    } // end of method CL1::set_Test
+} // end of class CL1
+";
+            var source = @"
+using System;
+
+class Module1
+{
+    static void Main()
+    {
+        CL2 cl2 = new CL2();
+        CL1 cl1 = cl2;
+        cl1.Test += (d) => Console.WriteLine(d[0] + "" and "" + d[1]);
+        cl2.Raise();
+
+        CL3 cl3 = new CL3();
+        cl1 = cl3;
+        cl1.Test += (d) => Console.WriteLine(""Charlie"");
+        cl3.Raise();
+    }
+}
+
+class CL2 : CL1
+{
+    public override event Action<dynamic[]> Test;
+    public void Raise() => Test(new string[] { ""Alice"", ""Bob"" });
+}
+
+class CL3 : CL1
+{
+    public override event Action<object[]> Test;
+    public void Raise() => Test(null);
+}
+";
+            var compilation = CreateCompilationWithCustomILSource(source, ilSource, new[] { CSharpRef, SystemCoreRef }, options: TestOptions.ReleaseExe);
+
+            var cl2 = compilation.GetTypeByMetadataName("CL2");
+            var test2 = cl2.GetMember<EventSymbol>("Test");
+            Assert.Equal("event System.Action<dynamic modopt(System.Runtime.CompilerServices.IsConst) []> CL2.Test",
+                         test2.ToTestDisplayString());
+
+            var cl3 = compilation.GetTypeByMetadataName("CL3");
+            var test3 = cl3.GetMember<EventSymbol>("Test");
+            Assert.Equal("event System.Action<System.Object modopt(System.Runtime.CompilerServices.IsConst) []> CL3.Test",
+                         test3.ToTestDisplayString());
+
+            CompileAndVerify(compilation, expectedOutput: 
+@"Alice and Bob
+Charlie");
+        }
     }
 }
\ No newline at end of file