Roslyn: Explicit implementation on interface with nullable reference support

[foxesknow]

I've got an interface which supports nullable reference types:

interface ICache
{
  bool TryOpen(string name, [NotNullWhen(true) ICacheItem? cacheItem);
}

In Visual Studio 16.6.5 if I implement the interface and let Visual Studio implement the interface for me I get this:

class TestCache : ICache
{
  bool TryOpen(string name, [NotNullWhen(true) ICacheItem? cacheItem)
  {
    throw new NotImplementedException();
  }
}

However, if I opt to explicitly implement the interface then Visual Studio doesn't add the nullable attributes:

 class TestCache : ICache
 {
   bool ICache.TryOpen(string name, ICacheItem? cacheItem)
   {
     throw new NotImplementedException();
   }
 }

This causes a warning/error complaining that "cacheItem" doesn't match the implemented member.

[yaakov-h]

Sounds like an issue for the Roslyn repo.

[kurmanov]

I've checked what's happening in the debugger and it turns out that when interface is being implemented explicitly all attributes for method parameters are being omitted

`srcWorkspacesCSharpPortableCodeGenerationParameterGenerator.cs:144

    private static SyntaxList<AttributeListSyntax> GenerateAttributes(
        IParameterSymbol parameter, bool isExplicit, CodeGenerationOptions options)
    {
        if (isExplicit)
        {
            return default;
        }

        var attributes = parameter.GetAttributes();
        if (attributes.Length == 0)
        {
            return default;
        }

        return AttributeGenerator.GenerateAttributeLists(attributes, options);
    }

`

Blaming the file didn't help since this logic was there from the very start of Roslyn repo. After I removed that check quick action started working as expected but several tests got broken in Microsoft.CodeAnalysis.Editor.CSharp.UnitTests.ImplementInterface.ImplementInterfaceTests: TestAttributesExplicit,TestIUnknownIDispatchAttributes2,TestOptionalDateTime2

If you look closely at the test suite code you'll notice that omitting attributes is expected behavior for " Implement interface explicitly" option - there are instances of these tests for implicit option and those have attributes placed in generated classes, but explicit version lack them. I wonder what was the logic behind that feature. There's a link to internal Microsoft TFS http://vstfdevdiv:8080/DevDiv2/DevDiv/_workitems/edit/530265 maybe it says something about it.