Combine of covarince/contravariance delegates

[iskiselev]

Currently C# allow combine (using +/- operators) delegates with different compatible (covarince/contravariance) types, but .Net Delegate.Combine/Delegate.Remove doesn't allow it. So, this sample legal C# code (that will be compiled even without warnings) will result in run-time exception:

using System;
internal class Program
{

    private static void Main(string[] args)
    {
        Func<string, object> a1 = SomeFunction;
        Func<object, object> b1 = SomeFunction;
        var c1 = a1 + b1;

        Func<object, string> a2 = SomeFunction;
        Func<object, object> b2 = SomeFunction;
        var c2 = a2 - b2;
    }

    public static string SomeFunction(object input)
    {
        return string.Empty;
    }
}

[svick]

Are you proposing to change the behavior of the runtime? If that's the case, Roslyn can't do anything about it, and you should probably make the proposal at the CoreCLR repo.

[HaloFour]

The only thing that I think that the C# compiler could do in this case would be to wrap the delegate, e.g.:

Func<string, object> a1 = SomeFunction;
Func<object, object> b1 = SomeFunction;
var c1 = a1 + b1;  // implicitly treated as a1 + new Func<string, object>(b1);

That would avoid the ArgumentException but it would increase the overhead of invoking the delegate.

[iskiselev]

I believe, that it should be fixed on CoreCLR end. At the same time, my preference here for now - see compile time error or warning. Even fixing it in CoreCLR will leave problem for source compiled against .Net Framework 4.6 or earlier.

[gafter]

Moved to CLR issue https://github.com/dotnet/coreclr/issues/1712