Question

Consider the following:

using System;
using System.Dynamic;

namespace DynamicTest
{
    class Program
    {
        static void Main(string[] args)
        {
            dynamic d = new DynamicTest();
            var v = d.foo();
            Console.WriteLine(v.Value);

            Console.ReadKey();
        }
    }

    public interface IValueProvider<T>
    {
        T Value
        {
            get;
        }
    }

    public class DynamicTest : DynamicObject
    {
        public override bool TryInvokeMember(InvokeMemberBinder binder, object[] args, out object result)
        {
            result = new ValueProvider<int>(32);
            return true;
        }

        private class ValueProvider<T> : IValueProvider<T>
        {
            public ValueProvider(T t)
            {
                this.Value = t;
            }

            public T Value
            {
                get;
                private set;
            }
        }
    }
}

This fails at runtime with:

Microsoft.CSharp.RuntimeBinder.RuntimeBinderException was unhandled
  Message='object' does not contain a definition for 'Value'
  Source=Anonymously Hosted DynamicMethods Assembly
  StackTrace:
       at CallSite.Target(Closure , CallSite , Object )
       at System.Dynamic.UpdateDelegates.UpdateAndExecute1[T0,TRet](CallSite site, T0 arg0)
       at DynamicTest.Program.Main(String[] args)
       at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
       at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
       at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
       at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean ignoreSyncCtx)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
       at System.Threading.ThreadHelper.ThreadStart()
  InnerException: 

If I change the ValueProvider<T> class to public rather than private it works as expected:

// this allows it to work
public class ValueProvider<T> : IValueProvider<T>

Or, instead, if I cast the result to IValueProvider<int> it also works:

// this also allows it to work
dynamic d = new DynamicTest();
var v = (IValueProvider<int>)d.foo();

If I try using the interface as the compile-time type in TryInvokeMember, it doesn’t work (didn’t really expect it to, but thought I’d try):

// this doesn't help
public override bool TryInvokeMember(InvokeMemberBinder binder, object[] args, out object result)
{
    var valueProvider = (IValueProvider<int>)new ValueProvider<int>(32);
    result = valueProvider;
    return true;
}

Can anyone point out what I’m missing here? Is there a way for me to get the code above working without resorting to making all my types public and without casting?

Answer 1

Interestingly, it works if the accessibility is internal:

internal class ValueProvider<T> : IValueProvider<T> { ... }

which as Hassan notes (comments) makes sense, as that is the accessibility that would normally be required for Program to use the API members of DynamicTest (in the same assembly). If we move Main to DynamicTest and leave it private it again works, which is the correct accessibility we should expect.

Maybe use internal for now.

Of course, an even better approach here would be to just use the interface. IMO you don’t need dynamic here; what you perhaps need is a non-generic IValueProvider:

public interface IValueProvider
{
    object Value { get; }
}
public interface IValueProvider<T> : IValueProvider
{
    new T Value{get; }
}

with (in the class):

object IValueProvider.Value { get { return Value; } }

and:

var v = (IValueProvider)d.foo();
Console.WriteLine(v.Value);