Dynamically implementing an interface in .NET 4.0 (C#)

The opensource framework Impromptu-Interface was designed to do this. It generates a cached lightweight proxy with a static interface and uses the dlr to forward the invocation to the original object.

using ImpromptuInterface;

public interface ISimpeleClassProps
{
    string Prop1 { get;  }

    long Prop2 { get; }

    Guid Prop3 { get; }
}

-

dynamic tOriginal= new ExpandoObject();
tOriginal.Prop1 = "Test";
tOriginal.Prop2 = 42L;
tOriginal.Prop3 = Guid.NewGuid();

ISimpeleClassProps tActsLike = Impromptu.ActLike(tOriginal);

As far as I know, it's not possible without manual intervention to write or generate code that forwards the interface members to the wrapped instance. If you would like to see Microsoft-provided support for this sort of thing, you might want to consider voting at https://connect.microsoft.com/VisualStudio/feedback/details/526307/add-automatic-generation-of-interface-implementation-via-implementing-member .

I think I wrote a library that does what you want... It is called DynamicWrapper (on CodePlex) and it will automatically wrap a a class so it implements an interface. Is this what you want?

Explicit casting, as, and is fail because of type comparison would against your proxy base class, but implicit cast can trigger DynamicObject.TryConvert, such that you can then return inner object in-lieu of the dynamic object.
- TryConvert MSDN Documentation

While the code below works, this isn't interface delegation per se, only exposure of internal state. It sounds like you might be looking for something like an interception pattern such as Brian's DynamicWrapper.

dynamic wrapper = new Proxy(new Foo());
IFoo foo = wrapper;
foo.Bar();

class Proxy : DynamicObject
{
    ...

    public override bool TryConvert(ConvertBinder binder, out object result)
    {
        Type bindingType = binder.Type;
        if (bindingType.IsInstanceOfType(target))
        {
            result = target;
            return true;
        }
        result = null;
        return false;

    }

}

Complementing the answer from @jbtule I have created my CustomActivator that is able to create a dynamic object at runtime and make it implements a desired interface. I also use the Impromptu-Interface framework to accomplish that.

The call is simple:

CustomActivator.CreateInstance<MyInterface>();

I put it on github.