How do I express a void method call as the result of DynamicMetaObject.BindInvokeMember?

I'm trying to give a short example of IDynamicMetaObjectProvider for the second edition of C# in Depth, and I'm running into issues.

I want to be able to express a void call, and I'm failing. I'm sure it's possible, because if I dynamically call a void method using the reflection binder, all is fine. Here's a short but complete example:

using System;
using System.Dynamic;
using System.Linq.Expressions;

class DynamicDemo : IDynamicMetaObjectProvider
{
    public DynamicMetaObject GetMetaObject(Expression expression)
    {
        return new MetaDemo(expression, this);
    }

    public void TestMethod(string name)
    {
        Console.WriteLine(name);
    }
    
}

class MetaDemo : DynamicMetaObject
{
    internal MetaDemo(Expression expression, DynamicDemo demo)
        : base(expression, BindingRestrictions.Empty, demo)
    {
    }
    
    public override DynamicMetaObject BindInvokeMember
        (InvokeMemberBinder binder, DynamicMetaObject[] args)
    {
        Expression self = this.Expression;
        
        Expression target = Expression.Call
            (Expression.Convert(self, typeof(DynamicDemo)),
             typeof(DynamicDemo).GetMethod("TestMethod"),
             Expression.Constant(binder.Name));
 
        var restrictions = BindingRestrictions.GetTypeRestriction
            (self, typeof(DynamicDemo));
        
        return new DynamicMetaObject(target, restrictions);
    }
}

class Test
{
    public void Foo()
    {
    }
    
    static void Main()
    {
        dynamic x = new Test();
        x.Foo(); // Works fine!
        
        x = new DynamicDemo();
        x.Foo(); // Throws
    }
}

This throws an exception:

> Unhandled Exception: > System.InvalidCastException: The > result type 'System.Void' of the > dynamic binding produced by the object > with type 'DynamicDemo' for the binder > 'Microsoft.CSharp.RuntimeBinder.CSharpInvokeMemberBinder' > is not compatible with the result type 'System.Object' expected by the > call site.

If I change the method to return object and return null, it works fine... but I don't want the result to be null, I want it to be void. That works fine for the reflection binder (see the first call in Main) but it fails for my dynamic object. I want it to work like the reflection binder - it's fine to call the method, so long as you don't try to use the result.

Have I missed a particular kind of expression I can use as the target?

This is similar to:

https://stackoverflow.com/questions/1360097/dlr-return-type

You do need to match the return type specified by the ReturnType property. For all of the standard binaries this is fixed to object for almost everything or void (for the deletion operations). If you know you're making a void call I'd suggest wrapping it in:

Expression.Block(
    call,
    Expression.Default(typeof(object))
);

The DLR used to be quite lax about what it would allow and it would provide some minimal amount of coercion automatically. We got rid of that because we didn't want to provide a set of convensions which may or may not have made sense for each language.

It sounds like you want to prevent:

dynamic x = obj.SomeMember();

There's no way to do that, there'll always be a value returned that the user can attempt to continue to interact with dynamically.

I don't like this, but it seems to work; the real problem seems to be the binder.ReturnType coming in oddly (and not being dropped ("pop") automatically), but:

if (target.Type != binder.ReturnType) {
    if (target.Type == typeof(void)) {
        target = Expression.Block(target, Expression.Default(binder.ReturnType));
    } else if (binder.ReturnType == typeof(void)) {
        target = Expression.Block(target, Expression.Empty());
    } else {
        target = Expression.Convert(target, binder.ReturnType);
    }
}
return new DynamicMetaObject(target, restrictions);

Perhaps the callsite expects null to be returned but discards the result - This enum looks interesting, particularly the "ResultDiscarded" flag...

[Flags, EditorBrowsable(EditorBrowsableState.Never)]
public enum CSharpBinderFlags
{
    BinaryOperationLogical = 8,
    CheckedContext = 1,
    ConvertArrayIndex = 0x20,
    ConvertExplicit = 0x10,
    InvokeSimpleName = 2,
    InvokeSpecialName = 4,
    None = 0,
    ResultDiscarded = 0x100,
    ResultIndexed = 0x40,
    ValueFromCompoundAssignment = 0x80
}

Food for thought...

UPDATE:

More hints can be gleaned from Microsoft / CSharp / RuntimeBinder / DynamicMetaObjectProviderDebugView which is used (I presume) as a visualizer for debuggers. The method TryEvalMethodVarArgs examines the delegate and creates a binder with the result discarded flag (???)

 Type delegateType = Expression.GetDelegateType(list.ToArray());
    if (string.IsNullOrEmpty(name))
    {
        binder = new CSharpInvokeBinder(CSharpCallFlags.ResultDiscarded, AccessibilityContext, list2.ToArray());
    }
    else
    {
        binder = new CSharpInvokeMemberBinder(CSharpCallFlags.ResultDiscarded, name, AccessibilityContext, types, list2.ToArray());
    }
    CallSite site = CallSite.Create(delegateType, binder);

... I'm at the end of my Reflector-foo here, but the framing of this code seems a bit odd since the TryEvalMethodVarArgs method itself expects an object as a return type, and the final line returns the result of the dynamic invoke. I'm probably barking up the wrong [expression] tree.

-Oisin

The C# binder (in Microsoft.CSharp.dll) knows whether or not the result is used; as x0n (+1) says, it keeps track of it in a flag. Unfortunately, the flag is buried inside a CSharpInvokeMemberBinder instance, which is a private type.

It looks like the C# binding mechanism uses ICSharpInvokeOrInvokeMemberBinder.ResultDiscarded (a property on an internal interface) to read it out; CSharpInvokeMemberBinder implements the interface (and property). The job appears to be done in Microsoft.CSharp.RuntimeBinder.BinderHelper.ConvertResult(). That method has code that throws if the aforementioned ResultDiscarded property doesn't return true if the type of the expression is void.

So it doesn't look to me like there's an easy way to tease out the fact that the result of the expression is dropped from the C# binder, in Beta 2 at least.