CODEWITHSUNDEEP
c#reflectiondelegates
Sean Anderson
Sean Anderson

Reputation: 29331

"Object does not match target type" on basic Delegate example

I'm playing around with Delegates, but I'm clearly not grasping something basic.

I've got the following:

class Test
{
    delegate bool Foo();

    public void Run()
    {
        Delegate foo = new Foo((delegate () { return true; }));
        bool result = (bool)foo.Method.Invoke(foo, null);
    }
}

Test test = new Test();
test.Run();

In the above example I encounter an exception: "Object does not match target type."

The following works alright, though:

class Test
{
    delegate bool Foo();

    public void Run()
    {
        Foo foo = new Foo((delegate () { return true; }));
        bool result = foo.Invoke();
    }
}

Test test = new Test();
test.Run();

What's incorrect with my first implementation? Shouldn't I be invoking foo's method, as foo, with no parameters? This would seem equivalent to the latter code.

Upvotes: 0

Views: 469

Answers (1)

Luaan
Luaan

Reputation: 63772

You're bypassing the entire reason you'd use the delegate in the first place. As a rule of thumb, don't touch Method.

The first argument in MethodInfo.Invoke is the this instance. This can be null or something else (e.g. a closure, or the actual object instance when the delegate represents an instance method), but it will never be the delegate instance - that doesn't make any sense.

In your case, it is indeed a closure instance. But you have no reference to the closure outside of the delegate - it's created behind the scene by the compiler. So the only proper way to do the invocation (using MethodInfo) would be foo.Method.Invoke(foo.Target, null);. Of course, this is entirely redundant - just use foo(); like any sane human, and you'll be fine :)

The same way, stop using the old delegate syntax. There's little point in doing so. A simple

Func<bool> foo = () => true;
Console.WriteLine(foo());

will work fine. You can substitute your own delegate for Func<bool> if you really need to, but using custom delegates isn't very useful nowadays.

Upvotes: 3

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