Delegate Covariance Confusion Conundrum!

Question

Why does this not work? Do I not understand delegate covariance correctly?

public delegate void MyDelegate(object obj)

public class MyClass
{
    public MyClass()
    {
         //Error: Expected method with 'void MyDelegate(object)' signature
         _delegate = MyMethod;
    }

    private MyDelegate _delegate;

    public void MyMethod(SomeObject obj)
    {}

}

Answer 1

The MyDelegate type declares that you can pass any kind of object in. However, MyMethod only takes objects of type SomeObject. What happens if I try to invoke the delegate passing a different kind of object: _delegate("a string object")? According to the declaration of MyDelegate, this should be allowed, but your function MyMethod can't actually receive a string argument.

Answer 2

Correct - you don't understand covariance correctly - yet :) Your code would work if you had the same types but as return values, like this:

public delegate object MyDelegate()

public class MyClass
{
    public MyClass()
    {
         _delegate = MyMethod;
    }

    private MyDelegate _delegate;

    public SomeObject MyMethod() { return null; }
}

That would demonstrate covariance. Alternatively, you can keep it as parameters but switch the types around:

public delegate void MyDelegate(SomeObject obj)

public class MyClass
{
    public MyClass()
    {
         _delegate = MyMethod;
    }

    private MyDelegate _delegate;

    public void MyMethod(object obj) {}
}

This now demonstrates contravariance.

My rule of thumb is to ask myself, "given the delegate, what could I do with it? If I can pass in an argument which would break the method, the conversion should have failed. If the method can return something which would break the caller, the conversion should have failed."

In your code, you could have called:

_delegate(new object());

At that point, poor MyMethod has a parameter which is meant to be of type SomeObject, but is actually of type object. This would be a Very Bad Thing, so the compiler stops it from happening.

Does that all make more sense?

Answer 3

You need to use a generic.

EDIT: Why? Because as another poster noted, Object and SomeObject do not equate to the same thing as Object may not be SomeObject. This is the whole point of Generics in the language.

public delegate void MyDelegate<T>(T obj)

public class MyClass
{
    public MyClass()
    {
        _delegate = MyMethod;
    }

    private MyDelegate<SomeObject> _delegate;

    public void MyMethod(SomeObject obj)
    {
    }
}

Answer 4

Arguments are contravariant, return types are covariant. If the delegate were to be called with an object that is not an instance of SomeObject, you'd have a typing error. On the other hand, returning SomeObject from a routine wrapped in a delegate that returns object is fine.

Answer 5

From the MSDN link you provided

Covariance permits a method to have a more derived return type than what is defined in the delegate. Contravariance permits a method with parameter types that are less derived than in the delegate type.

You're attempting to use a more derived parameter type which isn't supported (although .NET 4.0 probably will since this has sorted out many covariance/contravariance issues).