#372 – What Delegates Are Good For

Creating an instance of a delegate that allows you to invoke a method through the delegate doesn’t seem to add much value.  Why not just invoke the method directly?

            // New delegate instance
            LoggerDelegate del1 = Method1;

            // Can still invoke the method directly
            Method1("Direct");

            // Or can invoke through the delegate instance
            del1("Via delegate");

A delegate allows us to treat the reference to the method as a real object, storing it in a variable or passing it to a method as a parameter.  This allows great flexibility, because a method can be told at runtime which child method to call.

        static void DoSomeCalculations(LoggerDelegate log)
        {
            // Do some stuff

            log("I did stuff");

            // Do other stuff

            log("And I did some other stuff");
        }

You then pass in the specific logging method when you invoke DoSomeCalculations and it will call that method.

            DoSomeCalculations(Method1);
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About Sean
Software developer in the Twin Cities area, passionate about software development and sailing.

9 Responses to #372 – What Delegates Are Good For

  1. kai zhou says:

    Nice work, thank you Sean.

  2. Kristof says:

    Shouldn’t you pass del1 instead of Method1 as the argument? As far as I understood from your post that’s the whole point of delegate!

  3. Sean says:

    Kristof, note that you can pass either a delegate instance or a method name to a function that has a delegate type as one of its parameters. (E.g. DoSomeCalculations()). Similar to if you had a parameter of type int and you could either pass a integer typed variable or just an integer constant. You can think of the method name as like a delegate type constant.

  4. Mike Lee says:

    Sean, thanks for this. It wasn’t obvious to me at first, but I think I understand the significance now of using the delegate to use pass a method as an argument. To follow up, would there need to be an instance of a delegate declared for each method before it is passed? (which seems, odd and unintuitive) Assuming you don’t have to, could you then pass any method through the delegate parameter?

    • Sean says:

      Thanks Mike. No, you don’t need to define a delegate instance at all, but just pass the method name to the method that takes a delegate type as a parameter. A delegate instance will get created for you.

  5. Yume says:

    Thank you, I’ve never understood the use of delegates before! Been on your blog all day.

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