Call a function without parenthesis?

Question

I was looking at the "Contose Cookbook" training for creating Windows 8.1 Apps.

Going through the first lab, I saw something like variable += function;. The function didn't have paranthesis, and as far as I can see, you can create functions without paranthesis, which are properties, but you have to call them with.

Here is a class where I found this:

namespace ContosoCookbook
{
    sealed partial class App : Application
    {
        public App()
        {
            this.InitializeComponent();
            this.Suspending += OnSuspending;
        }

        private async void OnSuspending(object sender, SuspendingEventArgs e) 
        {
            var deferral = e.SuspendingOperation.GetDeferral();
            await SuspensionManager.SaveAsync();
            deferral.Complete();
        }

        ...
    }
}

What type of calling or function is that that accepts this.

The Application.Suspending event from msdn.

Answer 1

The event handler in question takes a delegate which matches the method's arguments, allowing it to take a method group.

As a simple example, any Action variable can accept any void method with no parameters as a method group. The above event handler's delegate likely takes an object to indicate the sender and some sort of EventArguments.

The most common delegates anymore are Action<T>, Func<T> and Predicate<T>, which represent void methods, methods with a specified return type, and methods which return booleans respectively. They can also be assigned lambdas. All of them have varying numbers of parameters. They can be very useful in some situations.

Note that assigning a delegate does not invoke it; that can be done at a later time. Delegates can also be combined. It may also be worth noting that delegates can do strange things with local variables: if you assign a delegate with something like (name) => tempName += name where tempName is a local variable, your local variable will be modified on invocation of the delegate. This makes sense, but may not be obvious.