Question

I’ve been seing Func<> for sometime now, and I’ve manage to avoid it (for now). But, now it looks like I can’t dodge it forever. For instance, I tried Dynamic Linq, but almost everything was in terms of Func<>. I’ve tried one of my book (C# 2008/Deitel&Deitel) and also MSDN but I’m not getting it yet. They all jump straight in the subject.

1. What can be said (in few words) about Func<>
2. Can I get some links on the web that can get me started on that matter?

Thanks for helping

Answer 1

Func<> is a generic delegate – it is just very convenient to use, because you don’t have to create your own delegate for each argument/return type combination.
Earlier, you had to write something like:

public delegate long MyDelegate( int number );

public void Method( IEnumerable<int> list, MyDelegate myDelegate )
{
    foreach( var number in list )
    {
        myDelegate( number );
    }
}

You had to publish your delegate so that a user can call your method correctly. Especially when you need a bunch of different delegates you ended up publishing one for every argument list and return type.
With Func<> you just write:

public void Method( IEnumerable<int> list, Func<int, long> myDelegate )
{
    foreach( var number in list )
    {
        myDelegate( number );
    }
}

It means the same as the first code example – Func<int, long> defines a delegate that takes one integer argument and returns a long value.

Of course you can use longer parameter lists, too: Func<int, int, bool, long> will still return a long value while it takes two ints and a bool value. If you wish a delegate without return value you will have to use Action<>, which will have void as a return type.

EDIT (by request): How to call the method in my example:

For the caller, there is no difference between the solution with MyDelegate or Func<>. In both cases he has three options to call the method:

Using a lambda notation (C# 3.0 required, probably the best solution for short methods):

Method( myList, i => i * i );

By using an anonymous method (C# 2.0 required):

Method( myList, delegate( int i )
{
    return i * i;
} );

Or by using a real method as an argument:

Method( myList, Square );

private static long Square( int number )
{
    return number * number;
}