Question

I have a class that the user can pass an Action into (or not).

public class FooClass<T> : BaseClass<T>
{
    public FooClass()
        : this((o) => ()) //This doesn't work...
    {
    }

    public FooClass(Action<T> myAction)
        : base(myAction)
    {
    }
}

---

Basically, I can’t pass null into my base class for the Action. But, at the same time I don’t want to force my user to pass in an Action. Instead, I want to be able to create a “do-nothing” action on the fly.

Answer 1

You want to say

this(t => { })

Think of it like this. You need t => anonymous-expression-body. In this case, anonymous-expression-body is an expression or a block. You can’t have an empty expression so you can’t indicate an empty method body with an expression. Therefore, you have to use a block in which case you can say { } to indicate the a block has an empty statement-list and is therefore the empty body.

For details, see the grammar specification, appendix B.

And here’s another way to think of it, which is a way that you could use to discover this for yourself. An Action<T> is a method that takes in a T and returns void. You can define an Action<T> via an non-anonymous method or via an anonymous method. You are trying to figure out how to do it using an anonymous method (or rather, a very special anonymous method, namely a lambda expression). If you wanted to do this via a non-anonymous method you would say

private void MyAction<T>(T t) { }

and then you could say

this(MyAction)

which uses the concept of a method group. But now you want to translate this to a lambda expression. So, let’s just take that method body and make it a lambda expression. Therefore, we throw away the private void MyAction<T>(T t) and replace it with t => and copy verbatim the method body { }.

this(t => { })

Boom.