Action vs delegate event

Question

I have seen developers using the below codes quite alternatively. What is the exact difference between these, and which ones go by the standard? Are they same, as Action and Func<T> is a delegate as well:

public event Action<EmployeeEventAgs> OnLeave;
public void Leave()
{
    OnLeave(new EmployeeEventAgs(this.ID));
}

VS

public delegate void GoOnLeave(EmployeeEventAgs e);
public event GoOnLeave OnLeave;
public void Leave()
{
    OnLeave(new EmployeeEventAgs(this.ID));
}

Answer 1

Fwiw, neither example uses standard .NET conventions. The EventHandler<T> generic should declare the event:

public event EventHandler<EmployeeEventArgs> Leave;

The "On" prefix should be reserved for a protected method that raises the event:

protected virtual void OnLeave(EmployeeEventArgs e) {
    var handler = Leave;
    if (handler != null) handler(this, e);
}

You don't have to do it this way, but anybody will instantly recognize the pattern, understand your code and know how to use and customize it.

And it has the great advantage of not being forced to choose between a custom delegate declaration and Action<>, EventHandler<> is the best way. Which answers your question.

Answer 2

Action<T> is exactly the same as delegate void ... (T t)

Func<T> is exactly the same as delegate T ... ()

Answer 3

Action is just a shortcut for the full delegate declaration.

public delegate void Action<T>(T obj)

http://msdn.microsoft.com/en-us/library/018hxwa8.aspx

Which one to use would depend on your organizations coding standards/style.

Answer 4

In general, they are equivalent. But in the context of using a delegate for the type of an event, the convention is to use EventHandler (where T inherits EventArgs):

public event EventHandler<EmployeeEventArgs> Left;

public void Leave()
{
    OnLeft(this.ID);
}

protected virtual void OnLeft(int id)
{
    if (Left != null) {
        Left(new EmployeeEventArgs(id));
    }
}

Answer 5

You could have written these Action and Func generic delegates yourself, but since they're generally useful they wrote them for you and stuck them in .Net libraries.

Answer 6

Yes, Action and Func are simply convenience delegates that have been defined in the 3.5 clr.

Action, Func and lambdas are all just syntactical sugar and convenience for using delegates.

There is nothing magic about them. Several people have written simple 2.0 addon libraries to add this functionality to 2.0 code.