Question

Lets imagine simple delegate calls:

void Main()
{
    Func<int, int, string> tfunc = null;
    tfunc += Add; // bind first method
    tfunc += Sub; // bind second method 

    Console.WriteLine(tfunc(2, 2));
}

private string Add(int a, int b)
{
    return "Add: " + (a + b).ToString();
}

private string Sub(int a, int b)
{
    return "Sub: " + (a - b).ToString();
}

The result of this program is:

Sub: 0

So, why Add method was not called? I’m expecting to call Method Add, and then method Sub.

Answer 1

Add was correctly chained and called, take a look at the result of

void Main()
{
    Func<int, int, string> tfunc = null;
    tfunc += Add; // bind first method
    tfunc += Sub; // bind second method 

    Console.WriteLine(tfunc(2, 2));
}

private string Add(int a, int b)
{
    Console.WriteLine("Inside Add");
    return "Add: " + (a + b).ToString();
}

private string Sub(int a, int b)
{
    Console.WriteLine("Inside Sub");
    return "Sub: " + (a - b).ToString();
}

It is:

Inside Add
Inside Sub
Sub: 0

What is not chained, because there is no way to access it, is the result of the Add method. Delegates that return a value, in case of chaining, return the value of the last method invoked, that is the last method that was added to the delegate.

This is specified in part 15.4 of the C# 4.0 language specification

Invocation of a delegate instance whose invocation list contains
multiple entries proceeds by invoking each of the methods in the
invocation list, synchronously, in order. …
If the delegate invocation includes output parameters or a
return value, their final value will come from the invocation of the
last delegate in the list.