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.
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.
The problem is that the return value is not passed between the method invocations, so the the output only captures the last string returned. I.e. the return of Add is lost.
