Asynchronous Delegate Problem

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Hi... All, I want to ask. I have the following code :

internal static class Program
{    
    private static Int32 number = 0;        
    private static Boolean IsComplete = false; 
    private static void Main()
    {        
        Action<Int32> action = arg =>
        {
            for (Int32 i = 0; i < arg; i++)
            {                
                Thread.Sleep(700);
                Console.WriteLine("Result : " + (number += i));
            }                        
        };
 
        action.BeginInvoke(5,
            arg =>
            {                
                Console.WriteLine("action complete, result = {0}", (Int32)arg.AsyncState);                
                IsComplete = true;
            }, 
            number);//if number changed to 10, then "result = 10" printed 
        while (!IsComplete)
        {
            Thread.Sleep(500);
            Console.WriteLine("Main is doing more...");
        }
 
        Console.ReadLine();
    }
}

Why, when action has finished and it's callback method get called always
printed "action complete, result = 0"(number isn't passed-in to callback method).
But, when I pass-in 10, then it will passed-in to the call back method ?

Posted 9-Apr-11 20:57pm

adaapanya

Updated 8-Jun-11 2:45am

v8

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OriginalGriff · Accepted Answer · 2011-04-09T21:23:00

It works fine!

All you have to do is remember when things happen.

Action<int32> action = arg =>
{
    for (Int32 i = 0; i < arg; i++)
    {
        Thread.Sleep(700);
        Console.WriteLine("Result : " + (number += i));
    }
};</int32>

Sets up the delegate. Nothing gets executed yet.

action.BeginInvoke(5,
    arg =>
    {
        Console.WriteLine("action complete, result = {0}", (Int32)arg.AsyncState);
        IsComplete = true;
    },
    number);//if number changed to 10, then "result = 10" printed

Evaluates all parameters, then starts the invoke. The parameters evaluate to:
A constant: 5
An anonymous delegate.
The current value of Number: 0
The thread is then created, and starts. But, the anonymous delegate that is executed when it finishes already has it's parameters ready: 0

while (!IsComplete)
{
    Thread.Sleep(500);
    Console.WriteLine("Main is doing more...");
}

Waits for the invoked thread to complete.

So, when the thread complete, it prints zero, not the final value of number.
When you change "number" to "10" the parameter evaluation is of a constant value.

To prove it, make the initial value of number -6 and see what happens!