Monday, December 1, 2014

async and await in C#

Async and Await in C#
In this article we will discuss about the two new keywords that were introduced in C# 5.0, for implementing asynchronous programming. So let's start with some basic their definitions, rules we have to follow to implement it.
await: As per MSDN, await is
The await operator is applied to a task in an asynchronous method to suspend the execution of the method until the awaited task completes. The task represents ongoing work.
This means, if we apply this keyword to a method, and when the control reaches that method, that method will start running in asynchronous mode and the control will move on to the next statement in the program. This is simply done by using the syntax
await Method_Name()
async: Simply adding this keyword will not work. The method in which await is used, must be marked with the async keyword. So if we need to make a method (say Method1Async()) asynchronous ,using await, inside a method named Method2(), we call it using the code as:
function void Method2()
  await Method1Async();
Task: The task or function which is to be made asynchronous, has a return type of the type Task which represents void return type or Task<TResult>, where TResult is the type of data returned by the function. So, for our method above, i.e. Method2, assuming that it returns an integer value, the signature will be of the following type:
function Task<Int32> Method2()
    return 1;
So this was the basic discussion. Now let's convert the above code into an example. For this, our asynchronous method or Method2, will be something which takes time to complete and returns us an integer value. In our case, we will simply make it sleep using Thread.Sleep and then return an integer value. So it's signature will become like the following:

Next, we create another function inside which we will call this function using await keyword. The reason we are not calling this function in the Main function is that we need to make the parent function as async and Main function cannot be made async. So we have to introduce an intermediate function named Method2(). Next, we simply need to call this function in the Main function. We are also printing different values, which will help us to evaluate and understand the flow of the program in better way.  So our complete code becomes like the following:

Run this code and see the results:

Let's try to understand the flow of the program with the above output.
1. The program execution starts with the execution of the Main function and prints the very first line "Starting the ASYNCHRONOUS process".
2. Next, it moves on to call 
Method1, which in turns Method2. As the Method2 is marked with the await keyword, it starts its execution in an asyncrhonous mode, suspend the further execution of the Method1 and immediately returns to the calling statement in the Main function, without printing the message in the very next line.
3. Then it continues the 
Main function and starts processing the for loop and print the message.
4. Finally when execution of the asynchronous method is completed, it again resumes the execution of the 
Method2 and prints the results, with the return value received from Method2.
So this was about the use of the async and await keywords. Hope you enjoyed reading it...!!!

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