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Tasks in C#

A Task represents an asynchronous operation. Managing threads is a tedious job. Tasks provide more features than threads such as continuation. They also provide easier exception handling, and return values which makes Tasks easier to use. Tasks use threads behind the scenes to accomplish the job. Tasks were introduced in .NET Framework 4 and were enhanced in C# 5.0

By default, Tasks use pooled threads. Unlike foreground threads, a pooled thread is a background thread which ends as soon as the main thread in the program ends.

Creating a Task is quite straightforward. Just call the static Run method with an Action delegate or an equivalent lambda expression:

Task.Run (() => Console.WriteLine ("Hello Task!"));	// Hello Task!
 

The previous code is equivalent to the following:

        new Thread(() => Console.WriteLine ("Hello Task!")).Start();
    

If you are using .NET Framework 4.0, you cannot use the Run method. You can use Task.Factory.New method:

Task.Factory.StartNew (() => Console.WriteLine ("Hello Task!")); // Hello Task!
    

Task.Run returns a Task object which can be used to track the progress of the task:

void Main()
{
	 var task = Task.Run ( () => Console.WriteLine ("Hello Task!"));
	 
	 Console.WriteLine (task.IsCompleted);  // False
}

    

The previous returns false because the task has not completed yet. The Wait method blocks the current thread until the task completes:

void Main()
{
	 var task = Task.Run ( () => Console.WriteLine ("Hello Task!"));
	 
	 task.Wait (); // Wait until task completes
	 
	 Console.WriteLine (task.IsCompleted);  // True
}

A Task can also return a value. The generic class Task<TResult> Run method can accept a Func<TResult> delegate or an equivalent lambda expression:

void Main()
{
	 var task = Task<int>.Run ( () => 1000);	 	 
	 
	 Console.WriteLine (task.Result);  // 1000
	 	
}

The property Result holds the result and accessing this property implicitly waits for the task to complete.

Tasks propagate exceptions. This means if the code in a task throws an unhandled exception, the exception propagates to the caller of the task.

void Main()
{
	 int i = 0;
	 var task = Task<int>.Run ( () => 1 / i);	 	 
	 
	 try
	 {
	 	Console.WriteLine (task.Result);  // Exception
	 }
	 
	 catch(AggregateException ex)
	 {
	   if (ex.InnerException is DivideByZeroException)
	        Console.WriteLine ("Division by 0");
	   else
	    	Console.WriteLine (ex);
	 }
	 	
}
    

The exception is wrapped in AggregateException in order to fit well in Parallel programming.

A continuation tells a task to do something else whenever it completes. It is normally implemented using callbacks. There are two ways to implement continuation. The first one is by using an awaiter object (this is only available since Framework 4.5)

var task = Task.Run (() => Enumerable
       .Range(1, 1000000)
       .Count (n => (n & 1) == 0)); // Count the even integers between 0 and million
	 
	 var awaiter = task.GetAwaiter();
	 
	 awaiter.OnCompleted (() => {
	 
	 	int result = awaiter.GetResult();
	 	Console.WriteLine (result);
	 
	 });

    

The method GetAwaiter returns an awaiter object. The awaiter.OnCompleted method tells the antecedent task to execute a delegate whenever it completes or faults.

Note that the result was accessed using the GetResult method. We could have accessed it using task.Result. The difference is that if GetResult is called and the antecedent task throws an exception, the exception will not be wrapped in AggregateException and this makes the exception handler code cleaner.

The other way to attach a continuation method is by calling ContinueWith. The method ContinueWith returns a Task object, so the result can be accessed via the property Result:

 
var task = Task.Run (() => Enumerable
        .Range(1, 1000000)
        .Count (n => (n & 1) == 0)); // Count the even integers between 0 and million
	 
	task.ContinueWith ( t => {
	 
	 	int result = t.Result;
	 	Console.WriteLine (result);
	 
	 });

    

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