As we know, that the TPL is centered on the Task and Task<TResult> objects. We can execute an asynchronous task by calling the Task.Run method and execute a delegate method or a block of code asynchronously and use Wait or other methods on that task. However, this approach is not always adequate, and there are scenarios where we may have different approaches to executing asynchronous operations, and we may use an Event-based Asynchronous Pattern (EAP) or an Asynchronous Programming Model (APM). To implement TAP principles here, and to get the same control over asynchronous operations executing with different models, we can use the TaskCompletionSource<TResult> object.

The TaskCompletionSource<TResult> object is used to create a task that executes an asynchronous operation. When the asynchronous operation completes, we can use the TaskCompletionSource<TResult> object to set the result, exception, or state of the task.

Here is a basic example that executes the ExecuteTask method that returns Task, where the ExecuteTask method uses the TaskCompletionSource<TResult> object to wrap the response as a Task and executes the ExecuteLongRunningTask through the Task.StartNew method:

static void Main(string[] args) 
{ 
  var t = ExecuteTask(); 
  t.Wait(); 
  Console.Read(); 
} 
 
public static Task<int> ExecuteTask() 
{ 
  var tcs = new TaskCompletionSource<int>(); 
  Task<int> t1 = tcs.Task; 
  Task.Factory.StartNew(() => 
  { 
    try 
    { 
      ExecuteLongRunningTask(10000); 
      tcs.SetResult(1); 
    }catch(Exception ex) 
    { 
      tcs.SetException(ex); 
    } 
  }); 
  return tcs.Task; 
 
} 
 
public static void ExecuteLongRunningTask(int millis) 
{ 
  Thread.Sleep(millis); 
  Console.WriteLine("Executed"); 
}