How to run multiple asynchronous tasks in C# and wait for them all to complete?

Task.WaitAll blocks the current thread until all other tasks have completed execution.

Task.WhenAll method is used to create a task that will complete if and only if all other tasks have completed. In the first example, we can see that when using Task.WhenAll, task completion occurs before other tasks complete. This means that Task.WhenAll will not block execution. In the second example, we can see that when using Task.WaitAll, task completion is only executed after all other tasks have completed. This means that Task.WaitAll blocks execution.

Example

static void Main(string[] args){
   Task task1 = new Task(() =>{
      for (var i = 0; i < 5; i++){
         Console.WriteLine("Task 1 - iteration {0}", i);
         Task.Delay(1000);
      }
      Console.WriteLine("Task 1 complete");
   });
   Task task2 = new Task(() =>{
      for (var i = 0; i < 5; i++){
         Console.WriteLine("Task 2 - iteration {0}", i);
         Task.Delay(1000);
      }
      Console.WriteLine("Task 2 complete");
   });
   task1.Start();
   task2.Start();
   Console.WriteLine("Waiting for tasks to complete.");
   Task.WhenAll(task1, task2);
   Console.WriteLine("Both Tasks Completed.");
   Console.ReadLine();
}

Output

Waiting for tasks to complete.
Both Tasks Completed.
Task 1 - iteration 0
Task 2 - iteration 0
Task 2 - iteration 1
Task 2 - iteration 2
Task 2 - iteration 3
Task 1 - iteration 1
Task 1 - iteration 2
Task 1 - iteration 3
Task 1 - iteration 4
Task 1 complete
Task 2 - iteration 4
Task 2 complete

Example

static void Main(string[] args){
   Task task1 = new Task(() =>{
      for (var i = 0; i < 5; i++){
         Console.WriteLine("Task 1 - iteration {0}", i);
         Task.Delay(1000);
      }
      Console.WriteLine("Task 1 complete");
   });
   Task task2 = new Task(() =>{
      for (var i = 0; i < 5; i++){
         Console.WriteLine("Task 2 - iteration {0}", i);
         Task.Delay(1000);
      }
      Console.WriteLine("Task 2 complete");
   });
   task1.Start();
   task2.Start();
   Console.WriteLine("Waiting for tasks to complete.");
   Task.WaitAll(task1, task2);
   Console.WriteLine("Both Tasks Completed.");
   Console.ReadLine();
}

Output

Waiting for tasks to complete.
Task 1 - iteration 0
Task 2 - iteration 0
Task 1 - iteration 1
Task 1 - iteration 2
Task 1 - iteration 3
Task 1 - iteration 4
Task 1 complete
Task 2 - iteration 1
Task 2 - iteration 2
Task 2 - iteration 3
Task 2 - iteration 4
Task 2 complete
Both Tasks Completed

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