How to combine two tasks in the third task?

Question

How can I complete the System.Threading.Task task as a continuation of two or more other task results?

public Task<FinalResult> RunStepsAsync() { Task<Step1Result> task1 = Task<Step1Result>.Factory.StartNew(Step1); // Use the result of step 1 in steps 2A and 2B Task<Step2AResult> task2A = task1.ContinueWith(t1 => Step2A(t1.Result)); Task<Step2BResult> task2B = task1.ContinueWith(t1 => Step2B(t1.Result)); // Now merge the results of steps 2A and 2B in step 3 Task<FinalResult> task3 = task2A .ContinueWith( t2A => task2B.ContinueWith( t2B => Step3(t2A.Result, t2B.Result))) .Unwrap(); return task3; } 

This works, but double ContinueWith seems inefficient. Is there a better way to do this, possibly using TaskCompletionSource? (I do not want to use locks or Task.WaitAll.)

Answer 1

Use TaskFactory.ContinueWhenAll .

 class Step1Result {} class Step2AResult { public Step2AResult(Step1Result result) {} } class Step2BResult { public Step2BResult(Step1Result result) {} } class FinalResult { public FinalResult(Step2AResult step2AResult, Step2BResult step2BResult) {} } public Task<FinalResult> RunStepsAsync() { var task1 = Task<Step1Result>.Factory.StartNew(() => new Step1Result()); // Use the result of step 1 in steps 2A and 2B var task2A = task1.ContinueWith(t1 => new Step2AResult(t1.Result)); var task2B = task1.ContinueWith(t1 => new Step2BResult(t1.Result)); // Now merge the results of steps 2A and 2B in step 3 return Task <FinalResult> .Factory .ContinueWhenAll(new Task[] { task2A, task2B }, tasks => new FinalResult(task2A.Result, task2B.Result)); }