Task.WhenAll for parallel execution in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does Task.WhenAll do in C#?

Task.WhenAll waits for multiple tasks to complete in parallel and returns a single task that finishes when all the given tasks are done.

beginner

How do you use Task.WhenAll to run two tasks at the same time?

You create two tasks and pass them to Task.WhenAll. It runs both tasks in parallel and waits for both to finish.

var task1 = Task.Run(() => DoWork1());
var task2 = Task.Run(() => DoWork2());
await Task.WhenAll(task1, task2);

intermediate

What type does Task.WhenAll return when given tasks with results?

When given tasks that return results (like Task<int>), Task.WhenAll returns a Task<T[]> where T[] is an array of all results.

intermediate

What happens if one of the tasks passed to Task.WhenAll throws an exception?

Task.WhenAll will complete with an exception. The exception contains all exceptions from the tasks that failed.

beginner

Why use Task.WhenAll instead of awaiting tasks one by one?

Using Task.WhenAll runs tasks in parallel, saving time. Awaiting tasks one by one runs them sequentially, which is slower.

What does Task.WhenAll return when given multiple Task objects?

A An array of tasks

B The result of the first task that finishes

C A single <code>Task</code> that completes when all tasks finish

D A boolean indicating if all tasks succeeded

Task.WhenAll returns a single Task that completes when all the input tasks complete.

If you want to get results from multiple Task<int> tasks, what type does Task.WhenAll return?

A <code>Task&lt;int&gt;</code>

B <code>Task&lt;int[]&gt;</code>

C <code>int[]</code>

D <code>Task</code>

When given tasks with results, Task.WhenAll returns a Task that produces an array of those results.

What happens if one task throws an exception when using Task.WhenAll?

A The returned task completes with an exception containing all errors

B The exception is ignored

C Only the first exception is thrown immediately

D The program crashes

Task.WhenAll aggregates exceptions from all failed tasks into one.

Why is Task.WhenAll better than awaiting tasks one after another?

A It runs tasks in parallel, saving time

B It runs tasks sequentially, which is faster

C It only runs the first task

D It cancels all tasks automatically

Running tasks in parallel reduces total wait time compared to sequential awaits.

Which of these is a correct way to use Task.WhenAll?

A await task1; await task2;

B Task.WhenAll(task1, task2);

C Task.WaitAll(task1, task2);

D await Task.WhenAll(task1, task2);

You must await the Task.WhenAll to wait for all tasks to finish asynchronously.

Explain how Task.WhenAll helps run multiple tasks in parallel and why this is useful.

Think about waiting for many things to finish together instead of one after another.

Describe what happens when one or more tasks fail while using Task.WhenAll.

Consider how errors from multiple sources are reported together.