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.

Practice

1.

What does Task.WhenAll do in C#?

easy

A. Runs multiple tasks at the same time and waits for all to finish

B. Runs tasks one after another in sequence

C. Cancels all running tasks immediately

D. Runs only the first task and ignores others

Solution

1. Step 1: Understand Task.WhenAll purpose

   Task.WhenAll is designed to run multiple tasks in parallel and wait until all complete.

2. Step 2: Compare options with behavior

   Only Runs multiple tasks at the same time and waits for all to finish correctly describes running tasks simultaneously and waiting for all to finish.

3. Final Answer:

   Runs multiple tasks at the same time and waits for all to finish -> Option A

4. Quick Check:

   Task.WhenAll = parallel run and wait [OK]

Hint: Think: all tasks run together and wait for all done [OK]

Common Mistakes:

• Thinking tasks run one by one
• Believing it cancels tasks
• Assuming it runs only one task

2.

Which of the following is the correct syntax to wait for multiple tasks task1 and task2 to complete using Task.WhenAll?

?

easy

A. await Task.WhenAll{task1, task2};

B. await Task.WhenAll(task1, task2);

C. await Task.WhenAll[task1, task2];

D. await Task.WhenAll();

Solution

1. Step 1: Recall Task.WhenAll syntax

   Task.WhenAll takes tasks as parameters inside parentheses and returns a Task to await.

2. Step 2: Check each option's syntax

   Only await Task.WhenAll(task1, task2); uses parentheses correctly and awaits the combined task.

3. Final Answer:

   await Task.WhenAll(task1, task2); -> Option B

4. Quick Check:

   Correct syntax uses parentheses () [OK]

Hint: Use parentheses () to pass tasks to WhenAll [OK]

Common Mistakes:

• Using square brackets [] instead of parentheses
• Using curly braces {} incorrectly
• Using angle brackets <> wrongly

3.

What will be the output of this C# code?

using System;
using System.Threading.Tasks;

class Program {
    static async Task Main() {
        Task<int> t1 = Task.FromResult(5);
        Task<int> t2 = Task.FromResult(10);
        int[] results = await Task.WhenAll(t1, t2);
        Console.WriteLine(string.Join(",", results));
    }
}

medium

A. Compilation error

B. 10,5

C. 5,10

D. Task[]

Solution

1. Step 1: Understand Task.FromResult and Task.WhenAll

   Task.FromResult creates completed tasks with values 5 and 10. Task.WhenAll waits and returns results as an int array.

2. Step 2: Check output of Console.WriteLine

   Results array contains [5, 10], so joining with comma prints "5,10".

3. Final Answer:

   5,10 -> Option C

4. Quick Check:

   Task.WhenAll returns array of results [OK]

Hint: Task.WhenAll returns array of results in order [OK]

Common Mistakes:

• Assuming results are reversed
• Expecting a string instead of array
• Thinking code causes error

4.

Identify the error in this code snippet using Task.WhenAll:

async Task RunTasks() {
    var t1 = Task.Delay(1000);
    var t2 = Task.Delay(2000);
    Task.WhenAll(t1, t2);
    Console.WriteLine("Done");
}

medium

A. Missing await before Task.WhenAll, so tasks run but not waited

B. Task.Delay cannot be used with Task.WhenAll

C. Console.WriteLine should be inside Task.WhenAll

D. Task.WhenAll requires a list, not separate tasks

Solution

1. Step 1: Check usage of Task.WhenAll

   Task.WhenAll returns a Task that must be awaited to wait for all tasks to finish.

2. Step 2: Identify missing await

   Code calls Task.WhenAll but does not await it, so "Done" prints immediately before delays finish.

3. Final Answer:

   Missing await before Task.WhenAll, so tasks run but not waited -> Option A

4. Quick Check:

   Always await Task.WhenAll to wait [OK]

Hint: Always await Task.WhenAll to wait for tasks [OK]

Common Mistakes:

• Not awaiting Task.WhenAll
• Thinking Task.Delay can't be used
• Misplacing Console.WriteLine

5.

You want to run three tasks that return strings: Task<string> t1, t2, and t3. You want to get all results as a list of strings after all finish. Which code correctly does this?

hard

A. var list = await Task.WhenAll(new List { t1, t2, t3 });

B. var list = await Task.WhenAll(t1, t2, t3).ToList();

C. var list = Task.WhenAll(t1, t2, t3).Result.ToList();

D. var results = await Task.WhenAll(t1, t2, t3); var list = results.ToList();

Solution

1. Step 1: Understand Task.WhenAll return type

   Task.WhenAll returns a string array when awaited on Task<string> tasks.

2. Step 2: Convert array to list correctly

   var results = await Task.WhenAll(t1, t2, t3); var list = results.ToList(); awaits Task.WhenAll, then converts the array to a list using ToList(), which is correct.

3. Final Answer:

   var results = await Task.WhenAll(t1, t2, t3); var list = results.ToList(); -> Option D

4. Quick Check:

   Await Task.WhenAll then convert array to list [OK]

Hint: Await Task.WhenAll first, then convert results to list [OK]

Common Mistakes:

• Trying to call ToList() before awaiting
• Using .Result causing blocking
• Passing list directly without awaiting