Task.WhenAll for parallel execution in C Sharp (C#)

Introduction

Task.WhenAll helps you run many tasks at the same time and wait for all of them to finish. This makes your program faster when doing multiple jobs.

When you want to download several files at once instead of one by one.

When you need to run multiple calculations in parallel to save time.

When you want to call several web services at the same time and wait for all results.

When you have many independent tasks that can run together without waiting for each other.

Syntax

C Sharp (C#)

Task.WhenAll(params Task[] tasks)
Task.WhenAll(IEnumerable<Task> tasks)

// For tasks that return results:
Task.WhenAll(params Task<TResult>[] tasks)
Task.WhenAll(IEnumerable<Task<TResult>> tasks)

You pass a list or array of tasks to Task.WhenAll.

It returns a single task that completes when all given tasks finish.

Examples

Waits for three tasks to finish running in parallel.

C Sharp (C#)

await Task.WhenAll(task1, task2, task3);

Waits for all tasks in a list and collects their results in an array.

C Sharp (C#)

var results = await Task.WhenAll(taskList);

Sample Program

This program runs three tasks at the same time. Each task waits for a different time and then returns a number. Task.WhenAll waits for all tasks to finish and collects their results. Then it prints each result.

C Sharp (C#)

using System;
using System.Threading.Tasks;

class Program
{
    static async Task Main()
    {
        Task<int> task1 = Task.Run(() => { Task.Delay(1000).Wait(); return 1; });
        Task<int> task2 = Task.Run(() => { Task.Delay(500).Wait(); return 2; });
        Task<int> task3 = Task.Run(() => { Task.Delay(1500).Wait(); return 3; });

        int[] results = await Task.WhenAll(task1, task2, task3);

        Console.WriteLine("Results:");
        foreach (var result in results)
        {
            Console.WriteLine(result);
        }
    }
}

Important Notes

If any task fails, Task.WhenAll will throw an exception containing all errors.

Use Task.WhenAll to improve performance by running tasks in parallel instead of one after another.

Summary

Task.WhenAll runs many tasks at the same time and waits for all to finish.

It returns results from all tasks as an array if tasks produce values.

It helps make programs faster by doing work in parallel.

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