Task.WhenAll for parallel execution in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to run two tasks in parallel and wait for both to finish.

C Sharp (C#)

var task1 = Task.Run(() => Console.WriteLine("Task 1"));
var task2 = Task.Run(() => Console.WriteLine("Task 2"));
await [1](task1, task2);

Drag options to blanks, or click blank then click option'

ATask.WhenAll

BTask.WaitAll

CTask.RunAll

DTask.WaitAny

Attempts:

3 left

💡 Hint

Common Mistakes

Using Task.WaitAll which is synchronous and blocks the thread.

Using Task.WaitAny which waits for only one task to complete.

Task.WhenAll runs multiple tasks in parallel and returns a task that completes when all have finished.

2fill in blank

medium

Complete the code to start three tasks and wait for all to complete using Task.WhenAll.

C Sharp (C#)

var tasks = new List<Task> {
    Task.Run(() => DoWork(1)),
    Task.Run(() => DoWork(2)),
    Task.Run(() => DoWork(3))
};
await [1](tasks);

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ATask.WaitAll

BTask.WhenAll

CTask.RunAll

DTask.WaitAny

Attempts:

3 left

💡 Hint

Common Mistakes

Passing the list to Task.WaitAll which is synchronous.

Using Task.WaitAny which waits for only one task.

Task.WhenAll accepts a collection of tasks and returns a task that completes when all tasks finish.

3fill in blank

hard

Fix the error in the code to correctly wait for multiple tasks to complete asynchronously.

C Sharp (C#)

var taskA = Task.Run(() => Compute(10));
var taskB = Task.Run(() => Compute(20));
Task.WhenAll(taskA, taskB).Wait(); // Fix this line
Console.WriteLine("All tasks done");

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Aawait Task.WhenAny(taskA, taskB);

BTask.WaitAll(taskA, taskB);

CTask.WhenAny(taskA, taskB);

Dawait Task.WhenAll(taskA, taskB);

Attempts:

3 left

💡 Hint

Common Mistakes

Using Task.WaitAll which blocks the thread.

Using Task.WhenAny which waits for only one task.

Using 'await Task.WhenAll' waits asynchronously without blocking the thread, unlike '.Wait()' which blocks.

4fill in blank

hard

Fill both blanks to create a dictionary with task results after all tasks complete.

C Sharp (C#)

var tasks = new List<Task<int>> {
    Task.Run(() => 1),
    Task.Run(() => 2),
    Task.Run(() => 3)
};
var results = await [1](tasks);
var dict = results.[2]((result, index) => new { index, result }).ToDictionary(x => x.index, x => x.result);

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ATask.WhenAll

BSelect

CSelectIndexed

DSelectMany

Attempts:

3 left

💡 Hint

Common Mistakes

Using SelectMany which flattens collections incorrectly.

Using a non-existent method SelectIndexed.

Task.WhenAll waits for all tasks and returns results; Select projects each result with its index.

5fill in blank

hard

Fill in the blanks to run tasks in parallel, filter results greater than 5, and create a list of those results.

C Sharp (C#)

var tasks = new List<Task<int>> {
    Task.Run(() => 3),
    Task.Run(() => 7),
    Task.Run(() => 10)
};
var results = await [1](tasks);
var filtered = results.Where(x => x [2] 5).ToList();
Console.WriteLine(string.Join(", ", filtered));

Drag options to blanks, or click blank then click option'

ATask.WhenAll

B>

C<

DTask.WaitAll

Attempts:

3 left

💡 Hint

Common Mistakes

Using Task.WaitAll which blocks the thread instead of awaiting.

Using '<' which filters results less than 5.

Task.WhenAll waits for all tasks and returns an array of results; '>' filters results greater than 5.

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