Task.WhenAll for parallel execution in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Task.WhenAll example?

Consider the following C# code using Task.WhenAll to run tasks in parallel. What will be printed to the console?

C Sharp (C#)

using System;
using System.Threading.Tasks;

class Program
{
    static async Task Main()
    {
        var task1 = Task.Delay(100).ContinueWith(_ => 1);
        var task2 = Task.Delay(50).ContinueWith(_ => 2);
        var results = await Task.WhenAll(task1, task2);
        Console.WriteLine(string.Join(",", results));
    }
}

A Runtime exception

B 2,1

C Compilation error

D 1,2

💡 Hint

Remember that Task.WhenAll returns results in the order of the tasks passed, not the order they complete.

The Task.WhenAll method returns an array of results in the same order as the tasks were passed in. Even though task2 finishes earlier, the results array keeps the order task1, task2. So the output is 1,2.

🧠 Conceptual

intermediate

Which statement about Task.WhenAll is true?

Choose the correct statement about Task.WhenAll in C#.

A It waits for all tasks to complete and returns their results in the order tasks were passed.

B It returns results as soon as the first task completes.

C It cancels all tasks if any one task fails.

D It runs tasks sequentially, one after another.

💡 Hint

Think about how Task.WhenAll handles multiple tasks and their results.

Task.WhenAll runs tasks concurrently and waits for all to finish. It returns results in the order tasks were passed. It does not run tasks sequentially, nor does it return early or cancel others on failure.

🔧 Debug

advanced

Why does this Task.WhenAll code throw an exception?

Examine the code below. It throws an exception at runtime. What is the cause?

C Sharp (C#)

using System;
using System.Threading.Tasks;

class Program
{
    static async Task Main()
    {
        var task1 = Task.FromException<int>(new InvalidOperationException("Error in task1"));
        var task2 = Task.Run(() => 42);
        var results = await Task.WhenAll(task1, task2);
        Console.WriteLine(string.Join(",", results));
    }
}

A The code has a syntax error causing compilation failure.

B Task.WhenAll returns partial results ignoring failed tasks.

C Task.WhenAll throws AggregateException because one task failed.

D Task.WhenAll cancels all tasks when one fails, so no exception is thrown.

💡 Hint

Think about how exceptions in tasks are handled when using Task.WhenAll.

If any task throws an exception, Task.WhenAll throws an AggregateException containing all exceptions from failed tasks. It does not ignore failures or cancel other tasks automatically.

📝 Syntax

advanced

Which option correctly uses Task.WhenAll with async lambdas?

Identify the correct syntax to run two async lambdas in parallel and await their results using Task.WhenAll.

A var results = await Task.WhenAll(Task.Run(async () => await Task.Delay(10)), Task.Run(async () => await Task.Delay(20)));

B var results = await Task.WhenAll(async () => await Task.Delay(10), async () => await Task.Delay(20));

C var results = await Task.WhenAll(() => Task.Delay(10), () => Task.Delay(20));

D var results = Task.WhenAll(Task.Delay(10), Task.Delay(20));

💡 Hint

Remember that Task.WhenAll expects tasks, not delegates or lambdas directly.

Task.WhenAll requires a collection of Task objects. Wrapping async lambdas inside Task.Run creates tasks to pass. Option A correctly creates and awaits the tasks.

🚀 Application

expert

How many items are in the results array after this Task.WhenAll call?

Given the code below, how many elements does the results array contain after Task.WhenAll completes?

C Sharp (C#)

using System;
using System.Threading.Tasks;

class Program
{
    static async Task Main()
    {
        Task<int> t1 = Task.FromResult(5);
        Task<int> t2 = Task.Run(() => 10);
        Task<int> t3 = Task.Delay(100).ContinueWith(_ => 15);
        var results = await Task.WhenAll(t1, t2, t3);
        Console.WriteLine(results.Length);
    }
}

A 2

B 3

C 1

D 0

💡 Hint

Count how many tasks are passed to Task.WhenAll.

Three tasks (t1, t2, t3) are passed to Task.WhenAll. The results array contains one element per task, so length is 3.

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