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.