Task.WhenAny for first completion in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Task.WhenAny with different delays

What will be the output of this C# code snippet using Task.WhenAny to await the first completed task?

C Sharp (C#)

using System;
using System.Threading.Tasks;

class Program
{
    static async Task Main()
    {
        var task1 = Task.Delay(300).ContinueWith(_ => "Task1");
        var task2 = Task.Delay(100).ContinueWith(_ => "Task2");
        var task3 = Task.Delay(200).ContinueWith(_ => "Task3");

        var firstCompleted = await Task.WhenAny(task1, task2, task3);
        Console.WriteLine(await firstCompleted);
    }
}

A Task1

B Task2

C Task3

D Compilation error

💡 Hint

Think about which task finishes first based on the delay times.

Task2 has the shortest delay (100ms), so it completes first. Task.WhenAny returns the first completed task, so awaiting it yields "Task2".

🧠 Conceptual

intermediate

Understanding Task.WhenAny behavior

Which statement best describes what Task.WhenAny does in C#?

A It cancels all other tasks once the first task completes.

B It waits for all tasks to complete and returns their results as a list.

C It returns the first task that completes, regardless of success or failure.

D It throws an exception if any task fails before completion.

💡 Hint

Consider what happens if one task completes with an error before others finish.

Task.WhenAny returns the first task that finishes, whether it completed successfully, faulted, or was canceled. It does not wait for all tasks or cancel others.

🔧 Debug

advanced

Identify the runtime error with Task.WhenAny usage

What runtime error will this code produce when executed?

C Sharp (C#)

using System;
using System.Threading.Tasks;

class Program
{
    static async Task Main()
    {
        Task<string> task1 = Task.FromResult("Hello");
        Task<string> task2 = null;

        var first = await Task.WhenAny(task1, task2);
        Console.WriteLine(await first);
    }
}

A ArgumentNullException

B NullReferenceException

C InvalidOperationException

D No error, outputs "Hello"

💡 Hint

Check what happens if one of the tasks passed to Task.WhenAny is null.

Passing a null task to Task.WhenAny causes an ArgumentNullException at runtime because the method expects all tasks to be non-null.

❓ Predict Output

advanced

Output when first completed task faults

What will be the output of this code snippet?

C Sharp (C#)

using System;
using System.Threading.Tasks;

class Program
{
    static async Task Main()
    {
        var task1 = Task.Run(() => throw new InvalidOperationException("Error in task1"));
        var task2 = Task.Delay(100).ContinueWith(_ => "Success");

        var first = await Task.WhenAny(task1, task2);

        try
        {
            Console.WriteLine(await first);
        }
        catch (Exception ex)
        {
            Console.WriteLine(ex.GetType().Name);
        }
    }
}

A Success

B Compilation error

C TaskCanceledException

D InvalidOperationException

💡 Hint

Consider which task completes first and what happens when awaiting a faulted task.

The task that throws an exception completes first. Awaiting it rethrows the exception, so the catch block prints the exception type name.

🚀 Application

expert

Using Task.WhenAny to implement a timeout

Which code snippet correctly uses Task.WhenAny to implement a 500ms timeout for an asynchronous operation DoWorkAsync()? The code should print "Timeout" if the operation takes too long, or print the result if it completes in time.

A

var workTask = DoWorkAsync();
var timeoutTask = Task.Delay(500);
var first = await Task.WhenAny(workTask, timeoutTask);
if (first == timeoutTask) Console.WriteLine("Timeout");
else Console.WriteLine(await workTask);

B

var workTask = DoWorkAsync();
var timeoutTask = Task.Delay(500);
var first = await Task.WhenAny(workTask, timeoutTask);
if (first == workTask) Console.WriteLine("Timeout");
else Console.WriteLine(await timeoutTask);

C

var workTask = DoWorkAsync();
var timeoutTask = Task.Delay(500);
await Task.WhenAny(workTask, timeoutTask);
if (workTask.IsCompleted) Console.WriteLine("Timeout");
else Console.WriteLine(await workTask);

D

var workTask = DoWorkAsync();
var timeoutTask = Task.Delay(500);
var first = await Task.WhenAny(workTask, timeoutTask);
if (first == timeoutTask) Console.WriteLine(await timeoutTask);
else Console.WriteLine("Timeout");

💡 Hint

Check which task finished first and print accordingly.

Option A correctly compares the first completed task to the timeout task and prints "Timeout" if the timeout task finished first, otherwise prints the work result.