Task.WhenAny for first completion in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to start a task that returns 5.

C Sharp (C#)

var task = Task.Run(() => [1]);

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

AConsole.WriteLine(5)

B"5"

CTask.Delay(1000)

D5

Attempts:

3 left

💡 Hint

Common Mistakes

Using Console.WriteLine inside the lambda instead of returning a value.

Returning a string instead of an integer.

The lambda returns the integer 5, so the task completes with result 5.

2fill in blank

medium

Complete the code to await the first task to complete among two tasks.

C Sharp (C#)

var firstFinished = await Task.WhenAny([1], Task.Delay(1000));

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

ATask.Run(() => 10)

BTask.Delay(2000)

CTask.CompletedTask

DTask.Run(() => Console.WriteLine("Hi"))

Attempts:

3 left

💡 Hint

Common Mistakes

Using a delay longer than the other task, so it never finishes first.

Using a task that does not complete or returns void.

Task.WhenAny waits for the first task to finish. The task returning 10 finishes before the 1-second delay.

3fill in blank

hard

Fix the error in the code to get the result of the first completed task.

C Sharp (C#)

var firstFinished = await Task.WhenAny(task1, task2);
var result = firstFinished.[1];

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

AResult

BGetResult()

CResultAsync

DGetAwaiter()

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to call a method that does not exist.

Using GetAwaiter() without awaiting.

Task.WhenAny returns the first completed task. Access its Result property to get the value.

4fill in blank

hard

Fill both blanks to create two tasks and await the first to complete.

C Sharp (C#)

var task1 = Task.Run(() => [1]);
var task2 = Task.Run(() => [2]);
var first = await Task.WhenAny(task1, task2);

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

A100

BTask.Delay(500).Wait(); return 200;

CTask.Delay(1000).Wait(); return 300;

DConsole.WriteLine("Done")

Attempts:

3 left

💡 Hint

Common Mistakes

Using Console.WriteLine which returns void.

Making both tasks delay the same time.

Task1 returns 100 immediately. Task2 waits 500ms then returns 200. The first task finishes immediately.

5fill in blank

hard

Fill all three blanks to create tasks and get the result of the first completed task.

C Sharp (C#)

var task1 = Task.Run(() => [1]);
var task2 = Task.Run(() => [2]);
var first = await Task.WhenAny(task1, task2);
var result = first.[3];

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

ATask.Delay(1000).Wait(); return 42;

B500

CResult

DTask.Delay(200).Wait(); return 24;

Attempts:

3 left

💡 Hint

Common Mistakes

Accessing the result before awaiting the task.

Using wrong property or method to get the result.

Task2 returns 500 immediately. Task1 delays 200 ms then returns 24. The first completed task returns 500. Access Result to get the value.

Practice

1. What does Task.WhenAny do in C#?

easy

A. It creates a new task that never completes.

B. It waits for all tasks to complete before continuing.

C. It cancels all running tasks immediately.

D. It waits for the first task to complete among multiple tasks.

Solution

1. Step 1: Understand Task.WhenAny purpose

   Task.WhenAny waits for the first task to finish among many tasks.

2. Step 2: Compare with other options

   Unlike Task.WhenAll, it does not wait for all tasks, nor does it cancel tasks or create infinite tasks.

3. Final Answer:

   It waits for the first task to complete among multiple tasks. -> Option D

4. Quick Check:

   Task.WhenAny = first task completion [OK]

Hint: Remember: WhenAny means wait for one task only [OK]

Common Mistakes:

• Confusing Task.WhenAny with Task.WhenAll
• Thinking it cancels other tasks
• Assuming it waits for all tasks

2. Which of the following is the correct syntax to use Task.WhenAny with two tasks task1 and task2?

easy

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

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

C. var first = await Task.WhenAny{task1, task2};

D. var first = await Task.WhenAny task1, task2;

Solution

1. Step 1: Check correct method call syntax

   Task.WhenAny accepts params array of tasks, so calling with (task1, task2) is correct.

2. Step 2: Identify invalid syntax

   Square brackets, curly braces, or missing parentheses cause syntax errors.

3. Final Answer:

   var first = await Task.WhenAny(task1, task2); -> Option A

4. Quick Check:

   Correct method call uses parentheses and comma-separated tasks [OK]

Hint: Use parentheses and commas for Task.WhenAny arguments [OK]

Common Mistakes:

• Using square brackets instead of parentheses
• Using curly braces instead of parentheses
• Omitting parentheses around arguments

3. What will be the output of this code snippet?

var task1 = Task.Delay(3000).ContinueWith(_ => "Task1 done");
var task2 = Task.Delay(1000).ContinueWith(_ => "Task2 done");
var first = await Task.WhenAny(task1, task2);
Console.WriteLine(await first);

medium

A. Task2 done

B. No output, program hangs

C. Task1 done\nTask2 done

D. Task1 done

Solution

1. Step 1: Analyze task delays

   task1 completes after 3000ms, task2 after 1000ms, so task2 finishes first.

2. Step 2: Task.WhenAny returns first completed task

   first will be task2, so awaiting it prints "Task2 done".

3. Final Answer:

   Task2 done -> Option A

4. Quick Check:

   First completed task result = Task2 done [OK]

Hint: The shortest delay task finishes first [OK]

Common Mistakes:

• Assuming task1 finishes first because it is listed first
• Expecting both tasks to print
• Thinking program hangs waiting for all tasks

4. Identify the error in this code using Task.WhenAny:

var task1 = Task.Delay(2000);
var task2 = Task.Delay(1000);
var first = Task.WhenAny(task1, task2);
Console.WriteLine(await first.Result);

medium

A. Using await on first.Result causes a compile error.

B. Task.WhenAny should be awaited before accessing Result.

C. Task.Delay cannot be used with Task.WhenAny.

D. Console.WriteLine cannot print task results.

Solution

1. Step 1: Check how Task.WhenAny is awaited

   Task.WhenAny returns a Task<Task>, so it must be awaited to get the completed task.

2. Step 2: Identify misuse of Result property

   Accessing first.Result without awaiting first causes incorrect behavior or deadlock.

3. Final Answer:

   Task.WhenAny should be awaited before accessing Result. -> Option B

4. Quick Check:

   Await Task.WhenAny before accessing completed task [OK]

Hint: Always await Task.WhenAny before using its result [OK]

Common Mistakes:

• Accessing Result property without await
• Confusing Task<Task> with Task
• Ignoring async/await pattern

5. You have three tasks: taskA (2s delay), taskB (1s delay), and taskC (3s delay). You want to print the result of the first completed task and then cancel the other two tasks. Which code snippet correctly achieves this?

hard

A. var first = await Task.WhenAny(taskA, taskB, taskC);
cts.Cancel();
Console.WriteLine(await first);

B. var first = await Task.WhenAny(taskA, taskB, taskC);
Console.WriteLine(await first);
// No cancellation code

C. var cts = new CancellationTokenSource();
var taskA = Task.Delay(2000, cts.Token);
var taskB = Task.Delay(1000, cts.Token);
var taskC = Task.Delay(3000, cts.Token);
var first = await Task.WhenAny(taskA, taskB, taskC);
cts.Cancel();
Console.WriteLine(await first);

D. var first = Task.WhenAny(taskA, taskB, taskC).Result;
cts.Cancel();
Console.WriteLine(first.Result);

Solution

1. Step 1: Use CancellationTokenSource to cancel tasks

   Create a CancellationTokenSource and pass its token to all tasks so they can be cancelled.

2. Step 2: Await Task.WhenAny and cancel remaining tasks

   Await Task.WhenAny to get first completed task, then call cts.Cancel() to stop others, finally print the result.

3. Final Answer:

   Use CancellationTokenSource, await Task.WhenAny, then cancel and print. -> Option C

4. Quick Check:

   Cancel tasks after first completion using CancellationTokenSource [OK]

Hint: Pass cancellation token to tasks and cancel after first finishes [OK]

Common Mistakes:

• Not passing cancellation token to tasks
• Calling Cancel before awaiting Task.WhenAny
• Using .Result instead of await causing deadlocks