Task.WhenAll for parallel execution in C Sharp (C#) - Mini Project: Build & Apply

Task.WhenAll for parallel execution

📖 Scenario: You are building a simple program that fetches data from multiple sources at the same time. This helps save time because the program does not wait for one source to finish before starting the next.

🎯 Goal: Learn how to use Task.WhenAll to run multiple tasks in parallel and wait for all of them to complete.

📋 What You'll Learn

Create three tasks that simulate data fetching with delays

Use a configuration variable to set the delay time

Use Task.WhenAll to run all tasks in parallel

Print the results after all tasks complete

💡 Why This Matters

🌍 Real World

Fetching data from multiple web services or databases at the same time to improve performance.

💼 Career

Understanding parallel execution with Task.WhenAll is important for building efficient and responsive applications in C#.

1

Create three tasks simulating data fetching

Create three Task<string> variables named task1, task2, and task3. Each task should simulate fetching data by returning a string after a delay. Use Task.Delay with 1000 milliseconds inside each task to simulate the delay. Use async lambdas for the tasks.

C Sharp (C#)

# Your code here

Hint

Use Task.Run with an async lambda that awaits Task.Delay(1000) and then returns a string.

2

Add a delay configuration variable

Add an int variable named delayTime and set it to 1000. Then update the Task.Delay calls inside task1, task2, and task3 to use delayTime instead of the fixed number.

C Sharp (C#)

using System;
using System.Threading.Tasks;

class Program
{
    static async Task Main()
    {
        int delayTime = 1000;
        // Update the delay in tasks to use delayTime
        // Your code here
    }
}

Hint

Declare int delayTime = 1000; before the tasks. Replace Task.Delay(1000) with Task.Delay(delayTime) inside each task.

3

Use Task.WhenAll to run tasks in parallel

Use Task.WhenAll with task1, task2, and task3 to run all tasks in parallel and wait for all to complete. Store the result in a variable named results of type string[]. Use await with Task.WhenAll.

C Sharp (C#)

using System;
using System.Threading.Tasks;

class Program
{
    static async Task Main()
    {
        int delayTime = 1000;
        Task task1 = Task.Run(async () => { await Task.Delay(delayTime); return "Data from source 1"; });
        Task task2 = Task.Run(async () => { await Task.Delay(delayTime); return "Data from source 2"; });
        Task task3 = Task.Run(async () => { await Task.Delay(delayTime); return "Data from source 3"; });

        // Use Task.WhenAll to run tasks in parallel and get results
        // Your code here
    }
}

Hint

Use string[] results = await Task.WhenAll(task1, task2, task3); to run all tasks and get their results.

4

Print the results after all tasks complete

Use a foreach loop to print each string in the results array. Use Console.WriteLine inside the loop to display each result.

C Sharp (C#)

using System;
using System.Threading.Tasks;

class Program
{
    static async Task Main()
    {
        int delayTime = 1000;
        Task task1 = Task.Run(async () => { await Task.Delay(delayTime); return "Data from source 1"; });
        Task task2 = Task.Run(async () => { await Task.Delay(delayTime); return "Data from source 2"; });
        Task task3 = Task.Run(async () => { await Task.Delay(delayTime); return "Data from source 3"; });

        string[] results = await Task.WhenAll(task1, task2, task3);

        // Print each result
        // Your code here
    }
}

Hint

Use foreach (string result in results) { Console.WriteLine(result); } to print each result.

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