Async and await keywords in C Sharp (C#) - Mini Project: Build & Apply

Async and await keywords

📖 Scenario: You are building a simple console app that fetches data from a server. To keep the app responsive, you will use async and await keywords to handle the data fetching without blocking the main thread.

🎯 Goal: Create an asynchronous method to simulate fetching data, then call it using await and display the result.

📋 What You'll Learn

Create an asynchronous method called FetchDataAsync that returns a Task<string>.

Inside FetchDataAsync, simulate a delay of 2 seconds using Task.Delay.

Return a string message from FetchDataAsync after the delay.

In the Main method, call FetchDataAsync using await and store the result in a variable called data.

Print the data variable to the console.

💡 Why This Matters

🌍 Real World

Many apps need to fetch data from the internet or perform long tasks without freezing the screen. Using <code>async</code> and <code>await</code> helps keep apps smooth and responsive.

💼 Career

Understanding asynchronous programming is essential for software developers working on web apps, desktop apps, or any program that handles input/output or network calls efficiently.

1

Create the asynchronous method

Create an asynchronous method called FetchDataAsync that returns Task<string>. Inside it, simulate a 2-second delay using await Task.Delay(2000). After the delay, return the string "Data fetched successfully".

C Sharp (C#)

using System;
using System.Threading.Tasks;

class Program
{
    static async Task Main()
    {
    }

    // Your code here
}

Hint

Use async before the method return type and await Task.Delay(2000) to simulate waiting.

2

Call the asynchronous method with await

Inside the Main method, call FetchDataAsync using await and store the result in a variable called data.

C Sharp (C#)

using System;
using System.Threading.Tasks;

class Program
{
    static async Task Main()
    {
        // Your code here
    }

    static async Task FetchDataAsync()
    {
        await Task.Delay(2000);
        return "Data fetched successfully";
    }
}

Hint

Use string data = await FetchDataAsync(); inside Main.

3

Print the fetched data

Add a Console.WriteLine statement inside the Main method to print the data variable.

C Sharp (C#)

using System;
using System.Threading.Tasks;

class Program
{
    static async Task Main()
    {
        string data = await FetchDataAsync();
        // Your code here
    }

    static async Task FetchDataAsync()
    {
        await Task.Delay(2000);
        return "Data fetched successfully";
    }
}

Hint

Use Console.WriteLine(data); to show the result.

4

Run and see the output

Run the program and observe the output. It should print Data fetched successfully after a 2-second delay.

C Sharp (C#)

using System;
using System.Threading.Tasks;

class Program
{
    static async Task Main()
    {
        string data = await FetchDataAsync();
        Console.WriteLine(data);
    }

    static async Task FetchDataAsync()
    {
        await Task.Delay(2000);
        return "Data fetched successfully";
    }
}

// Run the program to see the output

Hint

Wait 2 seconds, then see the message printed.

Practice

1. What is the main purpose of the async keyword in C#?

easy

A. To make a method run faster by using multiple threads automatically

B. To mark a method that can run asynchronously without blocking the caller

C. To declare a method that returns void only

D. To pause the execution of a method until a task completes

Solution

1. Step 1: Understand the role of async

   The async keyword marks a method so it can run asynchronously, allowing other work to continue without waiting.

2. Step 2: Differentiate from other options

   It does not automatically make code faster or use multiple threads, nor does it pause execution by itself.

3. Final Answer:

   To mark a method that can run asynchronously without blocking the caller -> Option B

4. Quick Check:

   async marks asynchronous methods = A [OK]

Hint: Async marks methods that run without blocking [OK]

Common Mistakes:

• Thinking async makes code multithreaded automatically
• Confusing async with pausing execution
• Believing async only works with void methods

2. Which of the following is the correct way to declare an asynchronous method that returns a string in C#?

easy

A. public string async GetData()

B. public async string GetData()

C. public Task async<string> GetData()

D. public async Task<string> GetData()

Solution

1. Step 1: Recall async method return types

   Async methods that return a value must return Task<T>, where T is the return type, here string.

2. Step 2: Check syntax correctness

   public async Task<string> GetData() correctly uses async Task<string>. Other options have wrong order or missing Task.

3. Final Answer:

   public async Task<string> GetData() -> Option D

4. Quick Check:

   Async methods returning values use Task<T> = A [OK]

Hint: Async methods returning values use Task<T> type [OK]

Common Mistakes:

• Placing async after return type
• Returning just string instead of Task<string>
• Using async keyword incorrectly in method signature

3. What will be the output of the following C# code?

async Task<int> GetNumberAsync() { await Task.Delay(100); return 5; }
async Task Main() { var result = await GetNumberAsync(); Console.WriteLine(result); }

medium

A. 5

B. 0

C. Task

D. Compilation error

Solution

1. Step 1: Understand async method behavior

   The method GetNumberAsync waits 100ms then returns 5 asynchronously.

2. Step 2: Await the result and print

   The Main method awaits GetNumberAsync and prints the returned value, which is 5.

3. Final Answer:

   5 -> Option A

4. Quick Check:

   Await returns method result = 5 [OK]

Hint: Await unwraps async result to actual value [OK]

Common Mistakes:

• Expecting Task object printed instead of value
• Ignoring await causing wrong output
• Thinking async delays block main thread

4. Identify the error in this code snippet:

async Task<int> CalculateAsync() { return 10; }
void Main() { var result = CalculateAsync(); Console.WriteLine(result); }

medium

A. Missing await keyword when calling CalculateAsync

B. CalculateAsync should return int, not Task<int>

C. Main method must be async to call CalculateAsync

D. CalculateAsync cannot return a value

Solution

1. Step 1: Analyze method call and return type

   CalculateAsync returns a Task<int>, but Main calls it without await, so result is a Task, not int.

2. Step 2: Understand how to get actual value

   To get the int value, await must be used, but Main is not async, so this is a problem.

3. Final Answer:

   Missing await keyword when calling CalculateAsync -> Option A

4. Quick Check:

   Call async method with await = C [OK]

Hint: Always await async calls to get results [OK]

Common Mistakes:

• Calling async method without await
• Expecting async method to return direct value
• Making Main async without proper signature

5. You want to run two independent async tasks and get their results as soon as both finish. Which code correctly does this in C#?

hard

A. var result1 = await Task1(); var result2 = await Task2();

B. var result1 = Task1().Result; var result2 = Task2().Result;

C. var task1 = Task1(); var task2 = Task2(); var result1 = await task1; var result2 = await task2;

D. await Task.WhenAll(Task1(), Task2()); var result1 = Task1().Result; var result2 = Task2().Result;

Solution

1. Step 1: Understand running tasks concurrently

   Starting both tasks without awaiting immediately lets them run in parallel.

2. Step 2: Await both tasks after starting

   Awaiting each task after starting both ensures you wait for both to finish and get results efficiently.

3. Step 3: Analyze other options

   var result1 = await Task1(); var result2 = await Task2(); awaits sequentially, running tasks one after another. var result1 = Task1().Result; var result2 = Task2().Result; blocks synchronously causing deadlocks. await Task.WhenAll(Task1(), Task2()); var result1 = Task1().Result; var result2 = Task2().Result; awaits but then calls Result again, causing redundant calls.

4. Final Answer:

   var task1 = Task1(); var task2 = Task2(); var result1 = await task1; var result2 = await task2; -> Option C

5. Quick Check:

   Start tasks first, then await both = B [OK]

Hint: Start tasks first, then await both to run concurrently [OK]

Common Mistakes:

• Awaiting tasks one by one causing sequential execution
• Using .Result causing blocking and deadlocks
• Calling tasks multiple times causing duplicate work