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Home Interviews What is the async and await pattern in C#? When should you use it? – MindStick
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ICSM Computer
ICSM Computer 12 Jun 2025
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The async and await pattern in C# is a language feature used to write asynchronous, non-blocking code in a clean, readable way.

Instead of using callbacks or threads manually, you can write asynchronous code as if it were synchronous, while still freeing up threads to do other work (like serving more web requests).

Basic Concept

  • async keyword: Marks a method as asynchronous.
  • await keyword: Tells the compiler to pause execution until the awaited task completes.
public async Task<string> GetDataAsync()
{
    HttpClient client = new HttpClient();
    string result = await client.GetStringAsync("https://example.com");
    return result;
}

Behind the scenes, the method gets "split" into parts that run before and after the await. The thread is returned to the pool while waiting.

Components of the Pattern

Component Role
async Makes the method return a Task, Task<T>, or ValueTask<T>
await Pauses method until the awaited task completes
Task/Task<T> Represents an asynchronous operation

Rules to Remember

  • You can only use await inside a method marked with async.
  • An async method must return:
    • Task (if it returns nothing),
    • Task<T> (if it returns a value),
    • or void (only for event handlers).
  • await only works with Task, Task<T>, ValueTask, or custom awaitables.

Example: Asynchronous Delay

public async Task PrintMessageAsync()
{
    Console.WriteLine("Before delay");
    await Task.Delay(1000); // waits asynchronously for 1 second
    Console.WriteLine("After delay");
}

This method does not block the thread during the delay.

When Should You Use async/await?

Use it when:

  • Calling I/O-bound operations (e.g., HTTP requests, file I/O, database calls)
  • You want to keep the UI responsive (WinForms, WPF)
  • You’re building scalable web APIs that must serve many concurrent requests
// GOOD: Database call (I/O bound)
public async Task<User> GetUserAsync(int id)
{
    return await _dbContext.Users.FindAsync(id);
}

Don’t use it when:

  • The method is CPU-bound and does not involve any async operations
  • You’re just wrapping synchronous code in Task.Run() without a good reason
  • You don't have any await inside — this will show a compiler warning

Bad Example

public async Task<int> CalculateSync()
{
    // BAD: nothing async happening here
    return 1 + 2;
}

Better:

public int CalculateSync()
{
    return 1 + 2;
}

Rule of Thumb

Use async/await for I/O-bound operations to keep your app responsive and scalable. Avoid it for purely CPU-bound tasks unless you need to offload work using Task.Run.

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