What is the async and await pattern in C#? When should you use it?

Question

Anubhav Kumar · Answer

The async/await pattern in C# is used to write 
asynchronous, non-blocking code in a way that is easy to read and maintain — like synchronous code, but without blocking threads.
It’s built on top of the Task or Taskandlt;Tandgt; type and enables you to perform
I/O-bound or CPU-bound operations without freezing the app.
Basic Syntax
public async Taskandlt;stringandgt; GetDataAsync()
{
    // This does not block the calling thread
    await Task.Delay(2000); 
    return "Hello, async!";
}
async: marks a method as asynchronous.await: pauses execution until the awaited task completes.The return type is usually Task, Taskandlt;Tandgt;, or 
void (for event handlers only).
Example with HttpClient
public async Taskandlt;stringandgt; FetchWebsiteAsync()
{
    using (HttpClient client = new HttpClient())
    {
        string result = await client.GetStringAsync("https://example.com");
        return result;
    }
}
When Should You Use async/await?
Use Case
Use async/await?
Notes
I/O-bound operations
Yes
HTTP calls, file I/O, DB queries, etc.
CPU-bound operations
Prefer Task.Run
Don't use async to parallelize loops or math-heavy code.
UI applications (WPF, WinForms)
Yes
Prevents freezing the UI thread.
ASP.NET/Web API
Yes
Frees up threads for other requests.
Library/helper methods
Yes
Mark them async to compose cleanly with other async methods.
Common Mistakes
Mistake
Explanation
Using .Result or .Wait() on async methods
Can cause deadlocks, especially in UI apps
async void (except event handlers)
Not awaitable, can't catch exceptions
Mixing blocking and async code
Breaks scalability, introduces bugs
Composing Async Calls
public async Taskandlt;stringandgt; GetUserDataAsync()
{
    var profile = await GetProfileAsync();
    var settings = await GetSettingsAsync(profile.Id);
    return $"{profile.Name} - {settings.Theme}";
}
Benefits of async/await
Non-blocking execution — improves scalabilityBetter performance — frees up threads during I/OEasier to read — compared to callbacks or raw Task.ContinueWith()Improves UI responsiveness — avoids freezing
Summary
Feature
Description
async
Marks a method as asynchronous
await
Asynchronously waits for a task to complete
Return type
Usually Task, Taskandlt;Tandgt;, or void (event only)
Use case
Ideal for I/O-bound operations (file, web, db)
Avoid for
CPU-heavy work unless combined with Task.Run

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What is the async and await pattern in C#? When should you use it?

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1 Answers

Basic Syntax

Example with `HttpClient`

When Should You Use `async`/`await`?

Common Mistakes

Composing Async Calls

Benefits of `async`/`await`

Summary

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Use Case	Use `async/await`?	Notes
I/O-bound operations	Yes	HTTP calls, file I/O, DB queries, etc.
CPU-bound operations	Prefer `Task.Run`	Don't use `async` to parallelize loops or math-heavy code.
UI applications (WPF, WinForms)	Yes	Prevents freezing the UI thread.
ASP.NET/Web API	Yes	Frees up threads for other requests.
Library/helper methods	Yes	Mark them async to compose cleanly with other async methods.

Mistake	Explanation
Using `.Result` or `.Wait()` on async methods	Can cause deadlocks, especially in UI apps
`async void` (except event handlers)	Not awaitable, can't catch exceptions
Mixing blocking and async code	Breaks scalability, introduces bugs

Feature	Description
`async`	Marks a method as asynchronous
`await`	Asynchronously waits for a task to complete
Return type	Usually `Task`, `Task<T>`, or `void` (event only)
Use case	Ideal for I/O-bound operations (file, web, db)
Avoid for	CPU-heavy work unless combined with `Task.Run`