10.1 Async and Await

Let’s go through async and await in C# — the key to asynchronous programming.

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📌 1. What is Asynchronous Programming?

• Asynchronous programming allows tasks to run without blocking the main thread.

• Useful for I/O operations, web requests, file access, or database calls.

• async and await keywords make it simple and readable compared to callbacks.

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📌 2. async Keyword

• Marks a method as asynchronous

• Must return either:

  • Task → for methods that return nothing

  • Task<T> → for methods that return a value

  • void → only for event handlers

public async Task DoWorkAsync()
{
    await Task.Delay(1000); // Simulate work
    Console.WriteLine("Work done!");
}

• Allows awaiting asynchronous operations inside

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📌 3. await Keyword

• Pauses execution of the async method until the awaited task completes

• Does not block the calling thread

public async Task MainAsync()
{
    Console.WriteLine("Start");
    await Task.Delay(2000); // Wait 2 seconds asynchronously
    Console.WriteLine("End");
}

Output:

Start
...2 seconds...
End

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📌 4. Example: Async File Reading

using System.IO;
using System.Threading.Tasks;
 
public async Task ReadFileAsync()
{
    using StreamReader reader = new StreamReader("example.txt");
    string content = await reader.ReadToEndAsync();
    Console.WriteLine(content);
}

• ReadToEndAsync() is non-blocking

• Program can do other work while waiting for I/O

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📌 5. Returning Values

public async Task<int> CalculateSumAsync(int a, int b)
{
    await Task.Delay(1000); // Simulate async operation
    return a + b;
}
 
int result = await CalculateSumAsync(5, 10);
Console.WriteLine(result); // 15

• Use Task<T> for returning values from async methods

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📌 6. Async with Parallel Tasks

public async Task FetchDataAsync()
{
    Task<string> task1 = Task.Run(() => "Data 1");
    Task<string> task2 = Task.Run(() => "Data 2");
 
    string[] results = await Task.WhenAll(task1, task2);
    foreach (var r in results) Console.WriteLine(r);
}

• Task.WhenAll() waits for multiple tasks concurrently

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📌 7. Best Practices

1. Avoid async void except in event handlers

2. Prefer Task or Task<T> return types

3. Use ConfigureAwait(false) in libraries to avoid context capture

4. Handle exceptions with try-catch inside async methods

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📌 8. Summary Table

Keyword	Description
async	Marks a method as asynchronous
await	Waits for task completion without blocking
Return Types	Task, Task<T>, void (event handlers only)
Use Case	I/O, network calls, database operations
Best Practice	Avoid async void, handle exceptions, return Task

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✅ Tip:

• Use async/await to keep your UI responsive and improve throughput in I/O-heavy applications

• Combine with Task.WhenAll or Task.WhenAny for concurrency