Lære Async Method Signatures and Return Types | Async/Await Fundamentals and Converting Sync to Async

Understanding how to structure asynchronous methods in C# is crucial for writing robust and maintainable code. When you declare an async method, you must carefully choose its return type. The three primary async return types are Task, Task<T>, and void. Each serves a specific purpose and comes with its own best practices.

A method returning Task is used when you want the method to be asynchronous but do not need to return a value. This is the most common return type for async methods that perform work but do not compute a result. On the other hand, Task<T> is used when the async method computes and returns a value of type T. This allows you to await the method and retrieve the result once the asynchronous operation is complete. The void return type is special: it should only be used for async event handlers, such as those responding to button clicks in a UI framework. Using void for non-event-handler async methods is discouraged because it makes error handling and flow control difficult.

Definition

In async programming, Task represents an ongoing operation that does not produce a result, while Task<T> represents an ongoing operation that produces a result of type T. Both allow you to await completion and handle exceptions.

Program.cs


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using System;
using System.Threading.Tasks;

namespace ConsoleApp
{
    public class Program
    {
        public static async Task Main(string[] args)
        {
            int result = await CalculateSumAsync(5, 7);
            Console.WriteLine($"The sum is: {result}");
        }

        public static async Task<int> CalculateSumAsync(int a, int b)
        {
            await Task.Delay(500); // Simulate asynchronous work
            return a + b;
        }
    }
}

When you call an async method that returns a value using Task<T>, you use the await keyword to pause execution until the result is available. The awaited result can then be stored in a variable and used like any other value. This pattern makes it easy to work with asynchronous operations as if they were synchronous, improving code readability and maintainability.

If you need to call an async method inside another async method, simply use await and handle the result as needed. If you call an async method without await, you receive the Task object itself, which can be stored, passed around, or awaited later. However, failing to await a task may lead to unhandled exceptions or code that executes out of order.

ExampleEventHandler.cs


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using System;
using System.Threading.Tasks;

namespace ConsoleApp
{
    public class ExampleEventHandler
    {
        // This pattern is only appropriate for event handlers
        public async void OnButtonClick(object sender, EventArgs e)
        {
            await Task.Delay(1000);
            Console.WriteLine("Button clicked!");
        }
    }
}

Exception handling in async methods depends on the return type. If an exception is thrown inside an async method that returns Task or Task<T>, the exception is captured and stored in the returned task. When you await the task, the exception is re-thrown, allowing you to handle it with a try-catch block. This makes error propagation predictable and manageable.

If an async method returns void, exceptions are not captured in a task and cannot be awaited. This means any exceptions thrown will crash the application if unhandled, making void a poor choice except for event handlers where the runtime expects a void signature. Always prefer Task or Task<T> for async methods to ensure proper error handling.

1. Which async method return type should be avoided except for event handlers?

2. How do you retrieve the result of an async method that returns Task?

3. What happens if an exception is thrown inside an async method?

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