EN CSharp Keyword async

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C#: async

Handbook C# · Practical Guide

Enables await in the callable member. The method can return a Task, Task, ValueTask or an async sequence; async does not itself start a separate thread.

Simple example (Example.cs)

Save this example separately as Example.cs. Run it independently of the other examples on this page.

Example.cs:

// PROJECT-EXAMPLE
async System.Threading.Tasks.Task<int> ReadAsync()
{
    await System.Threading.Tasks.Task.Delay(1);
    return 7;
}
System.Console.WriteLine(await ReadAsync());

Result:

7

Syntax

async Task MethodAsync() { await ...; }

Example 1: simple form

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using System.IO;
using System.Runtime.InteropServices;
using System.Runtime.CompilerServices;

public static class Program
{
    public static async Task Main(string[] args)
    {
        int result = await Jobs.DoubleAsync(4);
        Console.WriteLine(result);
    }
}
public static class Jobs { public static async Task<int> DoubleAsync(int value) { await Task.Delay(1); return value * 2; } }

Example 2: another practical form

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using System.IO;
using System.Runtime.InteropServices;
using System.Runtime.CompilerServices;

public static class Program
{
    public static async Task Main(string[] args)
    {
        Task<int>[] jobs = [Jobs.CountAsync(2), Jobs.CountAsync(3)];
        int[] results = await Task.WhenAll(jobs);
        Console.WriteLine(results.Sum());
    }
}
public static class Jobs { public static async Task<int> CountAsync(int value) { await Task.Yield(); return value; } }

Official source: Microsoft Learn.