EN CSharp Keyword using

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C#: using

Handbook C# · Practical Guide

Imports names, sets an alias, or manages an IDisposable/IAsyncDisposable resource. The import itself does not add the missing dependency.

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
using System;
using (var stream = new System.IO.MemoryStream())
{
    stream.WriteByte(65);
    Console.WriteLine(stream.Length);
}

Result:

1

Syntax

using Namespace; / using Alias = Type; / using var resource = ...; / await using ...;

the using declaration is valid until the end of the scope. For .csx use using (...) { ... } if the top-level declaration is ambiguous for script syntax.

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)
    {
        using var stream = new MemoryStream();
        stream.WriteByte(65);
        Console.WriteLine(stream.Length);
    }
}

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)
    {
        await using var stream = new MemoryStream();
        await stream.WriteAsync(new byte[] { 1, 2, 3 });
        Console.WriteLine(stream.Length);
    }
}

Official source: Microsoft Learn.