EN CSharp Keyword alias

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C#: alias

Handbook C# · Practical Guide

An external assembly alias adds a separate root namespace. It allows you to distinguish between identical fully qualified type names from different assemblies. Aliases are specified for reference in the project; in the code, the alias is declared via extern alias and used with the :: operator.

Simple example (Example.cs)

Save all shown files together and run Example.csproj. The standalone Example.cs does not have the Tools alias reference configured.

Example.cs:

// PROJECT-EXAMPLE
extern alias Tools;
System.Console.WriteLine(Tools::Demo.Number.Value);

Dependency.cs:

// PROJECT-EXAMPLE
namespace Demo; public static class Number { public static int Value => 7; }

Dependency.csproj:

<Project Sdk="Microsoft.NET.Sdk"><PropertyGroup><TargetFramework>net10.0</TargetFramework><EnableDefaultCompileItems>false</EnableDefaultCompileItems></PropertyGroup><ItemGroup><Compile Include="Dependency.cs" /></ItemGroup></Project>

Example.csproj:

<Project Sdk="Microsoft.NET.Sdk"><PropertyGroup><OutputType>Exe</OutputType><TargetFramework>net10.0</TargetFramework><EnableDefaultCompileItems>false</EnableDefaultCompileItems></PropertyGroup><ItemGroup><Compile Include="Example.cs" /><ProjectReference Include="Dependency.csproj"><Aliases>Tools</Aliases></ProjectReference></ItemGroup></Project>

dotnet run --project Example.csproj

Result:

7

Syntax

extern alias Legacy;
using OldLabel = Legacy::Shared.Label;

Accessing a local library via an alias

File Example.csproj:

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net10.0</TargetFramework>
    <LangVersion>14.0</LangVersion>
    <Nullable>enable</Nullable>
    
  </PropertyGroup>
  <ItemGroup>
    <Compile Remove="Dependency/**/*.cs" />
    <ProjectReference Include="Dependency/Dependency.csproj"><Aliases>Legacy</Aliases></ProjectReference>
  </ItemGroup>
</Project>

File Program.cs:

extern alias Legacy;
using System;

internal static class Program
{
    public static void Main()
    {
        var label = new Legacy::Shared.Label("Local library");
        Console.WriteLine(label.Text);
    }
}

File Dependency/Dependency.csproj:

<Project Sdk="Microsoft.NET.Sdk"><PropertyGroup><TargetFramework>net10.0</TargetFramework></PropertyGroup></Project>

File Dependency/Label.cs:

namespace Shared;
public sealed class Label(string text)
{
    public string Text { get; } = text;
}

Result:

Local library

Two libraries with the same fully qualified type name

File Example.csproj:

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net10.0</TargetFramework>
    <LangVersion>14.0</LangVersion>
    <Nullable>enable</Nullable>
    
  </PropertyGroup>
  <ItemGroup>
    <Compile Remove="LegacyDependency/**/*.cs;ModernDependency/**/*.cs" />
    <ProjectReference Include="LegacyDependency/LegacyDependency.csproj"><Aliases>OldApi</Aliases></ProjectReference>
    <ProjectReference Include="ModernDependency/ModernDependency.csproj"><Aliases>NewApi</Aliases></ProjectReference>
  </ItemGroup>
</Project>

File Program.cs:

extern alias OldApi;
extern alias NewApi;
using System;
using OldCounter = OldApi::Shared.Counter;
using NewCounter = NewApi::Shared.Counter;

internal static class Program
{
    public static void Main()
    {
        Console.WriteLine($"Old: {OldCounter.Scale(5)}");
        Console.WriteLine($"New: {NewCounter.Scale(5)}");
    }
}

File LegacyDependency/LegacyDependency.csproj:

<Project Sdk="Microsoft.NET.Sdk"><PropertyGroup><TargetFramework>net10.0</TargetFramework></PropertyGroup></Project>

File LegacyDependency/Counter.cs:

namespace Shared;
public static class Counter { public static int Scale(int value) => value * 2; }

File ModernDependency/ModernDependency.csproj:

<Project Sdk="Microsoft.NET.Sdk"><PropertyGroup><TargetFramework>net10.0</TargetFramework></PropertyGroup></Project>

File ModernDependency/Counter.cs:

namespace Shared;
public static class Counter { public static int Scale(int value) => value * 3; }

Result:

Old: 10
New: 15

Official source: Microsoft Learn.