名前空間、usingディレクティブ、global using（注意点）

名前空間と using の概要

名前空間と型の宣言

using System;

namespace MyApp.Models
{
  public class User
  {
    public string Name { get; private set; }
    public User(string name) { Name = name; }
  }
}

public class Program
{
  public static void Main(string[] args)
  {
    // Fully-qualified name without using
    MyApp.Models.User u = new MyApp.Models.User("Ada");
    Console.WriteLine(u.Name);
  }
}

using ディレクティブの基本

using System;
using MyApp.Models; // import the namespace declared previously

namespace MyApp.Models
{
  public class User
  {
    public string Name { get; private set; }
    public User(string name) { Name = name; }
  }
}

public class Program
{
  public static void Main(string[] args)
  {
    // Short type name thanks to using
    User u = new User("Alan");
    Console.WriteLine(u.Name);
  }
}

入れ子になった名前空間と FQN

using System;

namespace Company
{
  namespace Billing
  {
    public class Invoice { public int Number; }
  }

  namespace Support
  {
    public class Ticket { public int Id; }
  }
}

public class Program
{
  public static void Main(string[] args)
  {
    Company.Billing.Invoice inv = new Company.Billing.Invoice();
    inv.Number = 1001;
    Company.Support.Ticket t = new Company.Support.Ticket();
    t.Id = 42;
    Console.WriteLine("Invoice " + inv.Number + ", Ticket " + t.Id);
  }
}

using エイリアス

using System;

// Two classes with the same short name in different namespaces
namespace LibA { public class Logger { public string Name = "A"; } }
namespace LibB { public class Logger { public string Name = "B"; } }

// Create aliases to disambiguate
using ALogger = LibA.Logger;
using BLogger = LibB.Logger;

public class Program
{
  public static void Main(string[] args)
  {
    ALogger a = new ALogger();
    BLogger b = new BLogger();
    Console.WriteLine(a.Name + " & " + b.Name); // A & B
  }
}

Global using に関する注意とヒント

using ディレクティブの意味

まとめ