C# 编程/关键字/using

在 C# 中，using 关键字有两个完全不相关的含义，取决于它是用作指令还是语句。

指令

using 作为指令解析无限定类型引用，这样开发者就不必指定完整的命名空间。

例子

using System;
 
// A developer can now type ''Console.WriteLine();'' rather than ''System.Console.WriteLine()''.

using 还可以为引用类型提供命名空间别名。

例子

using utils = Company.Application.Utilities;

语句

using 作为语句会自动调用指定对象的 dispose。 该对象必须实现 IDisposable 接口。只要它们是相同类型，就可以在一个语句中使用多个对象。

例子

using (System.IO.StreamReader reader = new StreamReader("readme.txt"))
{
    // read from the file
}
 
// The file readme.txt has now been closed automatically.

using (Font headerFont = new Font("Arial", 12.0f),
            textFont = new Font("Times New Roman", 10.0f))
{
    // Use headerFont and textFont.
}

// Both font objects are closed now.

C# 关键字
abstract as base bool break byte case catch char checked class const continue decimal default delegate do double else enum event explicit extern false finally fixed float for foreach goto if implicit in int interface internal is lock long namespace new null object operator out override params private protected public readonly ref return sbyte sealed short sizeof stackalloc static string struct switch this throw true try typeof uint ulong unchecked unsafe ushort using var virtual void volatile while
C# 特殊标识符（上下文关键字）
add alias async await dynamic get global nameof partial remove set value when where yield
上下文关键字（用于查询）
ascending by descending equals from group in into join let on orderby select where