using 关键字有三个主要用途：
1.using 语句定义一个范围，在此范围的末尾将释放对象：

string filePath = "example.txt";
string textToWrite = "Hello, this is a test message!";
// Use the using statement to ensure the StreamWriter is properly disposed of
using (StreamWriter writer = new StreamWriter(filePath))
{writer.WriteLine(textToWrite);
}

2.using导入在其他命名空间中定义的类型：

using System;
using System.IO;

3.using 在命名空间下，为命名空间或类型或结构体创建别名

using Level = System.String;
using MyQuery = System.Linq;

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