在Using块离开后使用Dispose()，即使代码抛出异常。

因此，您通常将using用于需要在它们之后清理的类，例如IO。

所以，这个使用block:

using (MyClass mine = new MyClass())
{
  mine.Action();
}

会做同样的事情:

MyClass mine = new MyClass();
try
{
  mine.Action();
}
finally
{
  if (mine != null)
    mine.Dispose();
}

使用“使用”更简短，更容易阅读。

using (B a = new B())
{
   DoSomethingWith(a);
}

等于

B a = new B();
try
{
  DoSomethingWith(a);
}
finally
{
   ((IDisposable)a).Dispose();
}

using语句获取一个或多个资源，执行一条语句，然后处置资源。

从MSDN:

C#, through the .NET Framework common language runtime (CLR), automatically releases the memory used to store objects that are no longer required. The release of memory is non-deterministic; memory is released whenever the CLR decides to perform garbage collection. However, it is usually best to release limited resources such as file handles and network connections as quickly as possible. The using statement allows the programmer to specify when objects that use resources should release them. The object provided to the using statement must implement the IDisposable interface. This interface provides the Dispose method, which should release the object's resources.

换句话说，using语句告诉. net一旦不再需要using块中指定的对象，就释放它。

还要注意，通过using实例化的对象在using块中是只读的。请参考这里的官方c#参考。

如果该类型实现了IDisposable，它会自动释放该类型。

考虑到:

public class SomeDisposableType : IDisposable
{
   ...implmentation details...
}

它们是等价的:

SomeDisposableType t = new SomeDisposableType();
try {
    OperateOnType(t);
}
finally {
    if (t != null) {
        ((IDisposable)t).Dispose();
    }
}

using (SomeDisposableType u = new SomeDisposableType()) {
    OperateOnType(u);
}

第二种更容易阅读和维护。

自从c# 8以来，有了一个新的语法，可以使代码更具可读性:

using var x = new SomeDisposableType();

它没有自己的{}块，使用范围是从声明点到声明它的块的末尾。这意味着你可以避免这样的事情:

string x = null;
using(var someReader = ...)
{
  x = someReader.Read();
}

还有这个:

using var someReader = ...;
string x = someReader.Read();