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。

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

等于

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

using语句用于处理c#中实现IDisposable接口的对象。

IDisposable接口有一个名为Dispose的公共方法，用于释放对象。当我们使用using语句时，我们不需要在代码中显式地处理对象，using语句会处理它。

using (SqlConnection conn = new SqlConnection())
{

}

当我们使用上面的代码块时，内部生成的代码是这样的:

SqlConnection conn = new SqlConnection() 
try
{

}
finally
{
    // calls the dispose method of the conn object
}

更多细节请阅读:理解c#中的“using”语句。

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。