c# - "using"语句与花括号

Question

我想知道为什么我们在 C# 中使用 using 语句。查了一下，发现是用来执行语句，然后清理对象的。所以我的问题是:如果我们打开和关闭大括号 ( { } ) 来定义范围，这不是一回事吗？

使用语句:

using (SqlConnection conn = new SqlConnection(connString)) {
     SqlCommand cmd = conn.CreateCommand();
     cmd.CommandText = "SELECT * FROM Customers";
     conn.Open();
     using (SqlDataReader dr = cmd.ExecuteReader()) {
          while (dr.Read()) 
          // Do Something...
     }
}

花括号:

{
     SqlConnection conn = new SqlConnection(connString);
     SqlCommand cmd = conn.CreateCommand();
     cmd.CommandText = "SELECT * FROM Customers";
     conn.Open();
     {
          SqlDataReader dr = cmd.ExecuteReader();
          while (dr.Read()) 
          // Do Something...
     }
}

这两种方法有什么显着差异吗？

Answer 1

好吧，使用(当且仅当该类实现IDisposable 接口(interface)时才合法)

using (SqlConnection conn = new SqlConnection(connString)) {
  // Some Code
  ...
}

等于这段代码

SqlConnection conn = null;

try {
  SqlConnection conn = new SqlConnection(connString);

  // Some Code
  ...
}
finally {
  if (!Object.ReferenceEquals(null, conn))
    conn.Dispose();
}

C# 不具有与 C++ 相同的行为，因此不要像在 C++ 中那样在 C# 中使用 {...} 模式:

{
  SqlConnection conn = new SqlConnection(connString);
  ...
  // Here at {...} block exit system's behavior is quite different:
  //
  // C++: conn destructor will be called, 
  // resources (db connection) will be safely freed
  //
  // C#:  nothing will have happened! 
  // Sometimes later on (if only!) GC (garbage collector) 
  // will collect conn istance and free resources (db connection). 
  // So, in case of C#, we have a resource leak 
}