c# "using"语句后跟 try 语句 在这种情况下可以省略括号吗？

Question

  using System.Net;       // (See Chapter 16)
   ...
  string s = null;
  using (WebClient wc = new WebClient()) // why there is no brackets after this using statement
      try { s = wc.DownloadString ("http://www.albahari.com/nutshell/");  }
      catch (WebException ex)
      {
        if (ex.Status == WebExceptionStatus.Timeout)

          Console.WriteLine ("Timeout");
        else
          throw;     // Can't handle other sorts of WebException, so rethrow
      }

上面的代码是从 page 153 c# in a Nutshell 中复制的，我不明白为什么在 using 语句后缺少 { }，这是书中的错字(不太可能)还是根本不需要？由于语法是 using 需要在 {} 内跟一个代码块。

我希望这段代码是:

  using System.Net;       // (See Chapter 16)
   ...
  string s = null;
  using (WebClient wc = new WebClient()) // why there is no brackets after this using statement
  {
      try { s = wc.DownloadString ("http://www.albahari.com/nutshell/");  }
      catch (WebException ex)
      {
        if (ex.Status == WebExceptionStatus.Timeout)

          Console.WriteLine ("Timeout");
        else
          throw;     // Can't handle other sorts of WebException, so rethrow
      }
  }

Answer 1

如果您查看 C# Specification 中 using 语句的语法，您会看到 using 语句后跟(或者更确切地说，它们的主体由)“embedded_statements”。

using_statement
    : 'using' '(' resource_acquisition ')' embedded_statement
    ;

嵌入语句定义如下:

embedded_statement
    : block
    | empty_statement
    | expression_statement
    | selection_statement
    | iteration_statement
    | jump_statement
    | try_statement
    | checked_statement
    | unchecked_statement
    | lock_statement
    | using_statement
    | yield_statement
    | embedded_statement_unsafe
    ;

所以是的，这不是错字。在 using (...) 之后，可以有任何在 embedded_statement 中定义的语句。无论如何，要查看这是否是拼写错误，您可以简单地尝试编译示例代码。