string.Format 上的 {{{0}}} 是做什么的？

Question

在命名空间 MS.Internal 中，有一个名为 NamedObject 的 class。

它有一个奇怪的代码块：

public override string ToString()
{
  if (_name[0] != '{')
  {
    // lazily add {} around the name, to avoid allocating a string 
    // until it's actually needed
    _name = String.Format(CultureInfo.InvariantCulture, "{{{0}}}", _name);
  }

  return _name;
}

我特别好奇这条评论：

    // lazily add {} around the name, to avoid allocating a string 
    // until it's actually needed
    _name = String.Format(CultureInfo.InvariantCulture, "{{{0}}}", _name);

'lazy'怎么样？懒惰有什么用？

完整 class 来自 reference source:

//---------------------------------------------------------------------------- 
//
// <copyright file="NamedObject.cs" company="Microsoft">
//    Copyright (C) Microsoft Corporation.  All rights reserved.
// </copyright> 
//
// Description: Placeholder object, with a name that appears in the debugger 
// 
//---------------------------------------------------------------------------

using System;
using System.Globalization;
using MS.Internal.WindowsBase;

namespace MS.Internal
{
  /// <summary> 
  /// An instance of this class can be used wherever you might otherwise use
  /// "new Object()".  The name will show up in the debugger, instead of 
  /// merely "{object}"
  /// </summary>
  [FriendAccessAllowed]   // Built into Base, also used by Framework.
  internal class NamedObject
  {
    public NamedObject(string name)
    {
      if (String.IsNullOrEmpty(name))
        throw new ArgumentNullException(name);

      _name = name;
    }

    public override string ToString()
    {
      if (_name[0] != '{')
      {
        // lazily add {} around the name, to avoid allocating a string 
        // until it's actually needed
        _name = String.Format(CultureInfo.InvariantCulture, "{{{0}}}", _name);
      }

      return _name;
    }

    string _name;
  }
}

// File provided for Reference Use Only by Microsoft Corporation (c) 2007.
// Copyright (c) Microsoft Corporation. All rights reserved.

Answer 1

你escape a curly brace with a curly brace，即{{产生{，而}}产生}。

中间的 {0} 照常解释 - 即对索引零处参数的引用。

{{ {0} }}
^^ ^^^ ^^
|   |  |
|   |  +--- Closing curly brace
|   +------ Parameter reference
+---------- Opening curly brace

最终结果是用花括号括起来的参数零的值：

var res = string.Format("{{{0}}}", "hello"); // produces {hello}

How is that 'lazy'?

他们称此替代方案 "eager" 实现为懒惰：

internal class NamedObject {
    public NamedObject(string name) {
        if (String.IsNullOrEmpty(name))
            throw new ArgumentNullException(name);
        if (name[0] != '{') {
            // eagerly add {} around the name
            _name = String.Format(CultureInfo.InvariantCulture, "{{{0}}}", name);
        } else {
            _name = name;
        }
    }
    public override string ToString() {
        return _name;
    }
    string _name;
}

此实现会立即添加花括号，即使它不知道将需要用花括号括起来的名称。

Answer 2

{{ 和 }} 只是给你文字 { 和 }。（转义花括号）

所以，如果你有 {{{0}}}，并且你提供 foo，输出将是 {foo}

Answer 3

var value = "value";
String.Format(CultureInfo.InvariantCulture, "{{{0}}}", value); // will output {value}

Answer 4

How is that 'lazy'? What does it do to be lazy?

懒惰来自于它之前的if (_name[0] != '{')。

它只在第一次请求时改变_name字段。

正如大家已经指出的那样，String.Format("{{{0}}}", _name); 应该读作 "{{ {0} }}" 或 "\{ {0} \}"。内部 {0} 是要替换为第一个参数的实际字段，外部 {{ 和 }} 是获得单个 {}

的特殊符号

string.Format 上的 {{{0}}} 是做什么的？

What does {{{0}}} on string.Format do?

c#

lazy-evaluation