{{{0}}} 对字符串有什么作用.格式做
本文关键字:作用 格式 什么 字符串 | 更新日期: 2023-09-27 17:57:23
在命名空间MS.Internal
中,有一个名为NamedObject
的类。
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;
}
我对这个评论特别好奇:
// lazily add {} around the name, to avoid allocating a string
// until it's actually needed
_name = String.Format(CultureInfo.InvariantCulture, "{{{0}}}", _name);
这怎么是"懒惰"?懒惰有什么作用?
来自参考源的完整类:
//----------------------------------------------------------------------------
//
// <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.
你用大括号逃脱了大括号,即 {{
产生{
,}}
产生}
。
中间的{0}
像往常一样被解释 - 即对索引零处的参数的引用。
{{ {0} }}
^^ ^^^ ^^
| | |
| | +--- Closing curly brace
| +------ Parameter reference
+---------- Opening curly brace
最终结果是括在大括号中的参数零的值:
var res = string.Format("{{{0}}}", "hello"); // produces {hello}
这怎么是"懒惰"?
他们称其为这种替代的"渴望"实现的懒惰:
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;
}
此实现会立即添加大括号,即使它不知道需要括在大括号中的名称。
这怎么是"懒惰"?懒惰有什么作用?
懒惰来自它之前的if (_name[0] != '{')
。
它仅在首次请求时更改_name
字段。
就像每个人都已经指出的那样,String.Format("{{{0}}}", _name);
应该被解读为"{{ {0} }}"
或"'{ {0} '}"
.内{0}
是用第一个参数替换的实际字段,外{{
和}}
是获得单个{}
的特殊表示法
{{
,}}
只是给你字面上的{
和}
。(转义的大括号)
所以,如果你有{{{0}}}
,并且你给foo
,输出将是{foo}
var value = "value";
String.Format(CultureInfo.InvariantCulture, "{{{0}}}", value); // will output {value}