String.Format - 它是如何工作的以及如何实现自定义格式字符串 [英] String.Format - how it works and how to implement custom formatstrings

问题描述

使用 String.Format() 可以以多种不同的方式格式化例如 DateTime 对象.每次我寻找所需的格式时，我都需要在 Internet 上四处搜索.几乎总是我找到一个我可以使用的例子.例如:

With String.Format() it is possible to format for example DateTime objects in many different ways. Every time I am looking for a desired format I need to search around on Internet. Almost always I find an example I can use. For example:

String.Format("{0:MM/dd/yyyy}", DateTime.Now);          // "09/05/2012"

但我不知道它是如何工作的，以及哪些类支持这些神奇"的附加字符串.

But I don't have any clue how it works and which classes support these 'magic' additional strings.

所以我的问题是:

1. String.Format 如何将附加信息 MM/dd/yyyy 映射到字符串结果?
2. 是否所有 Microsoft 对象都支持此功能?
   这是否记录在某处?
3. 是否可以做这样的事情:
   String.Format("{0:MyCustomFormat}", new MyOwnClass())

1. How does String.Format map the additional information MM/dd/yyyy to a string result?
2. Do all Microsoft objects support this feature?
   Is this documented somewhere?
3. Is it possible to do something like this:
   String.Format("{0:MyCustomFormat}", new MyOwnClass())

推荐答案

String.Format 将字符串中的每个标记({0} 等)与对应对象:https://docs.microsoft.com/en-us/dotnet/api/system.string.format#overloads

String.Format matches each of the tokens inside the string ({0} etc) against the corresponding object: https://docs.microsoft.com/en-us/dotnet/api/system.string.format#overloads

可选地提供格式字符串:

A format string is optionally provided:

{ index[,alignment][ : formatString] }

如果提供了 formatString，则相应的对象必须实现 IFormattable，特别是接受 formatString 并返回相应格式化字符串的 ToString 方法:https://docs.microsoft.com/en-us/dotnet/api/system.iformattable.tostring

If formatString is provided, the corresponding object must implement IFormattable and specifically the ToString method that accepts formatString and returns the corresponding formatted string: https://docs.microsoft.com/en-us/dotnet/api/system.iformattable.tostring

IFormatProvider 也可用于捕获基本格式标准/默认值等.示例 此处 和 此处.

An IFormatProvider may also be used to capture basic formatting standards/defaults etc. Examples here and here.

请按顺序回答您的问题:

So the answers to your questions in order:

1. 它在 DateTime 对象上使用 IFormattable 接口的 ToString() 方法并传递 MM/dd/yyyy 格式字符串.正是该实现返回了正确的字符串.

1. It uses the IFormattable interface's ToString() method on the DateTime object and passes that the MM/dd/yyyy format string. It is that implementation which returns the correct string.

任何实现 IFormattable 的对象都支持此功能.你甚至可以自己写！

Any object that implement IFormattable supports this feature. You can even write your own!

是的，见上文.

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