Get-Date转换为字符串vs ToString（） [英] Get-Date cast to string vs ToString()

问题描述

我对PowerShell的字符串嵌入语法$（$ object）的理解一直是 $ object 转换为 [System.String] ，它调用 $ object.ToString（）。但是，我在Windows 8.1上使用PowerShell 4.0，在 [DateTime] 类中注意到这个好奇的行为。

My understanding of PowerShell's string embedding syntax "$($object)" has always been that $object is cast to [System.String], which invokes $object.ToString(). However, I've noticed this curious behavior with the [DateTime] class using PowerShell 4.0 on Windows 8.1.

PS> $x = Get-Date

PS> $x.GetType() | select -ExpandProperty Name
DateTime

PS> $x.ToString()
2015-05-29 13:36:06

PS> [String]$x
05/29/2015 13:36:06

PS> "$($x)"
05/29/2015 13:36:06

似乎$（$ object）给出与转换为字符串相同的行为，但显然产生了与 $ object.ToString不同的结果（）。 $ x.ToString（）与intl.cpl（yyyy-MM-dd）中设置的短日期格式一致。 [String] $ x 似乎使用了en-US默认值。

It seems that "$($object)" gives the same behavior as casting to string, but is clearly producing a different result from $object.ToString(). $x.ToString() is consistent with the short date format set in intl.cpl (yyyy-MM-dd). [String]$x appears to use the en-US default.

这可能是一个bug在DateTime类中，但我更惊讶的是，将对象转换为字符串的不同方法会产生不同的结果。如果不调用 ToString（），则将对象转换为字符串的规则是什么？ DateTime类只是一个特殊情况，因为它的重载 ToString（String）？

It is possible this is simply a bug in the DateTime class, but I'm more surprised that the different methods of converting an object to a string produce different results. What are the rules for casting an object to a string, if not calling ToString()? Is the DateTime class simply a special case because of its overloading of ToString(String)?

推荐答案

如果一个对象实现了 IFormattable 接口，则PowerShell将调用 IFormattable.ToString 而不是 Object.ToString 用于投射操作。对于静态 Parse 方法来说，类似的事情就是发生：如果有一个重载，比 IFormatProvider 参数更重要。

If an object implements the IFormattable interface, then PowerShell will call IFormattable.ToString instead of Object.ToString for the cast operation. A similar thing happens for static Parse method: if there is an overload with IFormatProvider parameter, than it would be called.

Add-Type -TypeDefinition @'
    using System;
    using System.Globalization;
    public class MyClass:IFormattable {
        public static MyClass Parse(string str) {
            return new MyClass{String=str};
        }
        public static MyClass Parse(string str,IFormatProvider fp) {
            return new MyClass{String=str,FormatProvider=((CultureInfo)fp).DisplayName};
        }
        public string String {get;private set;}
        public string FormatProvider {get;private set;}
        public override string ToString() {
            return "Object.ToString()";
        }
        string IFormattable.ToString(string format,IFormatProvider fp) {
            return string.Format("IFormattable.ToString({0},{1})",format,((CultureInfo)fp).DisplayName);
        }
    }
'@
[String](New-Object MyClass) #Call IFormattable.ToString(null,CultureInfo.InvariantCulture)
[MyClass]'Test'              #Call MyClass.Parse("Test",CultureInfo.InvariantCulture)

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