VB.net Mid作为左操作员特殊(奇怪？)行为

Question

今天,在与我的同事交谈时,脑子里出现了一些奇怪的东西.

处理来自vb6的字符串的"秘密"方式,如:

 Dim strSomeString as String
 strSomeString = "i am phat" 
 Mid$(strSomeString, 6,4) = "hack"

这将放在i am hack里面strSomeString.

虽然对vb6中支持的这种奇怪感到惊讶,但当我读到它在VB.Net中也得到支持时(我可能与旧代码的兼容性),我完全被吹了.

Dim TestString As String
' Initializes string.
TestString = "The dog jumps"
' Returns "The fox jumps".
Mid(TestString, 5, 3) = "fox"
' Returns "The cow jumps".
Mid(TestString, 5) = "cow"
' Returns "The cow jumpe".
Mid(TestString, 5) = "cow jumped over"
' Returns "The duc jumpe".
Mid(TestString, 5, 3) = "duck"

我的问题是:它在技术上如何运作？什么是Mid像个在特定的情况呢？(方法？函数？扩展方法？)

Answer 1

它被转换为Microsoft.VisualBasic.CompilerServices.StringType中对此函数的MSIL调用

Public Shared Sub MidStmtStr ( _
    ByRef sDest As String, _
    StartPosition As Integer, _
    MaxInsertLength As Integer, _
    sInsert As String _
)

这个编译器技巧纯粹是为了向后兼容.它内置于编译器中,因此不是您可以在自己的类型上实现的技巧.

所以

Mid(TestString, 5, 3) = "fox"

变

MidStmtStr(TestString, 5, 3, "fox")

希望这可以帮助,