而在 .Net(C#,VB.Net) 下， 参数默认是使用 ByVal （即按值传送)  传送的，一直没怎么注意。

这些天在优化程序时发现 当传送 大变量时，使用默认方式（ByVal) 效率相当低

如传入的参数变量类型 是 大字符串，数组，集合，DataSet 等

测试的关键代码如下,我传入的字符串并没有特别大，变量越大，使用 ByRef 效率越高,当然，当传入得变量可以被修改或无其他作用时，可以改用 ByRef 传

 Private Declare Function GetTickCount Lib "kernel32" () As Int32

    Private Function TestByRef(ByRef aa As String) As String
        aa = "1" & aa
        TestByRef = aa

    End Function

    Private Function TestByVal(ByVal aa As String) As String
        aa = "1" & aa
        TestByVal = aa

    End Function

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

        Dim teststr As String
        Dim Newstr As String

        Dim t As Int32
        Dim II As Int32

        teststr = "wosdfsdfdsfdsfsfdsfsfsfsfsfsfdsfdsfcvxcvxcvcxvvcxvvvxvcvxv"

        t = GetTickCount

        For II = 1 To 10000
            Newstr = TestByRef(teststr)
        Next

        MsgBox("ByRef  " & CStr(GetTickCount - t))

        t = GetTickCount

        For II = 1 To 10000
            Newstr = TestByVal(teststr)
        Next

        MsgBox("ByVal  " & CStr(GetTickCount - t))

    End Sub