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