Why Doesn't Function Strip Out Newline

I wrote a VB function as:

Public Function StripNewLineFromString(str As String) As String
str.Replace(ControlChars.Cr, "")
str.tempStr.Replace(ControlChars.Lf, "")
Return str.replace(ControlChars.Lf, "")
End Function

and wrote a C# unit test to test it. It works fine. However, when it really run the VB application itself, it does not strip out newline. If I changed the function to this:

Public Function StripNewLineFromString(str As String) As String
Dim tempStr As String = str
tempStr = tempStr.Replace(ControlChars.Cr, "")

[Code]....

, run the VB application, it strips out newline.