string to ref string error ??[Resolved]

[Pirate]

This is weird . Below, the method signature is marked as ByRef . I exported this class to dll file . It works fine in VB. In C# , it gives me two strange errors .They say : Argument '1': cannot convert from 'string' to 'ref string'
and
Argument '2': cannot convert from 'string' to 'ref string'

Public Shared Function FindFile(ByRef RootPath As String, ByRef FileName As String) As String

I used it like this :

MessageBox.Show(FindFileViaAPI.FileClass.FindFile("c:\\" ,"setup.exe"));

Anyone knows why ?