This is similar, but not identical, to PathCombine and [Path.Combine]. For example, appending "C:\foo" with "\bar" yields "C:\foo\bar" rather than "C:\bar" or "\bar", respectively.

static extern string PathCombine([Out] StringBuilder lpszDest, string lpszDir, string lpszFile);

Declare Unicode Function PathCombine Lib "shlwapi.dll" Alias "PathCombineW" (<MarshalAs(UnmanagedType.LPTStr)> PathOut As System.Text.StringBuilder, <MarshalAs(UnmanagedType.LPTStr)> PathIn As String, <MarshalAs(UnmanagedType.LPTStr)> More As String) As IntPtr

Declare Function PathCombine Lib "shlwapi.dll" Alias "PathCombineA" (ByVal szDest As String, ByVal lpszDir As String, ByVal lpszFile As String) As Long

Current signature is incorrect, because return value is string and CLR trying to release the same string twice (return value and lpszDest). I have unxpected crashes. I think that type of return value should change to IntPtr. This page contains some discussion about this problem: http://objectmix.com/dotnet/747522-p-invoke-signature-pathcombine.html

    static extern string PathCombine([Out] StringBuilder lpszDest, string lpszDir, string lpszFile);

        return PathCombine(sb,basePath,relativePath);