VB.NET
Dim u1 As New Uri("C:\Windows\System\")
Dim u2 As New Uri("C:\Windows\file.txt")
Dim relativeUri As Uri = u1.MakeRelativeUri(u2)
Dim relativePath As String = relativeUri.ToString()
relativePath = relativePath.Replace("/"c, "\"c)
Console.WriteLine(relativePath)
C#
Uri u1 = new Uri("C:\\Windows\\System\\");
Uri u2 = new Uri("C:\\Windows\\file.txt");
Uri relativeUri = u1.MakeRelativeUri(u2);
string relativePath = relativeUri.ToString();
relativePath = relativePath.Replace('/', '\\');
Console.WriteLine(relativePath);