private string RelativePath(string absolutePath, string relativeTo)

        {

            string[] absoluteDirectories = absolutePath.Split('\\');

            string[] relativeDirectories = relativeTo.Split('\\');

            //Get the shortest of the two paths

            int length = absoluteDirectories.Length < relativeDirectories.Length ? absoluteDirectories.Length : relativeDirectories.Length;

            //Use to determine where in the loop we exited

            int lastCommonRoot = -1;

            int index;

            //Find common root

            for (index = 0; index < length; index++)

                if (absoluteDirectories[index] == relativeDirectories[index])

                    lastCommonRoot = index;

                else

                    break;

            //If we didn't find a common prefix then throw

            if (lastCommonRoot == -1)

                throw new ArgumentException("Paths do not have a common base");

            //Build up the relative path

            StringBuilder relativePath = new StringBuilder();

            //Add on the ..

            for (index = lastCommonRoot + 1; index < absoluteDirectories.Length; index++)

                if (absoluteDirectories[index].Length > 0)

                    relativePath.Append("..\\");

            //Add on the folders

            for (index = lastCommonRoot + 1; index < relativeDirectories.Length - 1; index++)

                relativePath.Append(relativeDirectories[index] + "\\");

            relativePath.Append(relativeDirectories[relativeDirectories.Length - 1]);

            return relativePath.ToString();

        }

System.Uri uri1 = new Uri(@"C:\filename.txt");

System.Uri uri2 = new Uri(@"C:\mydirectory\anotherdirectory\");

Uri relativeUri = uri2.MakeRelativeUri(uri1);

Console.WriteLine(relativeUri.ToString());