GetRelativePath

public static string BuildRelativePath(string referencePath, string destinationPath)
{
    // Check both are absolute :
    DirectoryInfo destination = new DirectoryInfo(destinationPath);
    string destinationAbsolutePath = destination.FullName;

    string referenceAbsolutePath;
    if (referencePath.EndsWith(@"\") || referencePath.LastIndexOf('.') <= referencePath.LastIndexOf('\\'))
    {   // It's a folder.
        referenceAbsolutePath = new DirectoryInfo(referencePath).FullName;
    }
    else
    {   // It's a file.
        referenceAbsolutePath = new FileInfo(referencePath).FullName;
    }

    // Attention aux UNC !
    string[] splittedTargetPath = destinationAbsolutePath.Split(@"\".ToCharArray(), StringSplitOptions.RemoveEmptyEntries);
    string[] splittedSourcePath = referenceAbsolutePath.Split(@"\".ToCharArray(), StringSplitOptions.RemoveEmptyEntries);

    if (splittedSourcePath[0] != splittedTargetPath[0])
    {
        // Aucune racine commune : On retourne le chemin absolu.
        return destinationAbsolutePath;
    }

    int deeperSimilarLevel = 0;

    while (splittedSourcePath[deeperSimilarLevel] == splittedTargetPath[deeperSimilarLevel])
    {
        deeperSimilarLevel++;
        if (deeperSimilarLevel >= splittedSourcePath.Length || deeperSimilarLevel >= splittedTargetPath.Length)
        {
            break;
        }
    }

    StringBuilder relativePath = new StringBuilder();
    for (int sourceRewind = deeperSimilarLevel; sourceRewind < splittedSourcePath.Length; sourceRewind++)
    {
        relativePath.Append(@"..\");
    }

    if (relativePath.Length == 0)
    {
        relativePath.Append(@".\");
    }

    for (int targetForward = deeperSimilarLevel; targetForward < splittedTargetPath.Length - 1; targetForward++)
    {
        relativePath.AppendFormat(@"{0}\", splittedTargetPath[targetForward]);
    }

    if (deeperSimilarLevel < splittedTargetPath.Length)
    {
        relativePath.AppendFormat(@"{0}", splittedTargetPath[splittedTargetPath.Length - 1]);
    }

    return relativePath.ToString();
}