GetUNCPath.cs

[DllImport("mpr.dll", CharSet = CharSet.Auto)]
public static extern int WNetGetConnection(
    [MarshalAs(UnmanagedType.LPTStr)] string localName,
    [MarshalAs(UnmanagedType.LPTStr)] StringBuilder remoteName,
    ref int length);

public static string GetUNCPath(string originalPath)
{
    StringBuilder sb = new StringBuilder(512);
    int size = sb.Capacity;

    // look for the {LETTER}: combination ...
    if (originalPath.Length > 2 && originalPath[1] == ':')
    {
        // don't use char.IsLetter here - as that can be misleading
        // the only valid drive letters are a-z && A-Z.
        char c = originalPath[0];
        if ((c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z'))
        {
            int error = WNetGetConnection(originalPath.Substring(0, 2),
                sb, ref size);
            if (error == 0 || error == 1201)
            {
                DirectoryInfo dir = new DirectoryInfo(originalPath);

                string path = Path.GetFullPath(originalPath)
                    .Substring(Path.GetPathRoot(originalPath).Length);
                return Path.Combine(sb.ToString().TrimEnd(), path);
            }
        }
    }

    return originalPath;
}