Hamed,
You can check it against the Empty constant on the string class, which
is an empty string. This is different from a null reference, which means
that you don't refer to any string at all. For the most part, they are
probably the same, but if you want to check against all cases, you can do
the following:
bool pblnEmpty = (str == null || str.Length == 0);
Hope this helps.