i have a text file witch contains values like this :
string FileLines = File.ReadAllLines(@"D:TestHash.txt");
Hashtable hashtable = new Hashtable();
foreach (string line in FileLines)
// dont know what to do here
Hashtable is a container for key-value-pairs. Since you only have values, not key-value-pairs, you don't need a hashtable, you need a
HashSet<string> fileLineSet = new HashSet<string>(FileLines);
Check MSDN on how to use a hash set (including an example).