Compare a string with one another from XML used by C# DataGridView

I have a XML file , simple like this : http://pastebin.com/baKcUbRB
So, I have a DataGridView which rows are from xml node string. 
An user has a TextBox and a Button. If he wrote a link (with or not with http://), the button click event has to check if that written link already exists in XML file (or DataGridView values).
I wrote this : http://pastebin.com/jsYam2kg


For example : In XML file (as above) exists a link http://www.stackoverflow.com
If the user write again same link the MessageBox must appear, otherwise that link will be inserted in XML file.


Perhaps, the problem is check_link function. Where is the mistake ?


Thank you