I am working on an examination project, where I am trying to conctrcut a Timr Reg system where the employees can recoed there starttime finish time and alos info like vavation or illness. I have made a windows aplication, in which I have created a Formwith a button and a text filed, I am scusessfull in makeing the button to function ti go to next form, but my question is how I can make a function where when entering the first window the validation for ESSN can be done so if a worng ESSN is entered,the system should shw an error box or messange the correct number should be ebtered, is there a validation needed for that how it can be done.

at the end when the number is enterded it opnes another form where there are three buttons one is for start and second is for finish and third is for recording illness or vecationtime or absence.

at the end all info is recoredeinto a file(which is text format or Xml)