C# .NET - httpcontext.current.user.identity.name

hi,
i design a db with one field user_id..then in a form i need fill user_id by log in with httpcontext.current.user.identity.name
but it show me all cast errors.and it dont let me i use from this tips.
for example
cmd.parameters.addwithvalue("user_id",httpcontext.current.user.identity.name);
  i convert it to int.parse still error
then i just used  httpcontext.current.user.identity still error
  i put it in string id and convert it to string still error...
  i appreciate if anyone can help me to solve it.