How to loop for id from data base using LINQ?

I try it by using datatable like that

for (int i = 0; i < dtp.Rows.Count; i++)
            {
                if (dtp.Rows[i]["UserId"].ToString() == formid.ToString())
              {
                   
              }
           }

Now I need it but by using LINQ How can I loop linq query

my linq query like that

int ID = (from s in con.TUsers
                                    where s.UserId.Equals(comboBox1.SelectedValue)
                                    select s.UserId).SingleOrDefault();

help me please
thanx in advance.

Tags: C#, SQL, LINQ, SQL Server, SQL Server 2008, ASP.NET4.0