How Do I Checked If Employee Exist Or Not

i have a web application,in which a user information form.i want to  check if employee exist or not when user a user roll no which is also  PK.
 THAT may be by using on text change or using button click event
asp:TextBox CssClass="form-control" runat="server" MaxLength="7" ID="txtRollNo"    />                                           

and codebehind is

 protected void btn_Click_Click(object sender, EventArgs e)         {             if (!string.IsNullOrEmpty(txtRollNo.Text))             {                 Employee emp = _service.GetAllEmployee().Where(x => x.Employee_Id==Employee.Cnic).LastOrDefault();                   if (emp != null)                 {                     MsgLitteral.ShowNotificationInline("Record Found", false);                   }                 else                 {                     MsgLitteral.ShowNotificationInline("Record Not Found", false);                 }               }         }

Problem is that when i press the button is always return Record found,although there is no record against that employee Id.