Entry property Error in entity fremwork

using (var dbCtx = new EmployeeEntities())
        {
            //Mark entity as modified
            dbCtx.Entry(stud).State = System.Data.EntityState.Modified;
            dbCtx.SaveChanges();
        }

private void Save()
    {

        EmployeeMast objEmp = new EmployeeMast();

        objEmp.EmpName = txtName.Text.Trim();
        objEmp.EmpAdd = txtAddress.Text.Trim();
        // objEmp.EmpBDate = Convert.ToDateTime(txtBDay.Text.Trim());
        objEmp.EmpDept = Convert.ToInt32(drpDepartment.SelectedValue);

        if (ViewState["EmpId"] == null)
        {
            obj.EmployeeMasts.AddObject(objEmp);
            obj.SaveChanges();
        }
        else
        {
            objEmp.EmpId = 1;
            obj.Entry(objEmp).State = System.Data.EntityState.Modified;
            obj.SaveChanges();
        }
    }
if i am update record then write code this line.

obj.Entry(objEmp).State = System.Data.EntityState.Modified;
then "Entry" not set ..Error display.