code help

Question

How would I call the Check ID in the add studentbutton every time I call the checkID inside the student button it displays both the student id is already taken and it says student is saved.

        private void addStudentButton_Click(object sender, EventArgs e)

        {

            if (firstNameAddTextBox.Text == string.Empty ||

                lastNameAddTextBox.Text == string.Empty ||

                idAddTextBox.Text == string.Empty)

            {

                System.Windows.Forms.MessageBox.Show("Please enter all required information");

            }

            else

            {

                Student temp = new Student(this.firstNameAddTextBox.Text,

                    this.lastNameAddTextBox.Text, this.dobAddTextBox.Value,

                    Convert.ToInt32(this.idAddTextBox.Text));

                data.AddStudent(temp);

checkId();

                System.Windows.Forms.MessageBox.Show("Your student has been stored");

            }

        }

        public void checkId()

        {

            foreach (Student stu in students)

            {

                if (stu.IdNumber.ToString().Equals(idAddTextBox.Text))

                {

                    MessageBox.Show("Sorry.Student id already in use");

                    return;

                }

            }

        }