public void GetAllTeacherByDepartment()
{
Teachers _Teachers = new Teachers();
_Teachers.DepartmentId = Convert.ToInt32(DepartmentDropDownList.SelectedValue);
TeacherDropDownList.DataSource = _CourseAssigneManager.GetAllTeacherByDepartment(_Teachers.DepartmentId);
TeacherDropDownList.DataTextField = "TeacherName";
TeacherDropDownList.DataValueField = "Id";
TeacherDropDownList.DataBind();
TeacherDropDownList.Items.Insert(0, new ListItem("Select Teachers", "0"));
}