先拉一个控件：DropDownList

<asp:DropDownList ID="ddl_Class" runat="server"  CssClass="form-control"></asp:DropDownList>

后台代码展示：

  using (var db = new DemoEntities())
                {
                    //1.查询出当前修改的学员信息
                    var model = db.Student.FirstOrDefault(s => s.Id == editId);
                    if (model != null)
                    {
                        //2.绑定学员信息到界面上
                        txt_Name.Text = model.Name;
                        txt_Age.Text = model.Age.ToString();
                        txt_Email.Text = model.Email;
                        txt_Mobile.Text = model.Mobile;

                        //3.查询出班级信息

                        //3.1.查询班级数据
                        var list = (from c in db.Class select c).ToList();
                        //3.2.将数据绑定到下拉框
                        ddl_Class.DataSource = list;
                        ddl_Class.DataTextField = "ClassName";
                        ddl_Class.DataValueField = "Id";
                        ddl_Class.DataBind();

                        //4.使班级下拉框选中当前学员的班级
                        ddl_Class.SelectedValue = model.ClassId.ToString();

                    }
                    
                }

运行结果：