为什么我的DropDownList在Postback上为空？

Question

我看过一些类似问题的答案,但我似乎仍然无法弄清楚这一点.我想我误解了ASP.NET的工作方式.

在标准ASP.Net 4.0"创建新帐户"表单中,我添加了一个DropDownList,其中包含要为新帐户选择的角色.在aspx页面中,控件如下所示:

<asp:DropDownList ID="RoleList" Width="100px" runat="server"></asp:DropDownList>

然后我在Page_Load事件中填充List:

protected void Page_Load(object sender, EventArgs e)
    {
        RegisterUser.ContinueDestinationPageUrl = Request.QueryString["ReturnUrl"];

        if (Page.IsPostBack)
        {
            return;
        }

        //Set the Role List Selections
        DropDownList roleList = (DropDownList)RegisterUser.CreateUserStep.ContentTemplateContainer.FindControl("RoleList");

        //set the role list
        String[] roles = Roles.GetAllRoles();
        foreach (String role in roles)
        {
            roleList.Items.Add(new ListItem(role, role));
        }
    }

我可以从生成的html中看到/选择一个角色.单击用于创建用户的"提交"按钮时会出现问题:

protected void RegisterUser_CreatedUser(object sender, EventArgs e)
    {
        FormsAuthentication.SetAuthCookie(RegisterUser.UserName, false /* createPersistentCookie */);

        string continueUrl = RegisterUser.ContinueDestinationPageUrl;
        if (String.IsNullOrEmpty(continueUrl))
        {
            continueUrl = "~/";
        }

        //set user role
        DropDownList roleList = (DropDownList)RegisterUser.CreateUserStep.ContentTemplateContainer.FindControl("RoleList");
        Roles.AddUserToRole(RegisterUser.UserName, roleList.SelectedValue);

        Response.Redirect(continueUrl);
    }

这里,roleList对象包含零项,并且没有选定的值.不知何故,我在选择项目和提交之间丢失了填充项目.知道我做错了什么吗？

Answer 1

将您的下拉列表加载到OnInit函数中 - 然后在调用RegisterUser_CreatedUser时应该正确加载它:

protected override void OnInit(EventArgs e)
{
    base.OnInit(e);

    //Set the Role List Selections
    DropDownList roleList = (DropDownList)RegisterUser.CreateUserStep.ContentTemplateContainer.FindControl("RoleList");

    //set the role list
    String[] roles = Roles.GetAllRoles();
    foreach (String role in roles)
    {
        roleList.Items.Add(new ListItem(role, role));
    }
}