Question

在btnSave - button事件处理程序中，在以下两行中，SelectedValue始终返回null。

not.TargetSessionCode = this.sessionsDropDownList1.SelectedValue;
not.CourseCode = this.coursesDropDownList1.SelectedValue;

谁能说出原因？

这是我的aspx代码：

<%@ Page Language="C#" MasterPageFile="~/MasterPage.Master" AutoEventWireup="true" CodeBehind="TeacherComposeNotice.aspx.cs" Inherits="Ice_Web_Portal_v_3_0_WebApplication.Teacher.TeacherComposeNotice" Title="Untitled Page" %>
<asp:Content ID="Content1" ContentPlaceHolderID="ContentPlaceHolder1" runat="server">
    <table style="width: 270px">
        <tr>
            <td style="text-align: right">
                Session:</td>
            <td colspan="2">
                <asp:DropDownList ID="sessionsDropDownList1" runat="server" Width="178px">
                </asp:DropDownList></td>
            <td colspan="1" style="width: 6px">
                <asp:Label ID="userNameLabel" runat="server" Text="usernameLabel" Width="167px"></asp:Label></td>
        </tr>
        <tr>
            <td style="text-align: right">
                Courses:</td>
            <td colspan="2">
                <asp:DropDownList ID="coursesDropDownList1" runat="server" Width="178px">
                </asp:DropDownList></td>
                <td colspan="1"><asp:Label id="labCourses" runat="server" Width="167px" Text="labCourses"></asp:Label></td>
        </tr>
        <tr>
            <td style="text-align: right">
                Subject:</td>
            <td colspan="3">
                <asp:TextBox ID="mailSubjectTextBox" runat="server" Width="343px"></asp:TextBox></td>
        </tr>
        <tr>
            <td style="text-align: right" valign="top">
                Content:</td>
            <td colspan="3">
                <asp:TextBox ID="mailContentTextBox" runat="server" Height="181px" Width="343px" Font-Names="Verdana"></asp:TextBox></td>
        </tr>
        <tr>
            <td style="text-align: right" valign="top">
                Expiry Date :</td>
            <td colspan="3">
            </td>
        </tr>
        <tr>
            <td style="text-align: right" valign="top">
            </td>
            <td colspan="3" style="text-align: center">
                &nbsp;</td>
        </tr>
        <tr>
            <td style="text-align: right" valign="top">
            </td>
            <td colspan="3" style="text-align: left">
                <asp:Label ID="errorMessageLabel" runat="server" ForeColor="#C00000" Text="***" Width="314px"></asp:Label></td>
        </tr>
        <tr>
            <td style="text-align: right" valign="top">
            </td>
            <td colspan="3" style="text-align: left">
                <asp:Button ID="btnBack" runat="server" OnClick="btnBack_Click" Text="Back" />
                <asp:Button ID="btnSave" runat="server" OnClick="btnSave_Click" Text="Save" /></td>
        </tr>
    </table>
</asp:Content>

这是我的代码隐藏：

public partial class TeacherComposeNotice : System.Web.UI.Page
        {
            string username = string.Empty;

            protected void Page_Load(object sender, EventArgs e)
            {
                username = (string)Request.QueryString["username"];

                if (!IsPostBack)
                {
                    LoadAllSessionsToDDL();
                    LoadAllCoursesToDDL();
                }

            }

            private void LoadAllSessionsToDDL()
            {
                this.sessionsDropDownList1.Items.Clear();

                List<Ice_Web_Portal.BO.Session_> items = Ice_Web_Portal.BO.Session_.GetAllSessions();

                this.sessionsDropDownList1.DataSource = items;
                this.sessionsDropDownList1.DataTextField = "SessionName";
                this.sessionsDropDownList1.DataValueField = "SessionCode";
                this.sessionsDropDownList1.DataBind();

                this.sessionsDropDownList1.SelectedIndex = 0;
            }

            private void LoadAllCoursesToDDL()
            {
                this.coursesDropDownList1.Items.Clear();

                List<Course> items = Course.GetCourses();

                this.coursesDropDownList1.DataSource = items;
                this.coursesDropDownList1.DataTextField = "CourseName";
                this.coursesDropDownList1.DataValueField = "CourseCode";
                this.coursesDropDownList1.DataBind();
            }

            protected void btnBack_Click(object sender, EventArgs e)
            {
                Server.Transfer("~/Teacher/TeacherControlPanel.aspx?username=" + username);
            }

            protected void btnSave_Click(object sender, EventArgs e)
            {
                Ice_Web_Portal.BO.Teacher teacher = Ice_Web_Portal.BO.Teacher.GetTeacherByUsername(username);

                if (teacher != null)
                {
                    Notice not = new Notice();
                    not.HangingDate = DateTime.Now;
                    not.TeacherCode = teacher.TeacherCode;
                    not.TargetSessionCode = this.sessionsDropDownList1.SelectedValue;
                    not.CourseCode = this.coursesDropDownList1.SelectedValue;
                    not.NoticeSubject = this.mailSubjectTextBox.Text;
                    not.NoticeContent = this.mailContentTextBox.Text;
                    not.ExpiryDate = DateTime.Now.AddMonths(1);
                    not.IsExpired = false;

                    bool success = Notice.Save(not);

                    if (success)
                    {
                        errorMessageLabel.Text = "Saved";
                    }
                    else
                    {
                        errorMessageLabel.Text = "Save failed";
                    }
                }
                else
                {
                    //labErrorMessage.Text = "No teacher found";
                }
            }
        }

Answer 1

很奇怪。你的代码似乎完全没问题。我尝试使用以下代码重建您的页面：

public partial class DropdownTests : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            LoadAllCoursesToDDL();
        }
    }

    private List<Course> GetDummyCourses()
    {
        return new List<Course>{
            new Course{ CourseCode = "A12", CourseName = "Introduction to Programming"},
            new Course{ CourseCode = "B112", CourseName = "Theory of Computing"}
        };
    }

    private void LoadAllCoursesToDDL()
    {
        this.coursesDropDownList1.Items.Clear();

        List<Course> items = GetDummyCourses();

        this.coursesDropDownList1.DataSource = items;
        this.coursesDropDownList1.DataTextField = "CourseName";
        this.coursesDropDownList1.DataValueField = "CourseCode";
        this.coursesDropDownList1.DataBind();
    }

    protected void btnBack_Click(object sender, EventArgs e)
    {
        //Server.Transfer("~/Teacher/TeacherControlPanel.aspx?username=" + username);
    }

    protected void btnSave_Click(object sender, EventArgs e)
    {
        string code = coursesDropDownList1.SelectedItem.Value;
        string code2 = coursesDropDownList1.SelectedValue;
    }
}

public class Course
{
    public string CourseCode { get; set; }
    public string CourseName { get; set; }
}

它工作得非常好。如果是，请尝试直接使用coursesDropdownList1.SelectedItem.Value代替“SelectedValue”。无所谓，但只是试一试。

作为尝试，我还会创建一个新页面，在那里复制aspx代码并为其创建代码隐藏。在我看来，必须有更多的东西导致它失败。您是否在运行时动态创建并向UI添加一些字段？一些自定义服务器控件等??您的问题可能是您丢失了ViewState。

Answer 2

我遇到了同样的问题，但是enableViewState = true是唯一能让它发挥作用的解决方案。很奇怪。

asp.net下拉列表没有选择任何值

2 个答案: