“gradeid”有一个无效 SelectedValue，因为它不在项目列表中 参数名: value

[luojiankeji]

protected void Button2_Click(object sender, EventArgs e)
{
this.etime.Text = Convert.ToDateTime(this.btime.Text).AddMinutes(45).ToString();
}
protected void band_nj()
{
SqlConnection conn = Sqlcon.openDatabase();
string cmdtext = "select * from T_grade ";
SqlDataAdapter da = new SqlDataAdapter(cmdtext, conn);
DataSet ds = new DataSet();
da.Fill(ds);
this.gradeid.DataSource = ds;
this.gradeid.DataTextField = "gradename";
this.gradeid.DataValueField = "id";
this.gradeid.DataBind();//
ListItem item = new ListItem();
item.Text = "---请选择年级---";
item.Value = "-1";
this.gradeid.Items.Insert(0, item);
conn.Close();
conn.Dispose();


}

[风之影子]

设断点，单步调试一下。

[暖枫无敌]

“gradeid”有一个无效 SelectedValue，因为它不在项目列表中

出错的地方并不在你贴出来的这些代码中，请贴你设置选中项的那段代码先。

[luojiankeji]

aspx文件中控件代码

                    当班投票的班级和年级：
                </td>
                <td>
                    <asp:UpdatePanel ID="UpdatePanel1" runat="server">
                        <ContentTemplate>
                            <asp:DropDownList ID="gradeid" runat="server" 
                                OnSelectedIndexChanged="Dradeid_SelectedIndexChanged" AutoPostBack="True">
                            </asp:DropDownList>
                            <asp:DropDownList ID="classid" runat="server">
                            </asp:DropDownList>
                        </ContentTemplate>
                    </asp:UpdatePanel>
                </td>

[luojiankeji]

CS文件代码

    protected void Button2_Click(object sender, EventArgs e)
    {
        this.etime.Text = Convert.ToDateTime(this.btime.Text).AddMinutes(45).ToString();
    }
    protected void band_nj()
    {
        SqlConnection conn = Sqlcon.openDatabase();
        string cmdtext = "select * from T_grade ";
        SqlDataAdapter da = new SqlDataAdapter(cmdtext, conn);
        DataSet ds = new DataSet();
        da.Fill(ds);
        this.gradeid.DataSource = ds;
        this.gradeid.DataTextField = "gradename";
        this.gradeid.DataValueField = "id";
        this.gradeid.DataBind();//
        ListItem item = new ListItem();
        item.Text = "---请选择年级---";
        item.Value = "-1";
        this.gradeid.Items.Insert(0, item); 
        conn.Close();
        conn.Dispose();


    }
    protected void band_bj1(string gradeid)
    {
        SqlConnection conn = Sqlcon.openDatabase();
        string cmdtext = "select * from T_class  where gradeid='" + gradeid + "'";
        SqlDataAdapter da = new SqlDataAdapter(cmdtext, conn);
        DataSet ds = new DataSet();
        da.Fill(ds);
        this.classid.DataSource = ds;
        this.classid.DataTextField = "classname";
        this.classid.DataValueField = "id";
        this.classid.DataBind();//
        ListItem item = new ListItem();
        item.Text = "---请选择年级---";
        item.Value = "-1";
        this.gradeid.Items.Insert(0, item);
        conn.Close();
        conn.Dispose();
    }

    protected void Dradeid_SelectedIndexChanged(object sender, EventArgs e)
    {
        string bigid = this.gradeid.SelectedValue.Trim();
        band_bj1(bigid);
    }
}

[暖枫无敌]

代码看起来没什么问题，问题是你的数据有问题

自己检查下：
是不是某个T_grade 下并没有任何的T_class

[孟子E章]

你应该有其他代码设置
this.gradeid.SelectedValue="xx"了吧？这样的语句是错误的