c# - 下拉列表中的选定值无效

Question

我一直在开发一个项目，并面临一个错误，因为当我在其选择索引上填充其中一个文本框时，我有两个文本框和一个下拉列表，更改了下拉列表被填充，发生的情况是当我单击重置按钮来重置下拉文本框为空，下拉索引值为 0，其显示错误为

“下拉列表有一个无效的 SelectedIndex，因为它不存在于项目列表中。 参数名称:值“

<td class="style4">
            <asp:Label ID="lbl_bookname" runat="server" Text="Book Name" 
                ForeColor="Black"></asp:Label>
        </td>
        <asp:AutoCompleteExtender ID="AutoCompleteExtender1" runat="server"  TargetControlID="txt_bookname" MinimumPrefixLength="1" EnableCaching="true" CompletionSetCount="1" CompletionInterval="1000" ServiceMethod="getbookname">
        </asp:AutoCompleteExtender>
        <td class="style4">
        <asp:ScriptManager ID="ScriptManager1" runat="server">
                        </asp:ScriptManager>

            <asp:TextBox ID="txt_bookname" runat="server" ValidationGroup="a" OnTextChanged="editiondrpfill_TextChanged" AutoPostBack="true"></asp:TextBox>
             <asp:AutoCompleteExtender ID="AutoCompleteExtender2" runat="server" TargetControlID="txt_bookname" MinimumPrefixLength="1" EnableCaching="true"  CompletionSetCount="1" CompletionInterval="1000" ServiceMethod="getbookname">
        </asp:AutoCompleteExtender> 
        </td>
        <td class="style4">
            <asp:RequiredFieldValidator ID="RequiredFieldValidator3" runat="server" 
                ErrorMessage="*" ForeColor="Red" ControlToValidate="txt_bookname" 
                ValidationGroup="a"></asp:RequiredFieldValidator>
        </td>
        <td class="style4">
            <asp:Label ID="lbl_condition" runat="server" ForeColor="Black" Text="Condition"></asp:Label>
            </td>
        <td class="style4">
            <asp:TextBox ID="txt_condition" runat="server" BackColor="White" 
                ReadOnly="True" Width="120px"></asp:TextBox>
            <asp:RequiredFieldValidator ID="RequiredFieldValidator8" runat="server" 
                ControlToValidate="txt_condition" ErrorMessage="*" ForeColor="Red" 
                ValidationGroup="a"></asp:RequiredFieldValidator>
            </td>
        <td>
            &nbsp;</td>
    </tr>
    <tr>
        <td class="style4">
            <asp:Label ID="lbl_edition" runat="server" ForeColor="Black" Text="Edition"></asp:Label>
        </td>
        <td class="style4">
            <asp:DropDownList ID="drp_edition" runat="server" ValidationGroup="a" 
                Width="120px" onselectedindexchanged="drp_edition_SelectedIndexChanged" 
                AutoPostBack="true" TabIndex="0">
            </asp:DropDownList>
        </td>
        <td class="style4">
            <asp:RequiredFieldValidator ID="RequiredFieldValidator7" runat="server" 
                ControlToValidate="drp_edition" ErrorMessage="*" ForeColor="Red" 
                ValidationGroup="a"></asp:RequiredFieldValidator>
        </td>

用于填充下拉列表的CS代码

  protected void editiondrpfill_TextChanged(object sender, EventArgs e)
{
   sql = "select bt.booktwo_id,bt.edition,b.bookname from library_book b , library_booktwo bt where bookname ='" + txt_bookname.Text+ "' and bt.book_id=b.book_id";
   ds = obj.openDataset(sql, Session["SCHOOLCODE"].ToString());
   drp_edition.Items.Clear();
    ListItem li = new ListItem();
    li.Text = "Select the value";
     li.Value = "0";
    drp_edition.Items.Add(li);
    for (int i = 0; i < ds.Tables[0].Rows.Count; i++)
    {
        li = new ListItem();
         li.Text= ds.Tables[0].Rows[i]["edition"].ToString();
         li.Value = ds.Tables[0].Rows[i]["booktwo_id"].ToString();
        drp_edition.Items.Add(li);
    }
}

重置按钮事件的cs代码

  protected void btn_reset_Click(object sender, EventArgs e)
{
    txt_bookname.Text = "";
    txt_condition.Text = "";
    txt_member_id.Text = "";
    txt_nameofstudent.Text = "";
    txt_quantity.Text = "";
    drp_edition.SelectedIndex = 0;
    drp_isbn.SelectedIndex = 0;
}

Answer 1

在您的 aspx.cs 页面上做一件事，添加一个值为 0 的 ListItem 到下拉列表

<asp:DropDownList ID="drp_edition" runat="server" ValidationGroup="a" 
                Width="120px" onselectedindexchanged="drp_edition_SelectedIndexChanged" 
                AutoPostBack="true" TabIndex="0">
                <asp:ListItem Value="0" Text="Select" ></asp:ListItem>
            </asp:DropDownList>

它会解决你的问题。

希望它能起作用。