无法将下拉列表设为空 [英] Cannot make empty a dropdownlist

问题描述

你好，
我有2个下拉列表:

1- ddlCity:显示数据库中的所有城市
2-ddlBranch:显示每个城市的所有分支.

因此，一旦我选择了一个城市，就应该选择相关的分支机构.

我在C#源代码中有一个方法可以清除包含2个下拉列表的表单.因此，一旦我单击清除"按钮，则2下拉列表应该为空:

1-ddlcity显示:"--Select--"
2-ddlBranch显示:"----"

现在的问题是，当我单击清除"按钮时，会出现错误消息:"

Hello,
i have 2 dropdown lists:

1- ddlCity: that display all cities in the database
2-ddlBranch:display all branches per city.

So once i select a city, related branches should be selected.

and i have a method in C# source code that clear my form which contains the 2 dropdownlists. So once i click on the "Clear" button, the 2 dropdown list should be empty:

1-ddlcity display: "--Select--"
2-ddlBranch display: "----"

Now the problem is that When i click on the button "Clear" an error message occurs: "

'ddlBranch' has a SelectedValue which is invalid because it does not exist in the list of items.
Parameter name: value

请我需要您的帮助，并在此先感谢您.

天冬氨酸净代码:

Please I need your help and thank you in advance.

Asp. Net Code:

 <asp:DropDownList ID="ddlCity" runat="server" Width="170px" 

        DataSourceID="tblCity" DataTextField="city" DataValueField="cityID" AutoPostBack="true" AppendDataBoundItems="true"

        onselectedindexchanged="ddlCity_SelectedIndexChanged">
        <asp:ListItem Text="--Select--" Value=""></asp:ListItem>

 <asp:SqlDataSource ID="tblCity" runat="server" ConnectionString="<%$ ConnectionStrings:SmartBookingEngineConn %>" SelectCommand="SELECT [cityID],[city] FROM [tblCity]"></asp:SqlDataSource>
 
<asp:DropDownList ID="ddlBranch" runat="server" Width="170px" AutoPostBack="true" 

        onselectedindexchanged="ddlBranch_SelectedIndexChanged"  >
         <asp:ListItem Text="----" Value="" Selected="True"></asp:ListItem>
    </asp:DropDownList>

<telerik:RadAjaxManagerProxy ID="rp"  runat="server">              
</telerik:RadAjaxManagerProxy>

C#代码:

C# code:

  protected void Page_Load(object sender, EventArgs e)
  {
 if (!IsPostBack)
                {
                                       
                    // dropdown branches per city load
                   
                    ddlCity.DataBind();
                    if (ddlCity.SelectedValue !="")
                    {
                    dtbranch = null;
                    string selectedIDCity = ddlCity.SelectedValue;
                    string mysqlSelectBranch = "select Branch,CityID,BranchID      from viewBranchPerCity where CityID=" + selectedIDCity;
                    dtbranch = db.sqlServer.SelectRecord(mysqlSelectBranch);
                    ddlBranch.DataSource = dtbranch;
                    ddlBranch.DataValueField = "BranchID";
                    ddlBranch.DataTextField = "Branch";
                    }
                    ddlBranch.DataBind();

                }
rp.AjaxSettings.AddAjaxSetting(ddlCity, ddlBranch, RadAjaxLoadingPanel1);

  }

protected void btnClear_Click(object sender, EventArgs e)
        {
                      clearForm();
        }

protected void clearForm()
       {
           ddlCity.Enabled = true;
           ddlBranch.Enabled = true;

           ddlCity.SelectedValue = string.Empty;
           ddlBranch.SelectedValue = string.Empty;
       }
protected void ddlCity_SelectedIndexChanged(object sender, EventArgs e)
        {
            dtbranch = null;
            string selectedIDCity = ddlCity.SelectedValue;
            string mysqlSelectBranch = "select Branch,CityID,BranchID from viewBranchPerCity where CityID=" + selectedIDCity;
            dtbranch = db.sqlServer.SelectRecord(mysqlSelectBranch);
            ddlBranch.DataSource = dtbranch;
            ddlBranch.DataValueField = "BranchID";
            ddlBranch.DataTextField = "Branch";
            ddlBranch.DataBind();
        }

推荐答案

ConnectionStrings:SmartBookingEngineConn％>" SelectCommand =" < > < asp:DropDownList ID =" runat 服务器" 宽度 170像素" AutoPostBack true" span> onselectedindexchanged =" ddlBranch_SelectedIndexChanged" > < asp:ListItem 文本 =" 值 =" 已选择 =" True" < /asp:ListItem > < /asp:DropDownList > < telerik:RadAjaxManagerProxy ID =" =" 服务器" < /telerik:RadAjaxManagerProxy >

ConnectionStrings:SmartBookingEngineConn %>" SelectCommand="SELECT [cityID],[city] FROM [tblCity]"></asp:SqlDataSource> <asp:DropDownList ID="ddlBranch" runat="server" Width="170px" AutoPostBack="true" onselectedindexchanged="ddlBranch_SelectedIndexChanged" > <asp:ListItem Text="----" Value="" Selected="True"></asp:ListItem> </asp:DropDownList> <telerik:RadAjaxManagerProxy ID="rp" runat="server"> </telerik:RadAjaxManagerProxy>

C#代码:

C# code:

  protected void Page_Load(object sender, EventArgs e)
  {
 if (!IsPostBack)
                {
                                       
                    // dropdown branches per city load
                   
                    ddlCity.DataBind();
                    if (ddlCity.SelectedValue !="")
                    {
                    dtbranch = null;
                    string selectedIDCity = ddlCity.SelectedValue;
                    string mysqlSelectBranch = "select Branch,CityID,BranchID      from viewBranchPerCity where CityID=" + selectedIDCity;
                    dtbranch = db.sqlServer.SelectRecord(mysqlSelectBranch);
                    ddlBranch.DataSource = dtbranch;
                    ddlBranch.DataValueField = "BranchID";
                    ddlBranch.DataTextField = "Branch";
                    }
                    ddlBranch.DataBind();

                }
rp.AjaxSettings.AddAjaxSetting(ddlCity, ddlBranch, RadAjaxLoadingPanel1);

  }

protected void btnClear_Click(object sender, EventArgs e)
        {
                      clearForm();
        }

protected void clearForm()
       {
           ddlCity.Enabled = true;
           ddlBranch.Enabled = true;

           ddlCity.SelectedValue = string.Empty;
           ddlBranch.SelectedValue = string.Empty;
       }
protected void ddlCity_SelectedIndexChanged(object sender, EventArgs e)
        {
            dtbranch = null;
            string selectedIDCity = ddlCity.SelectedValue;
            string mysqlSelectBranch = "select Branch,CityID,BranchID from viewBranchPerCity where CityID=" + selectedIDCity;
            dtbranch = db.sqlServer.SelectRecord(mysqlSelectBranch);
            ddlBranch.DataSource = dtbranch;
            ddlBranch.DataValueField = "BranchID";
            ddlBranch.DataTextField = "Branch";
            ddlBranch.DataBind();
        }

请按以下方式更新您的代码.

Hi,

Please update your code as below.

protected void Page_Load(object sender, EventArgs e)
{
   if (!IsPostBack)
   {
      //ddlCity.DataBind();
      if (ddlCity.SelectedValue !="")
      {
         dtbranch = null;
         string selectedIDCity = ddlCity.SelectedValue;
         string mysqlSelectBranch = "select Branch,CityID,BranchID      from viewBranchPerCity where CityID=" + selectedIDCity;
         dtbranch = db.sqlServer.SelectRecord(mysqlSelectBranch);
         ddlBranch.DataSource = dtbranch;
         ddlBranch.DataValueField = "BranchID";
         ddlBranch.DataTextField = "Branch";
         ddlBranch.DataBind();  
      }
      //ddlBranch.DataBind(); //Move this line in if Block
   }
   rp.AjaxSettings.AddAjaxSetting(ddlCity, ddlBranch, RadAjaxLoadingPanel1); 
}
 
protected void btnClear_Click(object sender, EventArgs e)
{
   clearForm();
}
 
protected void clearForm()
{
   //ddlCity.Enabled = true;
   //ddlBranch.Enabled = true; 
   ddlCity.SelectedIndex= 0;
   ddlBranch.Items.Clear();
   ddlBranch.DataBind();
   ListItem li = new ListItem("----","");
   ddlBranch.Items.Add(li);
   ddlBranch.SelectedIndex= 0;
   ddlBranch.DataBind();
}
protected void ddlCity_SelectedIndexChanged(object sender, EventArgs e)
{
   dtbranch = null;
   string selectedIDCity = ddlCity.SelectedValue;
   string mysqlSelectBranch = "select Branch,CityID,BranchID from viewBranchPerCity where CityID=" + selectedIDCity;
   dtbranch = db.sqlServer.SelectRecord(mysqlSelectBranch);
   ddlBranch.DataSource = dtbranch;
   ddlBranch.DataValueField = "BranchID";
   ddlBranch.DataTextField = "Branch";
   ddlBranch.DataBind();
}

希望对您有帮助.

Hope this will help you.

您好，您可以使用以下内容清除下拉列表.

Hi, You can use the following for clear dropdown list.

protected void clearForm()
       {
           ddlCity.Items.Clear();
           ddlBranch.Items.Clear();
           ddlCity.SelectedValue.Selected = false;
           ddlBranch.SelectedValue.Selected = false;
       }

谢谢.

Thank You.

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