Dropdownlist Datasource and adding extra item from

问题:

I have a DropDownList that is associated with a DataSource in the aspx page. I need to add one more item when the page is loaded.

My Code:

<asp:LabelDropDownList ID="ddlVisualTemplate" runat="server" LabelText="Visual Template:"      DataSourceID="VisualTemplateDataSource" DataTextField="Name" DataValueField="Id" AutoPostBack="true" OnSelectedIndexChanged="ddlVisualTemplate_SelectedIndexChanged"/>                         

<asp:EntityDataSource ID="VisualTemplateDataSource" runat="server" 
     ConnectionString="name=Entities" 
     DefaultContainerName="Entities" EnableFlattening="False" 
     EntitySetName="tbEmailVisualTemplates">

And I am trying to an extra item to it:

    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            ddlVisualTemplate.Items.Add(new ListItem("None", string.Empty));
        }
    }

If I debug the code, it goes through it. But When the page is displayed dropdown doesn't contain "None".

回答1:

It's most probably because you're adding the item before the DataBind(). If you want to add an item with

ddlVisualTemplate.Items.Add()

then you have to do it after the dropdown is being bound.

If you look at http://msdn.microsoft.com/en-us/library/ms178472.aspx then DataBind is being done in PreRenderComplete. So you have to add the element in some event that occurs after PreRenderComplete. Or you could do it on the ddlVisualTemplate.DataBound event.

回答2:

Probably too late for the original poster, but maybe useful for other users:

You can add the value "None", "Choose value", etc. in the designer (or in the code) and prevent DataBind from overwriting it, by setting AppendDataBoundItems="true". This will make DataBind append rather than clear.

For an example see Scott Guthrie's post ListControl.AppendDataBoundItems Property in ASP.NET 2.0.

回答3:

You could easily fix this by setting the datasource prgrammatically:

ddlVisualTemplate.DataSource = VisualTemplateDataSource;
ddlVisualTemplate.DataBind();

ddlVisualTemplate.Items.Add(new ListItem("None", string.Empty));

BTW, these datasource controls are a wrong thing in asp.net in my opinion. I don't like the idea of defining the data source and giving the control over db connections to the aspx page. For a better way of doing this just google about session per request pattern, separation of concerns and n-tier apps.

Update: instead of "VisualTemplateDataSource" you could call directly the data. I don't know EF, but it might be like this: "DataContext.tbEmailVisualTemplates". You have to set your datacontext. And then you can get rid of the datasource control.

回答4:

Use this to add an item in the bound dropdown list at 0 index

ddlTicketType.Items.Insert(0, new ListItem("All", "0"));

回答5:

This fixed it for me but it puts the value in the end.

protected void ddlTest_PreRender(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        ddlTest.Items.Add(new ListItem("All", string.Empty));
    }
}