DropDownList AppendDataBoundItems(第一项为空白且无重复项)

问题描述

I have a DropDownList inside an UpdatePanel that is populated on postback from a SqlDataSource. It has a parameter which is another control. I sometimes need multiple postbacks, but what happens is that each time the update panel refreshes, items are added to the DropDownList. So the DropDownList ends up having data that is incorrect, or repeated data.

I have the AppendDataBoundItems property set to true because I need the first item to be blank.

How can I overcome this problem? Is there another way to have a blank first item?

(This DropDownList is in an ASP.NET 2.0 web app, and codebehind is in C#)

Instead of using AppendDataboundItems='true' (which will cause the problem you are talking about), respond to the DataBound event for the DropDownList and then add your "blank" item to the top of the list.

<asp:DropDownList runat="server" ID="MyList"
  ondatabound="MyListDataBound"></asp:DropDownList>

Then in your code behind:

protected void MyListDataBound(object sender, EventArgs e)
{
    MyList.Items.Insert(0, new ListItem("- Select -", ""));
}

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