C# DropDownList AppendDataBoundItems（第一项为空且无重复项）

DropDownList AppendDataBoundItems (first item to be blank and no duplicates)

提问by zohair

I have a DropDownListinside an UpdatePanelthat 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 DropDownListends up having data that is incorrect, or repeated data.

我有一个DropDownList在UpdatePanel从SqlDataSource. 它有一个参数，它是另一个控件。我有时需要多次回发，但是每次更新面板刷新时，都会将项目添加到DropDownList. 所以DropDownList最终会得到不正确的数据或重复的数据。

I have the AppendDataBoundItemsproperty set to truebecause I need the first item to be blank.

我将AppendDataBoundItems属性设置为true因为我需要第一个项目为空。

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

我怎样才能克服这个问题？还有另一种方法可以让第一项空白吗？

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

（这DropDownList是在 ASP.NET 2.0 Web 应用程序中，代码隐藏在 C# 中）

采纳答案by Keltex

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

而不是使用AppendDataboundItems='true'（这将导致您正在谈论的问题），响应DataBound事件DropDownList，然后将您的“空白”项目添加到列表顶部。

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

Then in your code behind:

然后在你后面的代码中：

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

回答by gius

You probably bind that DropDownList in the code behind. So you should not do it after postback again:

您可能在后面的代码中绑定了 DropDownList。所以你不应该在再次回发后这样做：

// probably in Page_Load method
if (!Page.IsPostBack)
{
    // do data binding here
};

回答by bagher

<asp:DropDownList ID="DropDownList1" AppendDataBoundItems="true" runat="server"
  DataSourceID="SqlDataSource1" DataTextField="state" DataValueField="state">
    <asp:ListItem Text="(Select a State)" Value="" />
</asp:DropDownList>
<asp:SqlDataSource ID="SqlDataSource1" runat="server"
  ConnectionString="<%$ ConnectionStrings:pubsConnectionString %>"
  SelectCommand="SELECT DISTINCT [state] FROM [authors]">
</asp:SqlDataSource>

回答by Miguel

Here's an Idea.

这是一个想法。

There's a property in the drop down list called AutoPostBackset it to true and then in the code behind you put all the binding method inside the if(!Page.IsPostBack). That worked for me.

下拉列表中有一个名为AutoPostBack将其设置为 true的属性，然后在后面的代码中将所有绑定方法放在if(!Page.IsPostBack). 那对我有用。

regards.

问候。

回答by Terminator

Here is an idea, we can use 2 events: DataBoundand DataBinding:

这是一个想法，我们可以使用 2 个事件：DataBound和DataBinding：

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

protected void MyListDataBinding(object sender, EventArgs e)
{
  MyList.Items.Items.Clear();
}

回答by Tony L.

There are good answers here but I felt the need to include more information because there are multiple options that work and we need to decide which to use.

这里有很好的答案，但我觉得有必要包含更多信息，因为有多种可行的选择，我们需要决定使用哪个。

First, we should understand AppendDataBoundItems. If AppendDataBoundItems = "true", ListItemsare added to the DropDownListwithout clearing out the old ones. Otherwise, the DropDownListis cleared about before the next DataBind. MSDN AppendDataBoundItems doc

首先，我们应该了解AppendDataBoundItems。如果AppendDataBoundItems = "true",ListItems被添加到DropDownList而不清除旧的。否则，DropDownList将在下一个 之前清除DataBind。MSDN AppendDataBoundItems 文档

There are basically 2 options covered by most of the answers:

大多数答案基本上涵盖了两个选项：

1. Define a blank option in html and add the ListItems from the database to the DropDownList only once.

1.在html中定义一个空白选项，并且只将数据库中的ListItems添加到DropDownList中一次。

Notice 3 things here:

请注意这里的 3 件事：

• Blank ListItemis defined in html
• AppendDataBoundItems="true"
• DataBindis NOT called on postbacks or when the DropDownListitem count is > 1

• 空白ListItem在 html 中定义
• AppendDataBoundItems="true"
• DataBind不会在回发或DropDownList项目计数 > 1时调用

Source:

来源：

<asp:DropDownList ID="MyList" runat="server" AppendDataBoundItems="true" DataValueField="Id" DataTextField="Name" >
    <asp:ListItem Text="- Select One -" Value="" />
</asp:DropDownList>

Code behind:

后面的代码：

protected void Page_Load(object sender, System.EventArgs e)
{
    if (MyList.Items.Count <= 1 ) {
        MyList.DataSource = MyDataSource;
        MyList.DataBind();
    }
}

Note: I like the logic of checking the count vs checking IsPostBack. Though PostBacks are often the cause of duplicate databinding, it is possible to cause it other ways. Checking the item count is basically just checking to see if it's already been loaded.

注意：我喜欢检查计数与检查的逻辑IsPostBack。尽管回发通常是导致重复数据绑定的原因，但也可能以其他方式导致它。检查项目计数基本上只是检查它是否已经加载。

OR (option to use IsPostBackinstead)

或（IsPostBack替代使用的选项）

protected void Page_Load(object sender, System.EventArgs e)
{
    if (!IsPostBack) {
        MyList.DataSource = MyDataSource;
        MyList.DataBind();
    }
}

2. Clear and reload the DropDownList on each page refresh.

2. 在每次页面刷新时清除并重新加载 DropDownList。

Notice 3 differences from the first option:

请注意与第一个选项的 3 个不同之处：

• AppendDataBoundItems="false"(if it is not defined then falseis it's default value)
• Blank ListItemis is added in code behind. We can't define it in html because with AppendDataBoundItems="false", it would be cleared out.
• DataBindis called on every Page_Load

• AppendDataBoundItems="false"（如果未定义，false则为默认值）
• 空白ListItem是在后面的代码中添加的。我们不能在 html 中定义它，因为使用AppendDataBoundItems="false"，它会被清除。
• DataBind被称为每个 Page_Load

Source:

来源：

<asp:DropDownList ID="MyList" runat="server" DataValueField="Id"  DataTextField="Name" 
    OnDataBound="MyList_DataBound" >
</asp:DropDownList>

Code behind:

后面的代码：

protected void Page_Load(object sender, System.EventArgs e)
{
    MyList.DataSource = MyDataSource;
    MyList.DataBind();
}

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