ASP:DropDownList in ItemTemplate: Why is SelectedValue attribute allowed?

问题

This piece of code

<asp:DropDownList runat="server" ID="testdropdown" SelectedValue="2">
    <asp:ListItem Text="1" Value="1"></asp:ListItem>
    <asp:ListItem Text="2" Value="2"></asp:ListItem>
    <asp:ListItem Text="3" Value="3"></asp:ListItem>
</asp:DropDownList>

yields this error:

The 'SelectedValue' property cannot be set declaratively.

Yet, this is a legal and commonly used edit template for databound GridViews. The SelectedValue attribute certainly appears to be declaratively set here.

<EditItemTemplate>
    <asp:DropDownList runat="server" 
        ID="GenreDropDownList"
        DataSourceID="GenreDataSource" 
        DataValueField="GenreId"
        DataTextField="Name"
        SelectedValue='<%# Bind("Genre.GenreId") %>'>
    </asp:DropDownList>
</EditItemTemplate>

The question is: what is the difference between the cases when you are allowed to set it declaratively and those in which you are not? The error message implies that it's never allowed.

回答1:

It means you cannot set it through the designer.

The correct way is:

<asp:DropDownList runat="server" ID="testdropdown">
    <asp:ListItem Text="1" Value="1"></asp:ListItem>
    <asp:ListItem Text="2" Value="2" Selected></asp:ListItem>
    <asp:ListItem Text="3" Value="3"></asp:ListItem>
</asp:DropDownList>

The reason the bound method works is because the value isn't selected in design mode but at runtime after the control is bound to a datasource

The DropDownList.SelectedValue method is meant to be applied at runtime hence the error about not being able to set it 'decoratively'

回答2:

in markup use SelectedValue='<%# "32" %> syntax .(note the following example ):

 <asp:DropDownList  ID="ddlField" SelectedValue='<%# "32" %>' 
   runat="server"      DataTextField="Name" DataValueField="ID"  >
  </asp:DropDownList>

or in code-behind just after DataBinding .(example):

ddlField.DataSource = Fields.SelectAll();
        ddlField.DataBind();           
        ddlField.SelectedValue = "32";

来源：https://stackoverflow.com/questions/580487/aspdropdownlist-in-itemtemplate-why-is-selectedvalue-attribute-allowed