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.