Hi,
try with this,
<asp:dropdownlist id="DropDownList1" runat="server" width="200px" xmlns:asp="#unknown">
AppendDataBoundItems="true" DataSourceID="sdsdropdown" DataTextField="Name"
DataValueField="Id">
</asp:dropdownlist>
<asp:SqlDataSource ID="sdsdropdown" runat="server"
ConnectionString="<%$ ConnectionStrings:ConnectionStringname %>"
SelectCommand="SELECT [Id], [Name] FROM [table]">
</asp:SqlDataSource>
or
<asp:dropdownlist id="DropDownList1" runat="server" width="200px" xmlns:asp="#unknown">
DataSourceID="sdsdropdown" DataTextField="Name"
DataValueField="Id">
</asp:dropdownlist>
<asp:sqldatasource id="sdsdropdown" runat="server" connectionstring="<%$ ConnectionStrings:ConnectionStringname %>" xmlns:asp="#unknown">
SelectCommand="SELECT 0 as [Id] ,'--Select--' as [Name] union SELECT [Id], [Name] FROM [table]">
</asp:sqldatasource>