--استفسار بسيط جزاكم الله خيرا---

<asp:DropDownList ID="DropDownList1" runat="server"
DataSourceID="SqlDataSource1" DataTextField="CategoryName"
DataValueField="CategoryID" AutoPostBack="True">
</asp:DropDownList>
<asp:SqlDataSource ID="SqlDataSource1" runat="server"
ConnectionString="<%$ ConnectionStrings:NorthwindConnectionString %>"
SelectCommand="SELECT [CategoryID], [CategoryName] FROM [Categories]">
</asp:SqlDataSource>
<br />
<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="False"
DataKeyNames="ProductID" DataSourceID="SqlDataSource2">
<Columns>
<asp:BoundField DataField="ProductID" HeaderText="ProductID"
InsertVisible="False" ReadOnly="True" SortExpression="ProductID" />
<asp:BoundField DataField="ProductName" HeaderText="ProductName"
SortExpression="ProductName" />
<asp:BoundField DataField="UnitPrice" HeaderText="UnitPrice"
SortExpression="UnitPrice" />
</Columns>
</asp:GridView>
<asp:SqlDataSource ID="SqlDataSource2" runat="server"
ConnectionString="<%$ ConnectionStrings:NorthwindConnectionString %>"
SelectCommand="SELECT [ProductID], [ProductName], [UnitPrice] FROM [Products] WHERE ([CategoryID] = @CategoryID)">
<SelectParameters>
<asp:ControlParameter ControlID="DropDownList1" Name="CategoryID"
PropertyName="SelectedValue" Type="Int32" />
</SelectParameters>
</asp:SqlDataSource>