Question: I'm wondering if it is possible to use the OpenCV framework to recognise a building? 
For example, if I store an image of a building, is it possible to use OpenCV to detect this building through the iPhone camera?

Thanks!

Answer: 
You need to use **Ajax Control Tool kit's Cascaded Drop-down**

Here is the like for it:

<http://www.asp.net/AjaxLibrary/AjaxControlToolkitSampleSite/Walkthrough/CCDWithDB.aspx>

and here is the demo:

<http://www.asp.net/AjaxLibrary/AjaxControlToolkitSampleSite/CascadingDropDown/CascadingDropDown.aspx>

here is a tutorial link:

<http://www.dotnetfox.com/articles/ajax-cascading-dropdown-example-with-database-in-Asp-Net-1078.aspx>

Hope this material helps.
May be you don't have set **AutoPostBack** property to **true**. Try below code sample : 

**ASPX:**

```
<asp:DropDownList runat="server" ID="parentDropDown" OnSelectedIndexChanged="parentDropDown_SelectedIndexChanged" AutoPostBack="true">
    <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>
<asp:DropDownList runat="server" ID="child1">
</asp:DropDownList>
<asp:DropDownList runat="server" ID="child2">
</asp:DropDownList>

```

**Code Behind:**

```
protected void parentDropDown_SelectedIndexChanged(object sender, EventArgs e)
{
    string uom_Name = parentDropDown.SelectedValue;
    child1.Items.Add(uom_Name);
    child2.Items.Add(uom_Name);
}

```

**If you want to remove existing item from child before you add then:**

```
protected void parentDropDown_SelectedIndexChanged(object sender, EventArgs e)
{
    string uom_Name = parentDropDown.SelectedValue;

    child1.Items.Clear();
    child2.Items.Clear();

    child1.Items.Add(uom_Name);
    child2.Items.Add(uom_Name);
}

```