索引超出范围异常ASP C＃

Question

I'm building a products page for an e commence site. I have two drop down lists.

ddlProducts is connected to a SQL data source from which it retrieves the product information to populate it.

ddlCategory contains a list of the product categories.

What I'm trying to do is to allow the user to select a product category from ddlCategory and then ddlProducts will populate with only the items from the database table that match that specific category.

ddlProducts then uses a method GetSelectedProduct() to retrieve the needed fields, for the product selected, from the database to display the information on the rest of the web page.

Here's the problem. When I run the page, ddlProducts is populated with the products specific to the default selection from ddlCategory (in the case, the category is "Living Room"). Now as long as I do not try to change the selection in ddlCategory, then everything is fine. I can use ddlProducts and everything does what it is supposed to do.

However if I try to change the selection on ddlCategory, then GetSelectedProduct() throws an Index out of range exception at line

DataRowView row = productsTable[0];

I can't figure out why.

Can anybody offer any insight here?? Thanks.

Drop Down List code

<asp:DropDownList ID="ddlCategory" runat="server" AutoPostBack="true">
        <asp:ListItem Value="Living Room">Living Room</asp:ListItem>
        <asp:ListItem Value="Dining Room">Dining Room</asp:ListItem>
        <asp:ListItem Value="Bedroom">Bedroom</asp:ListItem>
    </asp:DropDownList>
    &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
    <asp:DropDownList ID="ddlProducts" runat="server"
        AutoPostBack="True" DataSourceID="SqlDataSource1"
        DataTextField="productName" DataValueField ="productID">
    </asp:DropDownList>
    <asp:SqlDataSource ID="SqlDataSource1" runat="server" 
        ConnectionString="<%$ ConnectionStrings:OneStopConnectionString %>" 
        ProviderName="<%$ ConnectionStrings:OneStopConnectionString.ProviderName %>" 
        SelectCommand="SELECT [productID], [productName], [productRetail], [productImage] FROM [product] WHERE ([productRoom] = ?)">
        <SelectParameters>
            <asp:ControlParameter ControlID="ddlCategory" Name="productRoom" PropertyName="SelectedValue" Type="String" />
        </SelectParameters>
    </asp:SqlDataSource>

Back code for the product page

public partial class _Default : System.Web.UI.Page
{
private Product selectedProduct;

protected void Page_Load(object sender, EventArgs e)
{
    //bind drop-down list on first load 
    //get and show product on every load 
    if (!IsPostBack) ddlProducts.DataBind();
    selectedProduct = this.GetSelectedProduct();
    lblName.Text = selectedProduct.Name;
    lblUnitPrice.Text = selectedProduct.UnitPrice.ToString() + " each";
    imgProduct.ImageUrl = "Images / Products /" + selectedProduct.ImageFile;
}

private Product GetSelectedProduct()
{
    //get row from SqlDataSource based on value in dropdown list 
    DataView productsTable = (DataView)
    SqlDataSource1.Select(DataSourceSelectArguments.Empty);
    productsTable.RowFilter = string.Format("productID = '{0}'", ddlProducts.SelectedValue);
    DataRowView row = productsTable[0];  <------line that is throwing the exception

    //create a new product object and load with data from row 
    Product p = new Product();
    p.ProductID = row["ProductID"].ToString();
    p.Name = row["ProductName"].ToString();
    p.UnitPrice = row["productRetail"].ToString();
    p.ImageFile = row["productImage"].ToString();
    return p;
}

Answer 1

You need to databind ddlProducts before calling GetSelectedProduct, otherwise it isn't initialized correctly and you can't reference the SelectedValue.

Since ddlProducts.SelectedValue is returning an unexpected value, your RowFilter doesn't find any rows, so when you go and reference the first row, it then throws an error.

Referencing the selectedValues of DropdownLists can be very tricky because they are usually initialized very late in the lifecycle of the request. Even setting the selectedValue often times can't be read back out during the same request because on set it stored the value in a temporary location for later use during the actual databinding. Unless forced, the databinding doesn't happen until right before render, which is way after Load_Page happens. Hope that clarifies the why.