ASP.Net GridView Implementing Cascading DropDownList on Edit Mode

Note that this is just an example to make this exercise work. You can always replace this with your DataTable, Entity objects, Custom classes and so on that holds your actual source of data.

The ProductMake class holds two main properties. This is where we store the list of makes of the product. The ProductModel class is where we store all the models for each make. The ProductOrder class is where we store the list of orders the user has chosen. Now let's go ahead and provide this class with some sample data. The following is the code blocks.
The GetUserProductOrder() fetches the list of orders. We will use this as our DataSource in the GridView later. The GetProductMakes() method gets all the available makes that, in this case, we just added 3 main items to. The GetProductModel() method gets all the available models for each make. The GetProductModelByMake() method gets the specific model item and its details are based on the ProductMakeID. This method uses the LINQ syntax to query the DataSource based on the parameter that was passed to it.

The Implementation

Now it looks as if we already have some sample source of data to work with. Now let's go ahead and do the highlight of this exercise (which is the implementation of the cascading dropdownlist). Here are the code blocks below.
The gvProducts_RowDataBound event is where we bind the DropDownList with the data from our DataSource. First we check the RowType to ensure that we are only manipulating the rows of type DataRow. Please note that a GridView is composed of several row types such as Header, DataRow, EmptyDataRow, Footer, Pager and Separator. The next line in that block is the critical part of the code and that is to determine the Edit state.

Accessing the controls from within <EditItemTemplate> is a bit tricky, especially if you are not really familiar with how the stuff works within a GridView. Equating the RowState to DataControlState.Edit isn't really accurate and you might get an exception when doing so. The RowState property is a bitwise combination. Therefore, the RowState could indicate that you are in the Edit state and the Alternate state. Hence, you cannot do a simple equality check when you are in Edit mode. Instead you must do something like this.
We use the bitwise “&” operator to determine if the GridView is in Edit mode and check the result if its greater than zero. For details about the Bitwise operator see: Bitwise Operators in C#.

Once we've manage to determine the edit-state then we can now begin accessing the controls using the FindControl() method and bind it with the corresponding DataSources. If you notice, I've set the SelectedValue for the ddlMake and ddlModel DropDownLists so that by the time the user clicks edit, the DropDownList will be pre-selected with the previous item the user has chosen.
The ddlProductMake_SelectedIndexChanged event is where we actually do the cascading feature by populating the second DropDownList based from the value selected from the first DropDownList. But before that we need to cast the sender of the object triggering the event to determine which DropDownList is triggered within the GridView row. We then cast the NamingContainer of the sender to a type of GridViewRow to determine the actual row the user is editing.

The ddlProductModel_SelectedIndexChanged event is where we update the Price value based on the selected Model from the second DropDownList. It basically uses LINQ syntax to query the DataSource and to get the Price based on the selected value of the ddlModel.

Binding the GridView

Finally let's call the BindGrid() method to populate the GridView. Here's the code block below.
The Output

Here are some screenshots from running this code in the page: