Setting DropDown Options Dynamically Based on OnSelectedIndexChanged Value in Custom Web Part

I personally think you may be trying to load the data in the wrong way and should use the 'category' OnSelectedIndexChanged event, such that when the Category is changed the 'parts' are loaded. If you look at the CountrySelector source you will see it uses the uniSelectorCountry_OnSelectionChanged() method to change the states.

The issue I believe you are experiencing is that the 'categories' are always being reloaded and hence the SelectValue is getting reset, thus you are always getting a 'categoryId' of 0.

Populate only first (category) drop down in the OnContentLoaded method. Use SelectedIndexChanged event of the first drop down to populate second one. Make sure AutoPostBack property of the drop down is set to true.

Agree with both Suneel and Roman. The selected value or index of the dropdown will not be available to be retrieved or set until after viewstate is loaded. So if you attempt to do it OnContentLoaded() it will fail every time. In the OnContentLoaded(), assign the selected event changed event to the dropdown and in that event, perform your actions needed. Do not check for postback in your OnContentLoaded() as this will not give you what you need.

Thanks for the feedback. I really appreciate your time. I originally tried this approach, but I had difficulty getting the part.SelectedValue on postback when I went to save the submitted data. I figured that since the part options weren't populated in OnContentLoaded the system wasn't able to determine the part.SelectedValue in my save data postback.

Brandon, Is there a difference in how Kentico handles the onSelectionChanged event if I set it on the category drop down control in my form versus setting it programmatically in the onContentLoaded event?

I'll revisit this approach and be sure I didn't have something else in my code preventing the part.SelectedValue from working.

Thanks again.

There should be no difference in how Kentico handles the OnSelectionChanged event when set in Code-Behind or via the form. However, the Kentico Web Part lifecycle is slightly different to the ASP.Net Page Lifecycle.

I believe that your issue is being cause by the fact the OnContentLoaded event is firing before the OnSelectionChanged event and hence you are actually recreating the 'catgories' dropdown which in turn removes the selected value.

I haven't tested this but the below should work:

public override void OnContentLoaded()
{
    base.OnContentLoaded();

    if (!IsPostBack)
    {
        InitializeCategories();
    }

    // THIS IS WHERE I NEED TO GET THE SELECTED VALUE OF CATEGORY
    // I'VE TRIED THIS
    if (IsPostBack) 
    {
        int categoryId = Int32.Parse(category.SelectedValue); // This doesn't seem to work
        DataSet partsResult = new DataQuery("Custom.Parts.selectall")
        .Columns("partId", "partName")
        .Where("categoryId", QueryOperator.Equals, categoryId)
        .Execute();
        DataTable parts = partsResult.Tables[0];
        if (parts.Rows.Count > 0)
        {
            part.DataSource = parts;
            part.DataTextField = "partName";
            part.DataValueField = "partId";
            part.DataBind();
        }
     }   
 }    

 private void InitializeCategories()
 {        
    DataSet categoryResult = new DataQuery("Custom.CategoriesTable.selectall")
        .Columns("CategoryId", "CategoryName")
        .Execute();
    DataTable categories = categoryResult.Tables[0];
    if (this.categories.Rows.Count > 0)
    {
        category.DataSource = countries;
        category.DataTextField = "CategoryName";
        category.DataValueField = "CategoryId";
        category.DataBind();
    }
}