Databinding Drop down list based on another dropdown list

[Afraz Ali]

Hi guys,
I am getting this strange error and I am really pulling my hair at the moment. My scenario is simple. I have a drop down list for Categories. When the user Selects and category, I populate another drop down list of subcategory based on the selected category.
Here is the code

private void ddlCategory_SelectedIndexChanged(object sender, Telerik.WinControls.UI.Data.PositionChangedEventArgs e)
       {
            this.PopulateSubCategories();
        }

 private void PopulateSubCategories()
        {
            DataTable dtSubCategories = new SubCategory().GetSubCategoryByCategoryID(Convert.ToInt64(ddlCategory.SelectedItem.Value));
            DataRow dr = dtSubCategories.NewRow();
            dr["ID"] = 0;
            dr["Name"] = "Any";
            dr["CategoryID"] = 0;
            dr["isACtive"] = true;
            dtSubCategories.Rows.InsertAt(dr, 0);

            this.ddlSubCategory.DataSource = dtSubCategories;
            this.ddlSubCategory.DisplayMember = "Name";
            this.ddlSubCategory.ValueMember = "ID";
            this.ddlSubCategory.SelectedIndex = 0;
        }

The problem is that ddlCategory.SelectedItem.Value statement throws exception, Unable to cast object of type 'System.Data.DataRowView' to type 'System.IConvertible'. If i skip this function first time and change the index to some other than it works fine. Why is it not returning the SelectedValue on first bind
Thanks
Afraz Ali

[Afraz Ali]

ok here is the update. Turns out that drop down List's "ValueMember" property is set to null when its data source is set for first time. So when i added this check it has started working.

 if (!String.IsNullOrEmpty(ddlCategory.ValueMember))

But I am still confused as to why it shows this behavior and would like someone to explain to me whats going on under the hood.
Thanks
Afraz

[Peter]

Hi Afraz Ali,

Thank you for the writing.

Assigning a DataSource to RadDropDownList for first time fires the SelectedIndexChanged event, because the Currency manager changes the current position and this is a normal behavior - at this moment you do not have a ValueMember set yet. So you should set first ValueMember and DisplayMember to avoid this scenario.

I hope this helps.

Greetings,
Peter
the Telerik team