Thank you for writing.
By default, WinForms controls are validated on Leave and this behavior is normal.
You can put your validation logic at the SelectedIndexChanged event, for example:
radDropDownList1.DataSource = dataTabel1;
//this will fire SelectedIndexChanged event with Selectedindex = 0
//subscribe for event after the databinging
radDropDownList1_SelectedIndexChanged(objectsender, Telerik.WinControls.UI.Data.PositionChangedEventArgs e)
I hope this helps. Don't hesitate to contact us if you have other questions.
All the best,
the Telerik team
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