I have created a fully runnable sample project following the requirements you have set - create a RadComboBox dynamically in a UserControl. I have created it in the Page_Load() event handler and added it to a div placeholder in the markup as follows:
And the Page_Load() event handler:
sender, EventArgs e)
RadComboBox combo =
combo.DataSource = GetComboSource();
Therefore, I used the UserControl in a WebForm and included a RadButton which only function is to trigger a PostBack.
After triggering a PostBack, the lastly selected value in the RadComboBox is saved.
Please download and run the sample to ensure that it works as expected. If you are still facing this issue, please modify the provided example and attach it to an official Support Ticket. That would allow us to investigate your exact scenario locally and help you more efficiently.
Once we have a solution, we can share it here for convenience and better visibility from the community.
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