Telerik RadAjax controls fully support server and client-side validation.

This example demonstrates how to achieve the desired functionality in a straightforward and simple way with the RadAjaxPanel control. You can also use the RadAjaxManager for the same purpose -- in order to use the server-side validation you would have to add the panels with the validator controls in the UpdatedControls collection for the Register Button:

C# code:

private void RegisterButton_Click(object sender, System.EventArgs e)
{

	...

	if (RadioButtonList1.SelectedIndex == 1) // Update the 
	panels with validators only if server-side validation is performed.

	{

	// RadAjaxManager1.AjaxSettings[0] 
	specifies the settings for Ajax requests initiated by the Register Button 
	control
	

	AjaxUpdatedControl updatedControl1 = new AjaxUpdatedControl();

	updatedControl1.ControlID = Panel1.ClientID;

	 
	RadAjaxManager1.AjaxSettings[0].UpdatedControls.Add(updatedControl1);
	AjaxUpdatedControl updatedControl2 = new AjaxUpdatedControl();

	updatedControl2.ControlID = Panel2.ClientID;

	 
	RadAjaxManager1.AjaxSettings[0].UpdatedControls.Add(updatedControl2);
 AjaxUpdatedControl updatedControl3 = new AjaxUpdatedControl();

	updatedControl3.ControlID = Panel3.ClientID;

	 
	RadAjaxManager1.AjaxSettings[0].UpdatedControls.Add(updatedControl3);

	}

}

Private Sub RegisterButton_Click (sender As Object, e As System.EventArgs)

	...

	If RadioButtonList1.SelectedIndex = 1 Then ' Update the panels 
	with validators only if server-side validation is performed.

	' RadAjaxManager1.AjaxSettings(0) specifies the 
	settings for Ajax requests initiated by the Register Button control
	

	Dim updatedControl1 As New AjaxUpdatedControl()

	updatedControl1.ControlID = Panel1.ClientID

	RadAjaxManager1.AjaxSettings(0).UpdatedControls.Add
	(updatedControl1)

	

	Dim updatedControl2 As New AjaxUpdatedControl()

	updatedControl2.ControlID = Panel2.ClientID

	RadAjaxManager1.AjaxSettings(0).UpdatedControls.Add
	(updatedControl2)

	

	Dim updatedControl3 As New AjaxUpdatedControl()

	updatedControl3.ControlID = Panel3.ClientID

	RadAjaxManager1.AjaxSettings(0).UpdatedControls.Add
	(updatedControl3)

	End If

End Sub 'RegisterButton_Click