In the ASP.NET version you can do it like this:
1. Set EventResizeHandling to "CallBack".
2. In the OnEventResize handler, perform your checks. If there is an overlap, use UpdateType.None and notify the user using UpdateWithMessage(). Do not update the database. Do not reload events.
protected void DayPilotScheduler1_EventResize(object sender, DayPilot.Web.Ui.Events.EventResizeEventArgs e)
{
bool allowed = .... ; // perform the check
if (allowed) {
DbResizeEvent(e.Id, e.NewStart, e.NewEnd); // update the db
DayPilotScheduler1.DataSource = LoadEvents(); // reload events
DayPilotScheduler1.UpdateWithMessage("Event resized");
}
else {
DayPilotScheduler1.UpdateWithMessage("Action denied, there is an overlap.");
DayPilotScheduler1.Update(CallBackUpdateType.None);
}
}