public event AppointmentAddingEventHandler AppointmentAdding

Public Event AppointmentAdding As AppointmentAddingEventHandler

AppointmentAdding += (d, e) => { 
    foreach(var apt in e.Appointments) { //each appointment which is about to be added
        bool res = Validate(apt); //is validated by a custom method 
        if(!res) //if the validation fails 
            e.CanceledEditAppointments.Add(apt); //this appointment is not added to the scheduler
    } 
}

AddHandler AppointmentAdding, Sub(d, e)
    For Each apt In e.Appointments 'each appointment which is about to be added
        Dim res As Boolean = Validate(apt) 'is validated by a custom method
        If Not res Then 'if the validation fails
            e.CanceledEditAppointments.Add(apt) 'this appointment is not added to the scheduler
        End If
    Next apt
End Sub

<dxsch:SchedulerControl.TimeRegionItems>
    <dxsch:TimeRegionItem
            Type ="Pattern" 
            Start="1/1/2019 13:00:00" End="1/1/2019 14:00:00" 
            RecurrenceInfo="{dxsch:RecurrenceDaily Start='1/1/2019 13:00:00', ByDay=WorkDays}" 
            BrushName="{x:Static dxsch:DefaultBrushNames.TimeRegion3Hatch}"/>
</dxsch:SchedulerControl.TimeRegionItems>

bool ProcessAppointments(IReadOnlyList<AppointmentItem> appts, IList<AppointmentItem> itemsToCancel) {
    var toCancel = new List<AppointmentItem>();
    foreach (var item in appts){
        var range = new DateTimeRange(item.Start, item.End);
        var startLunchRange = new DateTimeRange(item.Start.Date.AddHours(13), item.Start.Date.AddHours(14));
        var endLunchRange = new DateTimeRange(item.End.Date.AddHours(13), item.End.Date.AddHours(14));
        if (range.Intersect(startLunchRange).Duration.Ticks != 0 
            || range.Intersect(endLunchRange).Duration.Ticks != 0
            || range.Duration.Hours > 23)
            toCancel.Add(item);
    }

    if (toCancel.Count > 0) {
        var Cancel = new UICommand() { Caption = "Cancel", IsDefault = true };
        var CancelConflicts = new UICommand() { Caption = "Cancel Conflicts" };
        var Ignore = new UICommand() { Caption = "Ignore", IsCancel = true };
        var result = this.ShowDialog(new List<UICommand>() { Cancel, CancelConflicts, Ignore },
            "Warning",
            "WarningUserControl",
            "The following appointment(-s) intersects the lunch time:\n\n"
            + string.Join("\n", toCancel.Select(c => c.Subject)) +
            "\n\nClick 'Cancel' to discard all changes.\nClick 'Cancel Conflicts' to cancel changes only in these appointment(-s).");

        if (result == Cancel)
            return false;

        if (result == CancelConflicts)
            foreach (var item in toCancel)
                itemsToCancel.Add(item);
    }
    return true;
}

Private Function ProcessAppointments(ByVal appts As IReadOnlyList(Of AppointmentItem), ByVal itemsToCancel As IList(Of AppointmentItem)) As Boolean
    Dim toCancel = New List(Of AppointmentItem)()
    For Each item In appts
        Dim range = New DateTimeRange(item.Start, item.End)
        Dim startLunchRange = New DateTimeRange(item.Start.Date.AddHours(13), item.Start.Date.AddHours(14))
        Dim endLunchRange = New DateTimeRange(item.End.Date.AddHours(13), item.End.Date.AddHours(14))
        If range.Intersect(startLunchRange).Duration.Ticks <> 0 OrElse range.Intersect(endLunchRange).Duration.Ticks <> 0 OrElse range.Duration.Hours > 23 Then
            toCancel.Add(item)
        End If
    Next item

    If toCancel.Count > 0 Then
        Dim Cancel = New UICommand() With {
            .Caption = "Cancel",
            .IsDefault = True
        }
        Dim CancelConflicts = New UICommand() With {.Caption = "Cancel Conflicts"}
        Dim Ignore = New UICommand() With {
            .Caption = "Ignore",
            .IsCancel = True
        }
        Dim result = Me.ShowDialog(New List(Of UICommand)() From {Cancel, CancelConflicts, Ignore}, "Warning", "WarningUserControl", "The following appointment(-s) intersects the lunch time:" & vbLf & vbLf & String.Join(vbLf, toCancel.Select(Function(c) c.Subject)) & vbLf & vbLf & "Click 'Cancel' to discard all changes." & vbLf & "Click 'Cancel Conflicts' to cancel changes only in these appointment(-s).")

        If result Is Cancel Then
            Return False
        End If

        If result Is CancelConflicts Then
            For Each item In toCancel
                itemsToCancel.Add(item)
            Next item
        End If
    End If
    Return True
End Function

private void Scheduler_AppointmentAdding(object sender, AppointmentAddingEventArgs e) {
    e.Cancel = !ProcessAppointments(e.Appointments, e.CanceledAppointments);
}

private void Scheduler_AppointmentEditing(object sender, AppointmentEditingEventArgs e) {
    e.Cancel = !ProcessAppointments(e.EditAppointments, e.CanceledEditAppointments);
}

Private Sub Scheduler_AppointmentAdding(ByVal sender As Object, ByVal e As AppointmentAddingEventArgs)
    e.Cancel = Not ProcessAppointments(e.Appointments, e.CanceledAppointments)
End Sub

Private Sub Scheduler_AppointmentEditing(ByVal sender As Object, ByVal e As AppointmentEditingEventArgs)
    e.Cancel = Not ProcessAppointments(e.EditAppointments, e.CanceledEditAppointments)
End Sub