public Autodesk.Revit.UI.Result Execute(ExternalCommandData commandData,
    ref string message, ElementSet elements)
{
    try
    {
        Document doc = commandData.Application.ActiveUIDocument.Document;
        UIDocument uidoc = commandData.Application.ActiveUIDocument;
        // Delete selected elements

        ICollection<Autodesk.Revit.DB.ElementId> ids =
            doc.Delete(uidoc.Selection.GetElementIds());

        TaskDialog taskDialog = new TaskDialog("Revit"); 
        taskDialog.MainContent = 
           ("Click Yes to return Succeeded. Selected members will be deleted.\n" +
           "Click No to return Failed.  Selected members will not be deleted.\n" +
           "Click Cancel to return Cancelled.  Selected members will not be deleted.");
        TaskDialogCommonButtons buttons = TaskDialogCommonButtons.Yes | 
            TaskDialogCommonButtons.No | TaskDialogCommonButtons.Cancel;
        taskDialog.CommonButtons = buttons;
        TaskDialogResult taskDialogResult = taskDialog.Show();

        if (taskDialogResult == TaskDialogResult.Yes)
        {
            return Autodesk.Revit.UI.Result.Succeeded;
        }
        else if (taskDialogResult == TaskDialogResult.No)
        {
            ICollection<ElementId> selectedElementIds = uidoc.Selection.GetElementIds();
            foreach (ElementId id in selectedElementIds)
            {
                elements.Insert( doc.GetElement(id) );
            }
            message = "Failed to delete selection.";
            return Autodesk.Revit.UI.Result.Failed;
        }
        else
        {
            return Autodesk.Revit.UI.Result.Cancelled;
        }
    }
    catch
    {
        message = "Unexpected Exception thrown.";
        return Autodesk.Revit.UI.Result.Failed;
    }

}

Public Function Execute(commandData As ExternalCommandData, ByRef message As String, elements As ElementSet) As Autodesk.Revit.UI.Result Implements IExternalCommand.Execute
    Try
        Dim doc As Document = commandData.Application.ActiveUIDocument.Document
        Dim uidoc As UIDocument = commandData.Application.ActiveUIDocument
        ' Delete selected elements


        Dim ids As ICollection(Of Autodesk.Revit.DB.ElementId) = doc.Delete(uidoc.Selection.GetElementIds())

        Dim taskDialog As New TaskDialog("Revit")
        taskDialog.MainContent = ("Click Yes to return Succeeded. Selected members will be deleted." & vbLf & "Click No to return Failed.  Selected members will not be deleted." & vbLf & "Click Cancel to return Cancelled.  Selected members will not be deleted.")
        Dim buttons As TaskDialogCommonButtons = TaskDialogCommonButtons.Yes Or TaskDialogCommonButtons.No Or TaskDialogCommonButtons.Cancel
        taskDialog.CommonButtons = buttons
        Dim taskDialogResult__1 As TaskDialogResult = taskDialog.Show()

        If taskDialogResult__1 = TaskDialogResult.Yes Then
            Return Autodesk.Revit.UI.Result.Succeeded
        ElseIf taskDialogResult__1 = TaskDialogResult.No Then
            Dim selectedElementIds As ICollection(Of ElementId) = uidoc.Selection.GetElementIds()
            For Each id As ElementId In selectedElementIds
                elements.Insert(doc.GetElement(id))
            Next
            message = "Failed to delete selection."
            Return Autodesk.Revit.UI.Result.Failed
        Else
            Return Autodesk.Revit.UI.Result.Cancelled
        End If
    Catch
        message = "Unexpected Exception thrown."
        Return Autodesk.Revit.UI.Result.Failed
    End Try

End Function