// All and any transaction should be enclosed in a 'using'
// block or guarded within a try-catch-finally blocks
// to guarantee that a transaction does not out-live its scope.
using (Transaction transaction = new Transaction(doc))
{
   // Before we modify a document, we must start a transaction
   transaction.Start("Creating room");

   // obtain the current options and add failures preprocessor to it.
   // Note: preprocessor implements IFailuresProcessor and will receive 
   // a notification about failures eventually posted during the transaction.
   FailureHandlingOptions options = transaction.GetFailureHandlingOptions();
   options.SetFailuresPreprocessor(new RoomWarningSwallower());
   transaction.SetFailureHandlingOptions(options);

   // Modify the document and commit the transaction.
   if (null != doc.Create.NewRoom(level, UV.Zero))
   {
      // Now, before the the transaction is committed, if there happened to 
      // be any failure, the preprocessor would receive them first and would
      // have the option to analyze them, and ignore or re-qualify them.
      transaction.Commit();
   }
   else  // could not create the room; no need to commit the transaction
   {
      transaction.RollBack();
   }
}

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

    Dim collector As New FilteredElementCollector(doc)
    Dim elementCollection As ICollection(Of Element) = collector.OfClass(GetType(Level)).ToElements()
    Dim level As Level = elementCollection.Cast(Of Level)().ElementAt(0)


    ' All and any transaction should be enclosed in a 'using'
    ' block or guarded within a try-catch-finally blocks
    ' to guarantee that a transaction does not out-live its scope.
    Using transaction As New Transaction(doc)
        ' Before we modify a document, we must start a transaction
        transaction.Start("Creating room")

        ' obtain the current options and add failures preprocessor to it.
        ' Note: preprocessor implements IFailuresProcessor and will receive 
        ' a notification about failures eventually posted during the transaction.
        Dim options As FailureHandlingOptions = transaction.GetFailureHandlingOptions()
        options.SetFailuresPreprocessor(New RoomWarningSwallower())
        transaction.SetFailureHandlingOptions(options)

        ' Modify the document and commit the transaction.
        If doc.Create.NewRoom(level, UV.Zero) IsNot Nothing Then
            ' Now, before the the transaction is committed, if there happened to 
            ' be any failure, the preprocessor would receive them first and would
            ' have the option to analyze them, and ignore or re-qualify them.
            transaction.Commit()
        Else
            ' could not create the room; no need to commit the transaction
            transaction.RollBack()
        End If
    End Using


    Return Autodesk.Revit.UI.Result.Succeeded
End Function