public class LogMessage

Project.UndoContext.BeginUndoStep("Logger");
try
{
    // How to add categories to the Logger!
    // Add two categories.
    if (!Logger.CategoryCaptions.ContainsKey("MyKey"))
    {
        Logger.CategoryCaptions.Add("MyKey", "My Category");
        Logger.CategoryCaptions.Add("AnotherKey", "Another Category");
    }

    // Print a message in different categories.
    Logger.AddMessage(new LogMessage("This is just a test message!", "MyKey"));
    Logger.AddMessage(new LogMessage("This is just a test message!", "AnotherKey"));
}
catch
{
    Project.UndoContext.CancelUndoStep(CancelUndoStepType.Rollback);
    throw;
}
finally
{
    Project.UndoContext.EndUndoStep();
}