Error 19 The type or namespace name 'PDialogEventHandler' could not be found (are you missing a using directive or an assembly reference?

public delegate void AddEventHandlerToSPDialogEvent(object sender, PDialogEventHandler e);
public class SPDialogEventHandler : EventArgs
{
  public int dialogResult { get; set; } // 0 or 1
  public string ReturnValues { get; set; } 
  public SPDialogEventHandler(int result, string list)
  {
    ReturnValues = list;
    dialogResult = result;
  }
}