public enum OptionSetting : System.Enum, System.IComparable, System.IConvertible, System.IFormattable, System.ISpanFormattable  

{
  //Default options can be set regardless of the value of the "companion"
  //path (to a project, folder, gdb, toolbox, etc.). The path value is ignored if
  //the option setting does not use it. This is the same behavior as on the Pro UI.
  ApplicationOptions.GeneralOptions.StartupOption = StartProjectMode.ShowStartPage;
  ApplicationOptions.GeneralOptions.HomeFolderOption = OptionSetting.UseDefault;
  ApplicationOptions.GeneralOptions.DefaultGeodatabaseOption = OptionSetting.UseDefault;
  ApplicationOptions.GeneralOptions.DefaultToolboxOption = OptionSetting.UseDefault;//set default option first

  //path values can (optionally) be set (back) to null if their 
  //"companion" option setting is the default option.
  if (ApplicationOptions.GeneralOptions.StartupOption != StartProjectMode.WithDefaultProject)
    ApplicationOptions.GeneralOptions.StartupProjectPath = null;
  if (ApplicationOptions.GeneralOptions.HomeFolderOption == OptionSetting.UseDefault)
    ApplicationOptions.GeneralOptions.CustomHomeFolder = null;
  if (ApplicationOptions.GeneralOptions.DefaultGeodatabaseOption == OptionSetting.UseDefault)
    ApplicationOptions.GeneralOptions.CustomDefaultGeodatabase = null;
  if (ApplicationOptions.GeneralOptions.DefaultToolboxOption == OptionSetting.UseDefault)
    ApplicationOptions.GeneralOptions.CustomDefaultToolbox = null;
}