public ToolValidator(IToolboxService toolboxService) { RuleFor(x => x.VTFabrikat).NotNull(); RuleFor(x => x.LiggerIvtk) .NotEmpty() .Custom((toolBoxId, context) => { if (toolBoxId != null) { var toolbox = toolboxService.GetToolbox(toolBoxId.Value); if (toolbox.Result != null) { context.AddFailure("You must specify an existing toolbox"); } } }); }