public ValidationPageViewModel() { UserName = new ValidatableObject <string>(); UserName.AddRule(new StringNotEmptyRule("A username is required.")); UserName.AddRule(new StringLengthRule(3, 10, "Username must be 3-10 characters.")); Email = new ValidatableObject <string>(); Email.AddRule(new StringNotEmptyRule("Email is required.")); Email.AddRule(new EmailRule("Invalid Email.")); TermsAndCondition = new ValidatableObject <bool>(); TermsAndCondition.AddRule(new IsValueTrueRule("Please accept terms and condition")); _validationGroup = new ValidatableGroup(UserName, Email, TermsAndCondition); CheckNameCommand = new RelayCommand(CheckName); SubmitCommand = new AsyncCommand(SubmitAsync); }