Customize validation message in Bot framework C#

Question

How to customize validation messages in Bot framework form flow? Below is the sample code I'm working where if the user type any text other than the options provided I need to give them the choice list back saying that the option chosen is not correct.

    [Prompt("Please choose # category... {||}", ChoiceStyle = ChoiceStyleOptions.Buttons)]
    public string Category;

    [Prompt("Please choose  # sub category... {||}", ChoiceStyle = ChoiceStyleOptions.Buttons)]
    public string Subcategory;

    [Prompt("We need some more details to create the request, provide me your **contact number**...")]
    [Pattern(Validations.Phone)]
    public string ContactNumber;

    [Prompt("Please provide **Attachement** if you don't have attachemnt please enter ? {||}")]
    public ChoiceOptions? Attachment;

Currently, if the option is not on the list it simply throws a message like

' Userinput is not a contact/attachment/category option'

Answer 1

Jobin, you want to look at the more advanced features of formFlow. You can create business logic around your fields by using field validation functions. This is a sample of what that looks like taken from MS Online docs. You can see the full sample and docs here I find that this method way of creating my form is much more flexible than using attributes.

public static IForm<SandwichOrder> BuildForm()
{
...
return new FormBuilder<SandwichOrder>()
    .Message("Welcome to the sandwich order bot!")
    .Field(nameof(Sandwich))
    .Field(nameof(Bread))
    .Field(nameof(Toppings),
        validate: async (state, value) =>
        {
            var values = ((List<object>)value).OfType<ToppingOptions>();
            var result = new ValidateResult { IsValid = true, Value = values };
            if (values != null && values.Contains(ToppingOptions.Everything))
            {
                result.Value = (from ToppingOptions topping in Enum.GetValues(typeof(ToppingOptions))
                                where topping != ToppingOptions.Everything && !values.Contains(topping)
                                select topping).ToList();
            }
            return result;
        })
    .Message("For sandwich toppings you have selected {Toppings}.")
    ...
    .Build();
}