[Feature] Manually adding errors to ObservableValidator

[sherman89]

Thank you for this library first of all, really enjoying it so far! ❤️

Is it possible to add AddError and ClearErrors methods (or something similar) to ObservableValidator for custom validation? In my case it's not possible to use validation attributes since my validation involves calling an injected service that does complicated validation logic.

Forgive me if there's already a way to do this, but I just couldn't figure it out 😕

Describe the problem this feature would solve

It would allow for more complex validation scenarios where a validation attribute is not enough. For example:

[MaxLength(25, ErrorMessage = "Name is too long!")]
public string Name
{
    get => _name;
    set
    {
        ValidateName(value);
        SetProperty(ref _name, value, true);
    }
}

private void ValidateName(string name)
{
    ClearErrors(nameof(Name));

    bool isValid = _myService.ComplexValidation(name);
    if (!isValid)
    {
        AddError(nameof(Name), "This name is not allowed!");
    }
}

Describe the solution

A way to manually add/clear errors to the error collection manually. It would help in cases where it's not possible or elegant to use validation attributes.

Describe alternatives you've considered

I tried creating a custom attribute and using the ValidationContext somehow to access my service, but decided in the end that it's way too ugly and hacky, and not a very reusable solution. I've also considered simply not using the toolkits implementation of INotifyDataErrorInfo, but it would be really nice if I didn't have to do that.

Additional context & Screenshots

None.

[ghost]

Hello, 'sherman89! Thanks for submitting a new feature request. I've automatically added a vote 👍 reaction to help get things started. Other community members can vote to help us prioritize this feature in the future!

[michael-hawker]

@Sergio0694 you just added some helpers for this in the latest PR, eh? I think that was from #3742, #3729, and #3665, eh?

[Sergio0694]

Hey @sherman89, thank you for trying it out, really happy to hear you're enjoying it so far! 😄

I believe there's an easy way to achieve what you're asking for without making modifications to the API surface, in particular I think it would be best to avoid adding API to let consumers manually change the internal state of the validator like that, since the validation APIs are actually meant to be used through the Validator class and in a centralized way, and they're particularly flexible to help out exactly with this. This would also make the code easier to maintain, as you'd always have your validation logic in the same place, without being able to introduce bugs when externally adding errors to a given entity.

What you're asking can actually be done already in two different ways, here's one with the [CustomValidation] attribute:

public class ValidationWithServiceModel : ObservableValidator
{
    private readonly IFancyService service;

    public ValidationWithServiceModel(IFancyService service)
    {
        this.service = service;
    }

    private string name;

    [MaxLength(25, ErrorMessage = "The name is too long")]
    [CustomValidation(typeof(ValidationWithServiceModel), nameof(ValidateName))]
    public string Name
    {
        get => this.name;
        set => SetProperty(ref this.name, value, true);
    }

    public static ValidationResult ValidateName(string name, ValidationContext context)
    {
        bool isValid = ((ValidationWithServiceModel)context.ObjectInstance).service.Validate(name);

        if (isValid)
        {
            return ValidationResult.Success;
        }

        return new ValidationResult("The name contains invalid characters");
    }
}

You can see here the ValidateName can implement the custom logic you mentioned, and it's simply retrieving the injected service through the field it's stored in from the current instance of the viewmodel. This has the advantage of having all the validation being executed at the same time when the property is set, not having to worry about updating the state from the custom validation method (which only needs to worry about actually doing the validation), and it also lets you keep the nice one-liner for the property set, as you now only need to call SetProperty and let it do all the work behind the scenes 🚀

I've added some unit tests in fc39f6f to showcase and test this scenario, let me know if that helps and if this solution works for you! Alternatively you could also pass a custom validation context with an IServiceProvider instance, but that'd be more complex to setup in some cases. This should be pretty easy to wire up instead. Let me know what you think! 😊

[sherman89]

@Sergio0694 Thank you Sergio that's exactly what I needed! Just tested it and works great! I had no idea CustomValidation even existed! 😮

Thank you very much for the quick response as well, I posted the issue and went to sleep, didn't expect to get an answer so quick 😄

Will close this issue now :) Best regards!