ModelState.AddModelError in Blazor Form

[ryanelian]

Is your feature request related to a problem? Please describe.

I am trying to add custom validation error message AFTER the form has been submitted successfully for an existing field.

In Razor Pages I can do something like: ModelState.AddModelError("Form.Field", "Something happened!"); which allows the error message to be displayed when the page is returned. (Bonus Point doing it like this because it allows the field CSS to also be displayed as errored field)

I can't find a way to do this in Blazor.

Describe the solution you'd like

I need a way to do ModelState.AddModelError in Blazor. Is it possible to expose ModelState altogether in Blazor?

Additional context

We apparently cannot simply use custom validation annotation because the validation may be async. (aspnet/Mvc#7791 required for this behavior)

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Related to issue, might qualify as separate issue:

protected override ValidationResult IsValid(object value, ValidationContext validationContext)
{
    var service = (IExternalService) validationContext
                         .GetService(typeof(IExternalService));
    // use service
}

In Blazor, GetService returns NULL!

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EDIT 2: Solved the issue, although with a different approach:

https://github.com/ryanelian/FluentValidation.Blazor

This library allows writing validators registered to the .NET Core DI, which easily allow custom logic (even async) to be developed easily!

[RemiBou]

I implemented custom error message in this blog post https://remibou.github.io/Using-the-Blazor-form-validation/ (search for "Display validation error from the server").

[mkArtakMSFT]

Thanks for contacting us, @ryanelian .
Glad that you've managed to find a solution for this.