How to get/inject services in custom ValidationAttributes

[lfrancke]

Hello everyone,

we're on .NET Core 3.1.5 and this is a Blazor Server application.

We have a ValidationAttribute and need access to an external Service to validate the objects.

ValidationAttribute has the IsValid method:

protected override ValidationResult IsValid(object value, ValidationContext validationContext)

ValidationContext has a GetService method which delegates to an instance of ServiceProvider.
Unfortunately, the service provider field is never initialized and so we cannot retrieve any Services.

This was raised (and fixed) back in Mvc: aspnet/Mvc#6346
But our Validator is called via one of these two:

• https://github.com/dotnet/aspnetcore/blob/master/src/Components/Forms/src/EditContextDataAnnotationsExtensions.cs#L47
• https://github.com/dotnet/aspnetcore/blob/master/src/Components/Forms/src/EditContextDataAnnotationsExtensions.cs#L75

And later down the stack the service provider is also never set.
I hesitated to open a bug (but can do so) but this seems wrong to me (or at least should be documented).

Any Google search eventually ends up at this Blog post but as I just mentioned this doesn't work.

So our question is: What's the correct way of injecting services into a ValidationAttribute or more general what is the proper way to validate a field of a model that requires a call to an external service?

Also: I'm using C#/.NET for the first time in 15 or so years, please be gentle ;-)

Thank you for your help!

[lfrancke]

For anyone stumbling across this later: A colleague asked the same question on StackOverflow and received an answer here: https://stackoverflow.com/questions/62559817/how-to-get-inject-services-in-custom-validationattributes

Short version is: Subscribe to the OnFieldChanged event of the Form's EditContext instead.

[lfrancke]

I raised an issue about this: #23380
Maybe we can clarify there if it's intended or not.