Aspnetcore: ValidationAttribute.IsValid receives a ValidationContext with an empty ServiceProvider

[lfrancke]

Describe the bug

NOTE: We initially posted this as a question on StackOverflow and Github Discussions.
While we did receive an answer we're still certain that this is a bug either in implementation or in documentation.

When the IsValid method of a ValidationAttribute is called its ValidationContext has the service provider set to null so all calls to GetService return nothing.

I believe this was fixed for MVC: https://github.com/aspnet/Mvc/issues/6346
Every google search eventually ends up at this blog post: https://andrewlock.net/injecting-services-into-validationattributes-in-asp-net-core/
He illustrates that this technique does work there. Now with Blazor this seems to fail.

I believe this should either be documented or fixed, it surprised us and it's not super obvious what the alternatives are.

To Reproduce

https://github.com/lfrancke/aspnet-validation-minimal

https://github.com/lfrancke/aspnet-validation-minimal/blob/master/Pages/Index.razor#L18-L20
service is null here.

Further technical details

• ASP.NET Core version 3.1.4
• IDE Rider 2020.1 (and 2020.2 EAP 4)

.NET Core SDK (reflecting any global.json):
 Version:   3.1.300
 Commit:    b2475c1295

Runtime Environment:
 OS Name:     Mac OS X
 OS Version:  10.15
 OS Platform: Darwin
 RID:         osx.10.15-x64
 Base Path:   /Users/lars/.dotnet/sdk/3.1.300/

Host (useful for support):
  Version: 3.1.4
  Commit:  0c2e69caa6

.NET Core SDKs installed:
  3.1.300 [/Users/lars/.dotnet/sdk]

.NET Core runtimes installed:
  Microsoft.AspNetCore.App 3.1.4 [/Users/lars/.dotnet/shared/Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 3.1.4 [/Users/lars/.dotnet/shared/Microsoft.NETCore.App]

To install additional .NET Core runtimes or SDKs:
  https://aka.ms/dotnet-download

[javiercn]

@lfrancke thanks for contacting us.

Injecting services for validation is not a feature Blazor offers.

@pranavkm can correct me.

[lfrancke]

Thanks for the quick reply.

That's fine. In that case it'd be great if that could be documented somewhere (preferably in the ValidationAttribute docs).

[mrpmorris]

My validation library will let you use FluentValidation and dependency injection

https://github.com/mrpmorris/blazor-validation

[lfrancke]

I'm wondering: Shouldn't this behavior be documented in the code documentation somehow?

As far as I can tell you closed the issue without any actions.

Injecting services for validation is not a feature Blazor offers.

This is not obvious anywhere right now. Especially not on the ValidationAttribute class itself.

[javiercn]

@lfrancke there is already a validation issue that you filed. We don't plan to make any code change for this. Or do you have a specific suggestion?

[lfrancke]

Sorry, excuse my ignorance.

I meant this https://docs.microsoft.com/en-us/dotnet/api/system.componentmodel.dataannotations.validationattribute.isvalid?view=netcore-3.1#System_ComponentModel_DataAnnotations_ValidationAttribute_IsValid_System_Object_System_ComponentModel_DataAnnotations_ValidationContext_

Which I assume is generated from the documentation in the code itself.

Not sure what the .NET term is (the equivalent to Javadoc in Java) for in code documentation.

[javiercn]

@lfrancke we can't add docs there. That lives within the dotnet runtime itself and is applicable to many other contexts that are not Blazor. I'm ok adding a note on the docs about this in the Blazor docs, but I don't think it is a good idea to put it on the class itself since it only applies to Blazor.

Hope that helps.

[lfrancke]

Understood. Thank you for explaining.