runtime: MetadataTypeAttribute doesn't influence DataAnnotations validation result

This is much needed as in case of scaff folding it just delete and recreates the class again, we would have to re-write everything again. please consider to implement it, need for real life project

I’ve just been checking this, and I don’t think this issue is Blazor-specific. I think it’s a more general point that System.ComponentModel.DataAnnotations.MetadataTypeAttribute doesn’t actually do anything to influence the DataAnnotations validation result.

To repro my findings, create a new .NET Core Console Application (so that’s neither Blazor nor WebAssembly), containing:

    [MetadataType(typeof(PersonMetadata))]
    public class Person
    {
        public string Name { get; set; }
    }

    public class PersonMetadata
    {
        [Required]
        public string Name { get; set; }
    }

    class Program
    {
        static void Main(string[] args)
        {
            // Some people say you should do this, but it doesn't seem to make any difference
            TypeDescriptor.AddProviderTransparent(
                new AssociatedMetadataTypeTypeDescriptionProvider(typeof(Person), typeof(PersonMetadata)),
                typeof(Person));

            var person = new Person();
            var results = new List<ValidationResult>();
            var isValid = Validator.TryValidateObject(person, new ValidationContext(person), results);
            Console.WriteLine($"isValid: {isValid}");
        }
    }

I would have expected this to say isValid: False but actually it says isValid: True. I can only get it to respect the [Required] attribute by putting it directly onto Person, not PersonMetadata. Also notice that this has nothing to do with partial classes (I’m not using them above).

@pranavkm Do we have any contacts who work on DataAnnotations? This seems like a weirdness that a lot of people have complained about all over StackOverflow etc. I can’t tell whether it used to work in the past (e.g. on .NET Framework) since some people say it does if you use TypeDescriptor.AddProviderTransparent, but it certainly doesn’t appear to work now.

So I have coded two working solutions.

Solution 1

• produces the same results as .Net 4.8
• requires TypeDescriptor.AddProviderTransparent() method.
• doesn’t use the MetadataType attribute.
• could potentially be made so that any declaration of the MetadataType attribute would auto register the TypeDescriptor.AddProviderTransparent() method negating the need for code.

Solution 2

• produces the same results as .Net 4.8
• uses the MetadataType attribute
• doesn’t use the TypeDescriptor.AddProviderTransparent() method.

Both solutions require adding a couple of methods to the System.ComponentModel.Annotations.Validator class. This resides in .Net Standard 2.1. I’m guessing any fixes would come out in a .NET 6 beta and not to any older versions?

So I’m hoping for some assistance here on which implementation to use. Like to hear everyone’s vote?

I created a small test project to investigate the DataAnnotations behaviour. Attached zip has two projects with identical code. A .Net Framework project called DN and a .Net Core project called Core. TestMetadataTypeAttribute.zip

Some findings: To get the Metadata type to work in .Net Framework 4.5+ you need to ensure two things

• When using Validator.TryValidateObject() you need to set the validateAllProperties parameter to True. Otherwise only the Required attribute gets checked.
• You need to ensure you add a provider transport as shown in previous posts

TypeDescriptor.AddProviderTransparent(
                new AssociatedMetadataTypeTypeDescriptionProvider(typeof(Person), typeof(PersonMetadata)),
                typeof(Person));

You don’t need to use

[MetadataType(typeof(PersonMetadata))]

As it makes no difference to .Net Framework 4.5+

The Core project doesn’t produce the expected results.

I went through the .Net Core 5 code, specifically through the System.ComponentModel.Annotations solution. I couldn’t find any direct references to MetadataTypeAttribute or AssociatedMetadataType in the context of the TryValidateObject code.

I will keep digging.

this is a must have feature, especially in projects with automatic class generations like Entity Framework, to allow adding additional validation or business rules.

Hope it will be implemented as soon as possible

Just wanted to add a +1 for this - while we can setup a task to mix-in our data annotations after scaffolding for now, I feel this is a regression from MVC.

Agree, It would be really nice if MetadataType worked. It would allow us to add data annotations for validation to Models generated using Scaffold-DbContext

The listed owners of the area-System.ComponentModel.DataAnnotations area are @lajones @ajcvickers.

Solution 2. I wrote functions that work but when I do a full build I get an IL Linker error which I can’t suppress and I’m not sure I should.

@Xaeco - can you try rebasing your change on the latest main? I merged #49901 last week, which resolved the existing ILLink warnings in that library. If you are still seeing warnings, can you send them to me, along with your change? I can assist in getting them resolved.

Thank you to all who have commented. It looks like solution 2 is preferred. I hope to get a pull request in today.

I’m going to have a go at fixing it

This is really an important issue. One of the main advantages of Blazor versus frameworks like Angular is the rapid development due to capability of reusing the ‘server code’ for validation or generation of models in the client. @mkArtakMSFT

See also (and upvote) https://github.com/dotnet/efcore/issues/14544

Still waiting on my pull request to be reviewed. No further progress.

Excellent. I will give it a go and let you know. Thank you> The listed owners of the area-System.ComponentModel.DataAnnotations area are @lajones @ajcvickers.

Solution 2. I wrote functions that work but when I do a full build I get an IL Linker error which I can’t suppress and I’m not sure I should.

@Xaeco - can you try rebasing your change on the latest main? I merged #49901 last week, which resolved the existing ILLink warnings in that library. If you are still seeing warnings, can you send them to me, along with your change? I can assist in getting them resolved.

Thanks for the effort guys. Solution 2 would be the neater solution. As mentioned by others this issue has a considerable impacts. Any Idea, when can we expect the fix?

news on this issue?

We’ve moved this issue to the Backlog milestone. This means that it is not going to happen for the coming release. We will reassess the backlog following the current release and consider this item at that time. However, keep in mind that there are many other high priority features with which it will be competing for resources.

Those “high-priority” features won’t matter if big corporates decline to use Blazor because its model-binding support sucks.

@ajcvickers any chance this fix could be back-ported to .NET 5?

If so what is the process?

Update. Option 1 has been approved. Not sure if it will land in a .NET 6 preview or the final release.

Hi Guys,

Any updates on this?

The listed owners of the area-System.ComponentModel.DataAnnotations area are @lajones @ajcvickers.

Solution 2. I wrote functions that work but when I do a full build I get an IL Linker error which I can’t suppress and I’m not sure I should.

@Xaeco - can you try rebasing your change on the latest main? I merged #49901 last week, which resolved the existing ILLink warnings in that library. If you are still seeing warnings, can you send them to me, along with your change? I can assist in getting them resolved.

I’ve had a PR in for the fix for 29 days now but haven’t had anyone look at it yet. Not sure if I’ve missed part of the process or everyone is really busy and this is too low priority. Any assistance would be appreciated. Thanks.

@SteveSandersonMS I have two working solutions for this issue.

Solution 1. Is to replace two functions with their DN4.8 equivalents. They use TypeDescriptor, which as far as I can tell, is being avoided.

Solution 2. I wrote functions that work but when I do a full build I get an IL Linker error which I can’t suppress and I’m not sure I should.

I’d appreciate talking to someone re the best way forward.

I prefer Solution 2 , Its the same as old silverlight WCF RIA services used.

This seems to be generally the problem with decorating any autogenerated class with attributes. I am experiencing a similar issue with trying to add XmlElement and XmlAttribute to classes generated from a proto file that need to be deserialized from an xml payload. The MetadataType attribute seems to have no effect whatsoever.

I also tried to use MetadataType on a simple class and it seems to be just completely ignored, so it’s not something to do with autogenerated files or protobuf, it just seems to not work at all even in the simplest scenario.

(I am using .net standard 2.1 and System.ComponentModel.Annotations nuGet version 5.0.0)

Tagging subscribers to this area: @ajcvickers See info in area-owners.md if you want to be subscribed.

public partial class PersonViewModel
{
    [Required(ErrorMessage = "First name is required.")]
    public string Fname { get; set; }
}

public partial class PersonViewModel
{
    public string Fname { get; set; }
}

[MetadataType(typeof(PersonViewModelMetaData))]
public partial class PersonViewModel
{
}

public class PersonViewModelMetaData
{
    (ErrorMessage = "First name is required.")]
    public string Fname { get; set; }
}

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 Version:   3.1.300-preview-015048
 Commit:    13f19b4682

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 OS Version:  10.0.18363
 OS Platform: Windows
 RID:         win10-x64
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