MetadataTypeAttribute doesn't influence DataAnnotations validation result

[RichardBr]

Update by @SteveSandersonMS: please see the comment #46678 (comment) for a description of the underlying issue/question.

Please ignore the parts of this thread that are specific to Blazor. That's just how the issue was originally reported, but it looks like the underlying concern is about DataAnnotations itself.

---

Describe the bug

When I use the MetadataType attribute (from System.ComponentModel.DataAnnotations) on the class for the purpose of validation I get the following error in the console window
WASM: System.TypeLoadException: Could not resolve type with token 01000027 from typeref (expected class 'System.ComponentModel.DataAnnotations.MetadataTypeAttribute' in assembly 'System.ComponentModel.Annotations, Version=4.3.1.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a')

Greater details of the error are shown in this file...[console screenshot - validation error](https://user-images.githubusercontent.com/5938313/79199691-42595400-7e2d-11ea-95bc-3c2248d8b60e.png

Please note my sample application does works when I put all validation attributes into main the class.

For clarity:
This approach works...

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

This approach fails...

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; }
}

To Reproduce

Please run the attached sample blazor application. To see the error please click the navigation link labelled "validation - failure". On the page, just simply press the submit button to see the error.
BlazorAppValidation.zip

Further technical details

• Include the output of dotnet --info

.NET Core SDK (reflecting any global.json):
 Version:   3.1.300-preview-015048
 Commit:    13f19b4682

Runtime Environment:
 OS Name:     Windows
 OS Version:  10.0.18363
 OS Platform: Windows
 RID:         win10-x64
 Base Path:   C:\Program Files\dotnet\sdk\3.1.300-preview-015048\

Host (useful for support):
  Version: 3.1.3
  Commit:  4a9f85e9f8

.NET Core SDKs installed:
  3.1.101 [C:\Program Files\dotnet\sdk]
  3.1.201 [C:\Program Files\dotnet\sdk]
  3.1.300-preview-015048 [C:\Program Files\dotnet\sdk]

.NET Core runtimes installed:
  Microsoft.AspNetCore.All 2.1.16 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.App 2.1.16 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 3.1.1 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 3.1.2 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 3.1.3 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 2.1.16 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 3.1.1 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 3.1.2 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 3.1.3 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.WindowsDesktop.App 3.1.1 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 3.1.2 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 3.1.3 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

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

[javiercn]

@RichardBr thanks for contacting us.

This is not a feature that we currently support AFAIK.

/cc: @SteveSandersonMS @pranavkm

[nssidhu]

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

[mkArtakMSFT]

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.

[wmgdev]

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

[MattSturtivant]

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.

[Gambero81]

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

[IanKemp]

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.

[navidzehi]

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

[SteveSandersonMS]

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.

[Gambero81]

news on this issue?

[DiegoVenancioVieira]

news on this issue?

Nothing, there are many other priorities. This is just one more in the backlog list. Waiting too

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

Issue Details

---

Describe the bug

When I use the MetadataType attribute (from System.ComponentModel.DataAnnotations) on the class for the purpose of validation I get the following error in the console window
WASM: System.TypeLoadException: Could not resolve type with token 01000027 from typeref (expected class 'System.ComponentModel.DataAnnotations.MetadataTypeAttribute' in assembly 'System.ComponentModel.Annotations, Version=4.3.1.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a')

Greater details of the error are shown in this file...[console screenshot - validation error](https://user-images.githubusercontent.com/5938313/79199691-42595400-7e2d-11ea-95bc-3c2248d8b60e.png

Please note my sample application does works when I put all validation attributes into main the class.

For clarity:
This approach works...

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

This approach fails...

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; }
}

To Reproduce

Please run the attached sample blazor application. To see the error please click the navigation link labelled "validation - failure". On the page, just simply press the submit button to see the error.
BlazorAppValidation.zip

Further technical details

• Include the output of dotnet --info

.NET Core SDK (reflecting any global.json):
 Version:   3.1.300-preview-015048
 Commit:    13f19b4682

Runtime Environment:
 OS Name:     Windows
 OS Version:  10.0.18363
 OS Platform: Windows
 RID:         win10-x64
 Base Path:   C:\Program Files\dotnet\sdk\3.1.300-preview-015048\

Host (useful for support):
  Version: 3.1.3
  Commit:  4a9f85e9f8

.NET Core SDKs installed:
  3.1.101 [C:\Program Files\dotnet\sdk]
  3.1.201 [C:\Program Files\dotnet\sdk]
  3.1.300-preview-015048 [C:\Program Files\dotnet\sdk]

.NET Core runtimes installed:
  Microsoft.AspNetCore.All 2.1.16 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.App 2.1.16 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 3.1.1 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 3.1.2 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 3.1.3 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 2.1.16 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 3.1.1 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 3.1.2 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 3.1.3 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.WindowsDesktop.App 3.1.1 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 3.1.2 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 3.1.3 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

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

Author:	RichardBr
Assignees:	-
Labels:	area-System.ComponentModel.DataAnnotations, untriaged
Milestone:	-