Improve the experience for polymorphic model validation

[pamedasa]

Describe the bug

The annotations that we add for validations are not getting invoked for polymorphic input classes. They are getting invoked only for the non-polymorphic classes.
Data model validators are not being invoked for request input data which is of type polymorphic data. In general, all the validators are being fired up before passing to actual implementation of controller code.

To Reproduce

Steps to reproduce the behavior:

1. Using this version of ASP.NET Core 2.2
2. Run this code '....'
3. With these arguments '....'
4. See error

Expected behavior

in Asp.Net framework, irrespective on input data model (whether it is normal (or) polymorphic) all the validators gets invoked and thrown the exception in right format. After moving to asp.net core as well, validators should work as expected earlier.

Screenshots

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Additional context

Add any other context about the problem here.
Include the output of dotnet --info

[mkArtakMSFT]

Thanks for contacting us, @pamedasa .
@pranavkm can you please look into this? Thanks!

[pranavkm]

I just got around to writing a snippet in our docs that shows how to author a polymorphic binder: https://docs.microsoft.com/en-us/aspnet/core/mvc/advanced/custom-model-binding?view=aspnetcore-2.2#polymorphic-model-binding. Outside of the top level property being bound, MVC uses the declared model type unless the model binder explicitly tells it otherwise.:

bindingContext.Result = ModelBindingResult.Success(result);

// Add a ValidationStateEntry with the "correct" ModelMetadata so validation executes on the actual type, not the declared type.
var modelMetadataProvider = bindingContext.HttpContext.RequestServices.GetRequiredService<IModelMetadataProvider>();

bindingContext.ValidationState.Add(result, new ValidationStateEntry
{
    Metadata = modelMetadataProvider.GetMetadataForType(result.GetType()),
});

@rynowak \ @dougbu we might want to build a thing for 5.0, but for 3.1 would it help if ValidationVisitor printed a warning if the model.GetType() != Metadata.ModelType saying you might have missed this? It's not super obvious that a binder has to do this.

[pamedasa]

I have created custom model validator which works for complex inputs like polymorphic data. Taken the advantage of TryValidateModel provided by controllerbase from asp.net core MVC and override the method to add our own logic.

public override bool TryValidateModel(object inputObject)
{
Type type = inputObject.GetType();
foreach (var prop in type.GetProperties().Where(p => !p.GetIndexParameters().Any()))
{
object val = prop.GetValue(inputObject);
base.TryValidateModel(val);
// Skipping the model validation for primitive types and
// objects which don't have properties.
if (!val.GetType().IsPrimitive && val.GetType().GetProperties().Length != 0)
{
this.TryValidateModel(val);
}
}
return true;
}

[mkArtakMSFT]

Thanks for the details, @pamedasa.
We'll look into improving polymorphic model validation effectiveness for 5.0.

[pranavkm]

@pamedasa I wouldn't recommend using the example you have. It misses validators on primitive data types and ignores validation attributes applied to properties. Have a look at the sample, the change to make MVC's model validation pick up the correct model requires minimal changes to the model binder.