Injecting Non-User-Submitted Data For Use During Validation - validation

Question

From what I can tell, ASP.Net Core performs model state validation before calling the relevant controller action method. This means that code in the action method isn't given an opportunity to add data to the model before it is validated.
What is the ASP.Net Core way of giving a view model access to additional, non-user-submitted data prior to validation?
Example
What I'm trying to do (doesn't work).
The view model's Validate method expects data to be in ValidOptions. However, since validation occurs before the controller can set this property, validation causes the view model to throw an ArgumentNullException.
// From the Controller
[HttpPost]
[ValidateAntiForgeryToken]
public async Task<IActionResult> Process([Bind("SelectedId")]ViewModels.Import details)
{
// data needed for validation
details.ValidOptions = await service.ImportTypes.ToListAsync();
if (ModelState.ValidationState != ModelValidationState.Valid) {
// ...
}
}
// From ViewModels.Import
public IEnumerable<Option> ValidOptions { get; set; }
public int SelectdId {get; set; }
public IEnumerable<ValidationResult> Validate(ValidationContext validationContext)
{
// throws ArgumentNullException because ValidOptions hasn't been set when this is executed
var option = ValidOptions.Single(t => t.Id == SelectdId);
//...
}

Answer 1

Probably many ways to skin a cat here. But the easiest for you is probably custom model binders. It's a way to "supplement" or change the binding of your model before it hits the controller. I will say that some see it as extremely bad practice to call an external service/repository at the point of model binding, but it does work and can come in handy.
You need to implement a class that inherits from IModelBinder.
public class MyViewModelBinder : IModelBinder
{
public Task BindModelAsync(ModelBindingContext bindingContext)
{
//Bind here. Including calling external services if you want.
}
}
Then you need to implement a provider, this essentially says "when" to bind.
public class MyViewModelBinderProvider : IModelBinderProvider
{
public IModelBinder GetBinder(ModelBinderProviderContext context)
{
if (context.Metadata.ModelType == typeof(MyViewModel))
return new MyViewModelBinder();
return null;
}
}
In your configure method of your startup.cs, you need to add the provider to the ModelBinderProviders list.
public void ConfigureServices(IServiceCollection services)
{
// Add framework services.
services.AddMvc(config =>
config.ModelBinderProviders.Add(new MyViewModelBinderProvider())
);
}
Further Documentation :
http://dotnetcoretutorials.com/2016/12/28/custom-model-binders-asp-net-core/
http://intellitect.com/custom-model-binding-in-asp-net-core-1-0/
I don't think the official documentation has an article on custom model binders yet unfortunately.