Why isn't validate being called on my IValidatableObject?

Question

I have a view model:

public class SelectVendorViewModel : IValidatableObject
{
    [Display(Name = "Document Date")]
    [RequiredUnless("IsPidDv")]
    public DateTime? DocumentDate { get; set; }

    [Display(Name = "Document Number")]
    [RequiredUnless("IsPidDv")]
    public int? DocumentNumber { get; set; }

    [Display(Name = "Vendor")]
    [RequiredUnless("IsPidDv")]
    public Guid? VendorId { get; set; }

    public List<SelectListItem> Vendors { get; set; }

    [Display(Name="PID/DV")]
    public bool IsPidDv { get; set; }

    public Guid? SalesReportId { get; set; }

    public IEnumerable<ValidationResult> Validate(ValidationContext validationContext)
    {
        return SelectVendorViewModelValidator.ValidateSalesReport(validationContext, this);
    }
}

A custom model binder:

internal class SelectVendorViewModelBinder : DefaultModelBinder, IModelBinder<SelectVendorViewModel>
{
    private readonly IVendorUnitOfWork _uow;

    public SelectVendorViewModelBinder(IVendorUnitOfWork uow)
    {
        _uow = uow;
    }

    protected override void OnModelUpdated(ControllerContext controllerContext, ModelBindingContext bindingContext)
    {
        var model = bindingContext.Model as SelectVendorViewModel;

        if (model == null || !model.VendorId.HasValue || !model.DocumentDate.HasValue || !model.DocumentNumber.HasValue)
        {
            return;
        }

        var salesReport = _uow.SalesReportRepository.GetSalesReport(model.VendorId.Value, model.DocumentNumber.Value,
            model.DocumentDate.Value);
        if (salesReport != null)
        {
            model.SalesReportId = salesReport.Id;
        }
    }
}

And a validator:

internal class SelectVendorViewModelValidator
{
    internal static IEnumerable<ValidationResult> ValidateSalesReport(ValidationContext validationContext, SelectVendorViewModel viewModel)
    {
        if (viewModel.IsPidDv)
        {
            yield break;
        }

        if (!viewModel.SalesReportId.HasValue || viewModel.SalesReportId.Value == default(Guid))
        {
            yield return new ValidationResult("Sales report document does not exist.");
        }
    }
}

And the controller action that is being posted to:

[HttpPost]
public virtual ActionResult SelectVendor(SelectVendorViewModel selectVendorVM)
{
    selectVendorVM.Vendors = GetVendors();
    if (!ModelState.IsValid)
    {
        return View(selectVendorVM);
    }           

    return RedirectToAction(MVC.Licensing.Endorsements.Create(selectVendorVM.SalesReportId));
}   

The binder is running correctly, I can step through it in the debugger. But the SelectVendorViewModel.Validate method is never called. The property validation passes, and if I set a breakpoint in the controller action ModelState.IsValid is true. I thought it might be something with the custom RequiredUnless annotation, but even when I remove them the validation doesn't work. I use this same pattern in lots of places in this app but this is the only one that doesn't work. The only difference I could find between this and the others is the RequiredUnless annotation and I was able to rule that out. What am I missing?

EDIT: Here's how I register the model binders:

Custom IModelBinderProvider:

public class GenericModelBinder : IModelBinderProvider
{
    public IModelBinder GetBinder(Type modelType)
    {
        var genericBinder = typeof(IModelBinder<>).MakeGenericType(modelType);

        var binder = DependencyResolver.Current.GetService(genericBinder) as IModelBinder;

        return binder;
    }
}

In Global.asax Application_Start method:

ModelBinderProviders.BinderProviders.Add(new GenericModelBinder());

And in the Ninject config:

kernel.Bind<IModelBinder<SelectVendorViewModel>>().To<SelectVendorViewModelBinder>();

Answer 1

Oh (cringe) you're not calling the base method of the ModelBinder which in turn calls the Validate method on the model. ;)

protected override void OnModelUpdated(ControllerContext controllerContext, ModelBindingContext bindingContext)
    {
        var model = bindingContext.Model as SelectVendorViewModel;

        if (model == null || !model.VendorId.HasValue || !model.DocumentDate.HasValue || !model.DocumentNumber.HasValue)
        {
            return;
        }

        var salesReport = _uow.SalesReportRepository.GetSalesReport(model.VendorId.Value, model.DocumentNumber.Value,
            model.DocumentDate.Value);
        if (salesReport != null)
        {
            model.SalesReportId = salesReport.Id;
        }
       // this is important as we overrode but still need base 
       // functionality to effect a validate
       base.OnModelUpdated(controllerContext, bindingContext);
    }