Custom Model Binder to bind nested property values

Question

The reason I need this: In one of my controllers I want to bind all Decimal values in a different way than the rest of my application. I do not want to register a Model Binder in Global.asax (via ModelBinders.Binders.Add(typeof(decimal), new DecimalModelBinder());)

I have tried deriving from the DefaultModelBinder class and override its BindProperty method, but that only works for the model instance's immediate (not nested) Decimal properties.

I have the following example to demonstrate my problem:

namespace ModelBinderTest.Controllers
{
    public class Model
    {
        public decimal Decimal { get; set; }
        public DecimalContainer DecimalContainer { get; set; }
    }

    public class DecimalContainer
    {
        public decimal DecimalNested { get; set; }
    }

    public class DecimalModelBinder : DefaultModelBinder
    {
        protected override void BindProperty(ControllerContext controllerContext, ModelBindingContext bindingContext, System.ComponentModel.PropertyDescriptor propertyDescriptor)
        {
            if (propertyDescriptor.PropertyType == typeof (decimal))
            {                
                propertyDescriptor.SetValue(bindingContext.Model,  999M);
                return;
            }

            base.BindProperty(controllerContext, bindingContext, propertyDescriptor);
        }
    }

    public class TestController : Controller
    {

        public ActionResult Index()
        {
            Model model = new Model();
            return View(model);
        }

        [HttpPost]
        public ActionResult Index([ModelBinder(typeof(DecimalModelBinder))] Model model)
        {
            return View(model);
        }

    }
}

This solution only sets the Model's Decimal property to 999, but doesn't do anything to DecimalContainer's DecimalNested property. I realize this is because base.BindProperty is called in my DecimalModelBinder's BindProperty override, but I don't know how to convince the base class to use my Model Binder when dealing with decimal properties.

Answer 1

You could apply the model binder unconditionally in your Application_Start:

ModelBinders.Binders.Add(typeof(decimal), new DecimalModelBinder());

and then have a custom authorization filter (yes, authorization filter as it runs before the model binder) that will inject into the HttpContext some value that could later be used by the model binder:

[AttributeUsage(AttributeTargets.Method)]
public class MyDecimalBinderAttribute : ActionFilterAttribute, IAuthorizationFilter
{
    public void OnAuthorization(AuthorizationContext filterContext)
    {
        filterContext.HttpContext.Items["_apply_decimal_binder_"] = true;
    }
}

and then in your model binder test if the HttpContext contains the custom value befoire applying it:

public class DecimalModelBinder : DefaultModelBinder
{
    public override object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
    {
        if (controllerContext.HttpContext.Items.Contains("_apply_decimal_binder_"))
        {
            // The controller action was decorated with the [MyDecimalBinder]
            // so we can proceed
            return 999M;
        }

        // fallback to the default binder
        return base.BindModel(controllerContext, bindingContext);
    }
}

Now all that's left is to decorate your controller action with the custom filter to enable the decimal binder:

[HttpPost]
[MyDecimalBinder]
public ActionResult Index(Model model)
{
    return View(model);
}