Is there a way to have the DefaultModelBinder ignore empty items when binding to a List<Enum>

Question

I have a scenario where I'd like to change the behavior of the DefaultModelBinder in how it binds to a List of enums.

I have an enum...

public enum MyEnum { FirstVal, SecondVal, ThirdVal }

and a class for a model...

public class MyModel
{
    public List<MyEnum> MyEnums { get; set; }
}

and the POST body is...

MyEnums=&MyEnums=ThirdVal

Currently, after model binding, the MyEnums property will contain...

[0] = FirstVal
[1] = ThirdVal

Is there was a way to tell the model binder to ignore the empty value in the posted data so that MyEnums property could look like the following?

[0] = ThirdVal

Answer 1

You could write a custom model binder for MyModel:

public class MyModelModelBinder : DefaultModelBinder
{
    protected override void SetProperty(
        ControllerContext controllerContext, 
        ModelBindingContext bindingContext, 
        PropertyDescriptor propertyDescriptor, 
        object value)
    {
        if (value is ICollection<MyEnum>)
        {
            var myEnums = controllerContext.HttpContext.Request[propertyDescriptor.Name];
            if (!string.IsNullOrEmpty(myEnums))
            {
                var tokens = myEnums.Split(new [] { ',' }, StringSplitOptions.RemoveEmptyEntries);
                value = tokens.Select(x => (MyEnum)Enum.Parse(typeof(MyEnum), x)).ToList();
            }
        }
        base.SetProperty(controllerContext, bindingContext, propertyDescriptor, value);
    }
}

which is registered in Application_Start:

protected void Application_Start()
{
    AreaRegistration.RegisterAllAreas();
    RegisterRoutes(RouteTable.Routes);
    ModelBinders.Binders.Add(typeof(MyModel), new MyModelModelBinder());
}

---

UPDATE:

As requested in the comments section here's how to make the previous binder more generic:

protected override void SetProperty(
    ControllerContext controllerContext, 
    ModelBindingContext bindingContext, 
    PropertyDescriptor propertyDescriptor, 
    object value)
{
    var collection = value as IList;
    if (collection != null && collection.GetType().IsGenericType)
    {
        var genericArgument = collection
            .GetType()
            .GetGenericArguments()
            .Where(t => t.IsEnum)
            .FirstOrDefault();

        if (genericArgument != null)
        {
            collection.Clear();
            var enumValues = controllerContext.HttpContext
                .Request[propertyDescriptor.Name];
            var tokens = enumValues.Split(
                new[] { ',' }, StringSplitOptions.RemoveEmptyEntries);
            foreach (var token in tokens)
            {
                collection.Add(Enum.Parse(genericArgument, token));
            }
        }
    }
    base.SetProperty(controllerContext, bindingContext, propertyDescriptor, value);
}