ASP.NET MVC3 Custom Model Binder Issues

Question

I have an applicant model that contains a list of tags:

public class Applicant
{
    public virtual IList<Tag> Tags { get; protected set; }
}

When the form is submitted, there is an input field that contains a comma-delimited list of tags the user has input. I have a custom model binder to convert this list to a collection:

public class TagListModelBinder : IModelBinder
{
    public object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
    {
        var incomingData = bindingContext.ValueProvider.GetValue("tags").AttemptedValue;
        IList<Tag> tags = incomingData.Split(',').Select(data => new Tag { TagName = data.Trim() }).ToList();
        return tags;
    }
}

However, when my model is populated and passed into the controller action on POST, the Tags property is still an empty list. Any idea why it isn't populating the list correctly?

http://prideparrot.com/blog/archive/2012/6/customizing_property_binding_through_attributes — VJAI, Jun 28 '12 at 16:39
@Mark I don't see a reason to replace the entire model binder as your link might suggest. — Erik Philips, Jun 28 '12 at 16:49

score 2 · Answer 1 · answered Jun 28 '12 at 15:46

2

A model binder only binds submitted values. It does not bind values rendered in the view.

You need to create a custom EditorTemplate to render the tags as you need them.

answered Jun 28 '12 at 15:46

Erik Funkenbusch

92,674
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score 2 · Accepted Answer · answered Jun 28 '12 at 16:56

2

The problem is you have the protected set accessor in Tags property. If you change that into public as below things will work fine.

public class Applicant
{
    public virtual IList<Tag> Tags { get; set; }
}

answered Jun 28 '12 at 16:56

VJAI

32,167
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score 1 · Answer 3 · answered Jun 28 '12 at 15:56

1

MVC can already bind to a List, I would recommend using the built in technology that already does what you need.

I didn't notice any code about adding the binder, did you add your ModelBinder to the Binders?

protected void Application_Start()
{
  ModelBinders.Binders.Add(typeof(IList<Tag>), new TagListModelBinder());
}

answered Jun 28 '12 at 15:56

Erik Philips

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Yes, I did add it to the binders and it's being called. It just doesn't get populated in the action method parameter. – arknotts Jun 28 '12 at 16:55
I looked at your link about binding to a list. Unfortunately this won't work in my scenario since the javascript I use on the client side puts everything into one input field. – arknotts Jun 28 '12 at 18:08

ASP.NET MVC3 Custom Model Binder Issues

3 Answers3