ASP.net MVC - Model binding excludes class fields?

Question

In a recent project - i've hit an unexpected snag.
A class with simple public fields (note not properties) doesn't seem to want to play nice with the ASP.net MVC 3.0 model binder.

Is this by design?

besides changing the fields to properties - any options here?

update

The reason for the simple fields (instead of properties) is because I'm working with a shared library between MVC and a Script Sharp project. Script sharp supports properties - but it becomes a mess with javascript in the view (using knockout.js)

So despite what i'd love (which is to just use properties) i'm using public fields in my dto classes.

I wanted to avoid having multiple definitions of the same classes. sigh

Answer 1

Is this by design?

Yes, only properties with public getter/setters work with the default model binder.

besides changing the fields to properties - any options here?

That's what you should do as you should be using view models anyway. And view models are specifically designed for the views. So changing the fields to properties is the correct way to do. Now if you don't follow good practices and try to use your domain models as input/output to actions then you will have to write a custom model binder that works with fields. It will be a lot of work though.

Answer 2

I know it's against the c# team's philosophy, but I think fields are clean in poco classes. Seems like less moving parts to me. Anyway,

Here's a model binder that will load fields. It's fairly trivial. Note that you can use a new object with Activator.CreateInsance or start with an existing object as a starting point. I was using Yes/No dropdowns for bool, you may have check boxes, in which case unfortunately you'll have to loop through fieldinfos and look for missing form inputs b/c html doesn't submit form items if it's a false checkbox.

~/Binders/FieldModelBinder.cs

using System;
using System.Reflection;
using System.Web.Mvc;

namespace MyGreatWebsite.Binders
{
  public class FieldModelBinder : DefaultModelBinder
  {
    public override object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
    {
      var obj = controllerContext.HttpContext.Session["CurrentObject"];// Activator.CreateInstance(bindingContext.ModelType);
      var form = controllerContext.HttpContext.Request.Form;
      foreach (string input in form)
      {
        FieldInfo fieldInfo = obj.GetType().GetField(input);
        if (fieldInfo == null)
          continue;
        else if (fieldInfo.FieldType == typeof(bool))
          fieldInfo.SetValue(obj, form[input] == "Yes");
        else
          fieldInfo.SetValue(obj, Convert.ChangeType(form[input], fieldInfo.FieldType));
      }
      return obj;
    }
  }
}

Startup.cs

  public partial class Startup
  {
    public void Configuration(IAppBuilder app)
    {
      ConfigureAuth(app);
      ModelBinders.Binders.Add(typeof(FieldModelBinder), new FieldModelBinder());
    }
  }

Usage

[HttpPost]
public ActionResult MyGreatAction([ModelBinder(typeof(FieldModelBinder))] MyGreatProject.MyGreatNamespace.MyGreatType instance, string myGreatParameters)
{
  DoSomethingGreatWithMyInstance();
  return View();
}