ASP.NET MVC posted file model binding when parameter is Model

Question

Is there any way to get posted files (<input type="file" />) to take part in model binding in ASP.NET MVC without manually looking at the request context in a custom model binder, and without creating a separate action method which only takes a posted file as input?

I would have thought that this would work:

class MyModel {
  public HttpPostedFileBase MyFile { get; set; }
  public int? OtherProperty { get; set; }
}

<form enctype="multipart/form-data">
  <input type="file" name="MyFile" />
  <input type="text" name="OtherProperty" />
</form>

public ActionResult Create(MyModel myModel) { ... }

But given the above scenario, MyFile isn't even part of the value provider values in the binding context. (OtherProperty is, of course.) But it works if I do this:

public ActionResult Create(HttpPostedFileBase postedFile, ...) { ... }

So, why does no binding occur when the parameter is a model, and how can I make it work? I have no problem with using a custom model binder, but how can I do this in a custom model binder without looking at Request.Files["MyFile"]?

For consistency, clarity and testability, I'd like my code to provide automatic binding of all properties on a model, including those bound to posted files, without manually inspecting the request context. I am currently testing model binding using the approach Scott Hanselman wrote about.

Or am I going about this in the wrong way? How would you solve this? Or is this not possible by design due to the history of separation between Request.Form and Request.Files?

Answer 1

Have you looked at this post which he links to from the one you linked to (via another one...)?

If not, it looks quite simple. This is the model binder he uses:

public class HttpPostedFileBaseModelBinder : IModelBinder {
    public ModelBinderResult BindModel(ModelBindingContext bindingContext) {
        HttpPostedFileBase theFile =
            bindingContext.HttpContext.Request.Files[bindingContext.ModelName];
        return new ModelBinderResult(theFile);
    }
}

He registers it in Global.asax.cs as follows:

ModelBinders.Binders[typeof(HttpPostedFileBase)] = 
    new HttpPostedFileBaseModelBinder();

and posts with a form that looks like this:

<form action="/File/UploadAFile" enctype="multipart/form-data" method="post">
    Choose file: <input type="file" name="theFile" />
    <input type="submit" />
</form>

All the code is copied straight off the blog post...

Answer 2

You don't need to register a custom binder, HttpPostedFileBase is registered by default in the framework:

public ActionResult Create(HttpPostedFileBase myFile)
{
    ...
}

It helps to read a book every once in awhile, instead of relying solely on blogs and web forums.

Answer 3

It turns out the reason is that ValueProviderDictionary only looks in Request.Form, RouteData and Request.QueryString to populate the value provider dictionary in the model binding context. So there's no way for a custom model binder to allow posted files to participate in model binding without inspecting the files collection in the request context directly. This is the closest way I've found to accomplish the same thing:

public ActionResult Create(MyModel myModel, HttpPostedFileBase myModelFile) { }

As long as myModelFile is actually the name of the file input form field, there's no need for any custom stuff.

Answer 4

Another way is to add a hidden field with the same name as the input:

<input type="hidden" name="MyFile" id="MyFileSubmitPlaceHolder" />

The DefaultModelBinder will then see a field and create the correct binder.