Binding a dictionary containing a list in MVC4

Question

I am having trouble posting a form that contains a Dictionary that contains an int as a key and a list of objects as a value.

Originally this was just a List of Objects and that worked fine and the type was:

List<MyObject> Fields

the working markup was

Fields_{0}__Property1

where {0} is the index of the object. To get it to post back the List of Objects I rendered the object with hidden fields like this:

  @Html.HiddenFor(m => m.Property1, new { Name = string.Format("Fields[{0}].Property1", Model.Index), @id = string.Format("Fields_{0}__Property1", Model.Index) })

This worked well. Now however, we have a dictionary instead of a list and the list is inside the dictionary.

Now the type is:

Dictionary<int, List<MyObject>>.

I tested the format expected when we render the dictionary using Html.HiddenFor and so I've added hidden fields with the required format which now is:

  @Html.HiddenFor(m => m.Property1, new { Name = string.Format("Fields[{0}][{1}].Property1", Model.Index, Model.Position), @id = string.Format("Fields_{0}__{1}__Property1", Model.Index, Model.Position) })

now the field id is

Fields_{0}__{1}__Property1

where {0} is the key of the dictionary and {1} is the index of the object in the list.

However on postback I now get

 [InvalidCastException: Specified cast is not valid.]
   System.Web.Mvc.CollectionHelpers.ReplaceDictionaryImpl(IDictionary`2 dictionary, IEnumerable`1 newContents) +131

I am guessing MVC is smart enough to render the fields of this complex object on the view but not smart enough to collect them back into the viewmodel when we post back.

I found this other guy who had a similar problem here and he solved it by not using a dictionary but instead creating a complex object. I'm wondering, however, if there's a quicker way that won't require me to rewrite the entire system.

Any ideas?

Update

I solved it by taking the source code of DefaultModelBinder and adjusting it. I found the source here. I didn't create my own Binder because I want all the advanced functionality and validation rules to apply to all other elements. Once I got the DefaultModelBinder compiling and working I found the part where the dictionary was failing to cast the complex items and wrote a custom Dictionary update method that solved the problem

asymptoticFault asymptoticFault · Accepted Answer · 2013-08-20T14:51:03

You can always create a custom Model Binder to bind objects from request values exactly as you want. Simply create a class that implements the System.Web.Mvc.IModelBinder interface and implement the BindModel() method.

Binding a dictionary containing a list in MVC4

1 Answers