ASP.NET Web API List Parameter Binding

We chased our tails on this a bit today, so hopefully someone finds this useful.

The Problem

Your action needs to accept a list of primitives but your list parameter (of type T[], IEnumerable<T>, List<T>, etc) always comes back null. You’re pretty sure an identical parameter has always work fine for an MVC action.

The Solution

Add [ModelBinder] to the parameter:

public class ValuesController : ApiController
{
    // GET api/values?id=Hello&id=%20world!
    public string Get([ModelBinder]List<string> id)
    {
        return string.Join(",", id);
    }
}

Explanation

According to Mike Stall’s detailed post on Web API parameter binding, Web API only uses model binding for “simple types”, falling back on formatters for everything else. Adding [ModelBinder] forces the complex types to use model binding anyway.

Related but not constructive: it feels weird naming a list parameter “id”, but even more weird using the API if it were named “ids”. Maybe just “x” instead?