Question

I am curious why the ApiController handles default parameter values on actions differently than a ‘regular’ Controller.

This code works just fine, request to /Test means page gets value 1

public class TestController : Controller
{
    public ActionResult Index(int page = 1)
    {
        return View(page);
    }
}

This code doesn’t work when a request is made to /api/Values. It fails with:

“The parameters dictionary contains a null entry for parameter ‘page’ of non-nullable type ‘System.Int32’ for method ‘System.Collections.Generic.IEnumerable`1[System.String] Get(Int32)’ in ‘MvcApplication1.Controllers.Controllers.ValuesController’. An optional parameter must be a reference type, a nullable type, or be declared as an optional parameter.”

public class ValuesController : ApiController
{
    public IEnumerable<string> Get(int page = 1)
    {
        return new string[] { page.ToString() };
    }      
}

Any hints on why this is?

Answer 1

Try adding the [FromUri] or [FromForm] parameter attribute.

public class ValuesController : ApiController
{
    public IEnumerable<string> Get([FromUri]int page = 1)
    {
        return new string[] { page.ToString() };
    }      
}

Mike Stall has two good posts about parameter binding in Webapi which does not work as it does in ASP MVC. The big problem to get used to is that you can only read the request body once in your pipeline. So if you need to read more than 1 complex object as a parameter, you probably need to resort to ModelBinding by parameter. I had a problem similar to yours when I was reading the content body earlier in the pipeline for logging purposes and did not realize about the read once restriction above and had to solve with my own custom model binder.

Explains model binding at http://blogs.msdn.com/b/jmstall/archive/2012/04/16/how-webapi-does-parameter-binding.aspx and then suggests a way to make WebAPI model binding more like ASP MVC http://blogs.msdn.com/b/jmstall/archive/2012/04/18/mvc-style-parameter-binding-for-webapi.aspx