How to work with RouteValues with multiple values of the same name

Question

In my ASP.NET MVC 4 application I can filter on multiple tags. In HTML, it looks like this:

<form>
  <label>
    <input type="checkbox" name="tag" value="1">One
  </label>
  <label>
    <input type="checkbox" name="tag" value="2">Two
  </label>
  <label>
    <input type="checkbox" name="tag" value="3">Three
  </label>
  <input type="submit" name="action" value="Filter">
</form>

When checking the first and third checkbox, the querystring is serialized as ?tag=1&tag=3 and my controller nicely passes an object with the type of the following class:

// Filter class
public class Filter { 
    public ICollection<int> tag { get; set; }
}

// Controller method
public ActionResult Index(AdFilter filter)
{
    string url = Url.Action("DoFilter", filter);
    // url gets this value:
    // "/controller/Index?tag=System.Collections.Generic.List%601%5BSystem.Int32%5D"
    // I would expect this:
    // "/controller/Index?tag=1&tag=3"
    ...
 }

However, a call to Url.Action results in the typename of the collection being serialized, instead of the actual values.

How can this be done?

---

The standard infrastructure of MVC can handle the multi-keys described as input. Is there not standard infrastructure that can handle it the other way around? Am I missing something?

Answer 1

You can do it in the following way:

string url = Url.Action("DoFilter", TypeHelper2.ObjectToDictionary(filter) );

The TypeHelper2.ObjectToDictionary is a modified version of an internal method of .NET, and can be found in this two file gist.

Changed behavior: when an item implements IEnumerable, for each item an entry is created in the returned dictionary with as key "Name[index]" (the index is 0 based). This is possible because the binder of the MVC controller can handle both tag=1&tag=3 and tag[0]=1&tag[1]=3 query strings.

Answer 2

An easy yet no so elegant solution can be:

  public ActionResult Index(AdFilter filter)
  {
     string parameters = "?";
     foreach (var item in filter.tag)
        {
            parameters += string.Format("tag={0}&", item);
        }
     //trimming the last "&"
     parameters = parameters.TrimEnd(parameters[parameters.Length - 1]);
     string url = Url.Action("DoFilter") + parameters;

  }