Create RouteValueDictionary with multivalued keys - asp.net-mvc

Question

I'd like to return a RedirectToRouteResult that sends users to a URL like the following:
/MyController/MyAction/id?newid=3&newid=5&newid=7
The newid parameter has several values.
My call looks like: return RedirectToAction(string.Empty, routeValues);
Here is what I have tried so far, and that do not work:
// ...?newid%5B0%5D=3&newid%5B1%5D=5&newid%5B2%5D=7
var routeValues = new RouteValueDictionary {
{"id", myid},
{"newid[0]", 3},
{"newid[1]", 5},
{"newid[2]", 7},
};
// ...?newid=System.Int32%5B%5D
var routeValues = new { id = myid, newid = new int[] { 3, 5, 7 } };
// ...?newid=System.String%5B%5D
var routeValues = new { id = myid, newid = new string[] { "3", "5", "7" } };
// ...?newid=System.Int32%5B%5D
var routeValues = new RouteValueDictionary {
{"id", myid},
{"newid", new int[] { 3, 5, 7 } }
};
What is the secret to make this work?

Answer 1

That's one thing that's really missing from the framework. Your best bet is to manually roll it:
public ActionResult Foo()
{
var ids = new[] { 3, 5, 7 };
var url = new UriBuilder(Url.Action("MyAction", "MyController", new { id = "123" }, Request.Url.Scheme));
url.Query = string.Join("&", ids.Select(x => "newid=" + HttpUtility.UrlEncode(x.ToString())));
return Redirect(url.ToString());
}
Putting this into a custom extension method could increase the readability of course.

Answer 2

I was in the case where I don't even know the names of the keys that where provide multiple times. And since the querystring does accept multiple keys as a coma separated list. I found it helpful to write an extension method based on Darin's answer.
public static UriBuilder TransformToMultiplyKeyUrl(this RouteValueDictionary self, string baseUrl)
{
var url = new UriBuilder(baseUrl);
//Transform x=y,z into x=y&x=z
url.Query = String.Join("&",
self.SelectMany(
pair => ((string)pair.Value).Split(',')
.Select(v => String.Format("{0}={1}", pair.Key, HttpUtility.UrlEncode(v)))));
return url;
}