How do I accept an array as an ASP.NET MVC controller action parameter?

You can also use this URL format, and ASP.NET MVC will do everything for you. But, remember to apply URL encoding.

?param1[0]=3344&param1[1]=2222

The default model binder expects this url:

http://localhost:54119/Designs/Multiple?ids=24041&ids=24117

in order to successfully bind to:

public ActionResult Multiple(int[] ids)
{
    ...
}

And if you want this to work with comma separated values you could write a custom model binder:

public class IntArrayModelBinder : DefaultModelBinder
{
    public override object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
    {
        var value = bindingContext.ValueProvider.GetValue(bindingContext.ModelName);
        if (value == null || string.IsNullOrEmpty(value.AttemptedValue))
        {
            return null;
        }

        return value
            .AttemptedValue
            .Split(',')
            .Select(int.Parse)
            .ToArray();
    }
}

and then you could apply this model binder to a particular action argument:

public ActionResult Multiple([ModelBinder(typeof(IntArrayModelBinder))] int[] ids)
{
    ...
}

or apply it globally to all integer array parameters in your Application_Start in Global.asax:

ModelBinders.Binders.Add(typeof(int[]), new IntArrayModelBinder());

and now your controller action might look like this:

public ActionResult Multiple(int[] ids)
{
    ...
}

To extend on Darin Dimitrov's answer, something you can get away with is accepting a simple string in your URL parameter and converting it to an array yourself:

public ActionResult Multiple(string ids){
  int[] idsArray = ids.Split(',').Select(int.Parse).ToArray();
  /* ...process results... */
}

If you get a parse error while doing this (because someone passed you a malformed array), you can cause your exception handler to return a 400 Bad Request error instead of the default, more unfriendly 404 Not Found error that MVC returns when an endpoint is not found.