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

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)
{
    ...
}

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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.

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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

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I don't know where Groky's URL string was coming from, but I had the same problem with some javascript calling my controller/action. It would build up a URL of null, 1, or many "IDs" from a multiple-select list (which is unique to the solution I'm going to share).

I copy/pasted Darin's custom model binder and decorated my action/parameter, but it didn't work. I still got null valued int[] ids. Even in the "safe" case where I actually did have many IDs.

I ended up changing the javascript to produce an ASP.NET MVC friendly parameter array like

?ids=1&ids=2

I had to do some silly stuff, though

ids || []                 #=> if null, get an empty array
[ids || []]               #=> if a single item, wrap it in an array
[].concat.apply([], ...)  #=> in case I wrapped an array, flatten it

So, the full block was

ids = [].concat.apply([], [ids || []])
id_parameter = 'ids=' + ids.join('&ids=')

It's messy, but it's the first time I had to hack like this in javascript.

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.Net Core Answer

For those coming here in recent times, you can do this in .Net Core with:

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

and:

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

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