asp.net mvc 3 - Model binding comma separated query string parameter

Question

How can I bind query string parameter that is comma separated value

http://localhost/Action?ids=4783,5063,5305

to a controller action expecting a list?

public ActionResult Action(List<long> ids)
{
    return View();
}

Note! ids in the controller action must a list (or something IEnumerable based), so string ids is not accepted as an answer because these parameters are passed to many actions and parsing string to an array would add unwanted noise.

Answer 1

Here's my improved version of Nathan Taylor’s solution used in archil's answer.

1. Nathan’s binder could only bind sub-properties of complex models, while mine can also bind individual controller arguments.
2. My binder also gives you correct handling of empty parameters, by returning an actual empty instance of your array or IEnumerable.

To wire this up, you can either attach this to an individual Controller argument:

[ModelBinder(typeof(CommaSeparatedModelBinder))]

…or set it as the global default binder in Application_Start in global.asax.cs:

ModelBinders.Binders.DefaultBinder = new CommaSeparatedModelBinder();

In the second case it will try and handle all IEnumerables and fall back to ASP.NET MVC standard implementation for everything else.

Behold:

public class CommaSeparatedModelBinder : DefaultModelBinder
{
    private static readonly MethodInfo ToArrayMethod = typeof(Enumerable).GetMethod("ToArray");

    public override object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
    {
        return BindCsv(bindingContext.ModelType, bindingContext.ModelName, bindingContext)
                ?? base.BindModel(controllerContext, bindingContext);
    }

    protected override object GetPropertyValue(ControllerContext controllerContext, ModelBindingContext bindingContext, System.ComponentModel.PropertyDescriptor propertyDescriptor, IModelBinder propertyBinder)
    {
        return BindCsv(propertyDescriptor.PropertyType, propertyDescriptor.Name, bindingContext)
                ?? base.GetPropertyValue(controllerContext, bindingContext, propertyDescriptor, propertyBinder);
    }

    private object BindCsv(Type type, string name, ModelBindingContext bindingContext)
    {
        if (type.GetInterface(typeof(IEnumerable).Name) != null)
        {
            var actualValue = bindingContext.ValueProvider.GetValue(name);

            if (actualValue != null)
            {
                var valueType = type.GetElementType() ?? type.GetGenericArguments().FirstOrDefault();

                if (valueType != null && valueType.GetInterface(typeof(IConvertible).Name) != null)
                {
                    var list = (IList)Activator.CreateInstance(typeof(List<>).MakeGenericType(valueType));

                    foreach (var splitValue in actualValue.AttemptedValue.Split(new[] { ',' }))
                    {
                            if(!String.IsNullOrWhiteSpace(splitValue))
                                list.Add(Convert.ChangeType(splitValue, valueType));
                    }

                    if (type.IsArray)
                        return ToArrayMethod.MakeGenericMethod(valueType).Invoke(this, new[] { list });
                    else
                        return list;
                }
            }
        }

        return null;
    }
}