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;
    }
}

Answer 2

Default model binder expects simple type lists to be in the format

name=value&name=value2&name=value3

To use builtin binding, you should change your query string to

Action?ids=4783&ids=5063&ids=5305

Or create custom model binder. You may take a look at following article (code from there)

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

    protected override object GetPropertyValue(ControllerContext controllerContext, ModelBindingContext bindingContext, System.ComponentModel.PropertyDescriptor propertyDescriptor, IModelBinder propertyBinder)
    {
        if (propertyDescriptor.PropertyType.GetInterface(typeof(IEnumerable).Name) != null)
        {
            var actualValue = bindingContext.ValueProvider.GetValue(propertyDescriptor.Name);

            if (actualValue != null && !String.IsNullOrWhiteSpace(actualValue.AttemptedValue) && actualValue.AttemptedValue.Contains(","))
            {
                var valueType = propertyDescriptor.PropertyType.GetElementType() ?? propertyDescriptor.PropertyType.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[] { ',' }))
                    {
                        list.Add(Convert.ChangeType(splitValue, valueType));
                    }

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

        return base.GetPropertyValue(controllerContext, bindingContext, propertyDescriptor, propertyBinder);
    }
}

Answer 3

Archils answer gave some ideas how to implement my own model binder. I was able to slightly simplify the source code as there wasn't need for very generic CSV support. Instead of setting the received data to List<int> I am putting it to the class.

Model binder

public class FarmModelBinder : DefaultModelBinder
{
    public override object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
    {
        if (bindingContext.ModelType == typeof(FarmModel))
        {
            var newBindingContext = new ModelBindingContext()
            {
                ModelMetadata = ModelMetadataProviders.Current.GetMetadataForType(
                () => CreateFarmModel(controllerContext, bindingContext),
                typeof(FarmModel)
                ),
                ModelState = bindingContext.ModelState,
                ValueProvider = bindingContext.ValueProvider
            };

            return base.BindModel(controllerContext, newBindingContext);
        }

        return base.BindModel(controllerContext, bindingContext);
    }

    private FarmModel CreateFarmModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
    {
        var farmsIds = new List<int>();

        var value = bindingContext.ValueProvider.GetValue("farmData");
        if(value != null && value.AttemptedValue != null)
        {
            var array = value.AttemptedValue.Split(new [] {','});
            foreach (var s in array)
            {
                int result;
                if(int.TryParse(s, out result))
                {
                    farmsIds.Add(result);
                }
            }
        }
        return new FarmModel() { FarmIds = farmsIds };
    }
}

Model

public class FarmModel
{
    public IEnumerable<int> FarmIds { get; set; }
}

Adding of the custom binder

System.Web.Mvc.ModelBinders.Binders.Add(typeof(FarmModel), new FarmModelBinder());

Answer 4

Taken from my answer:

I will show you here a very simple custom model binder I have just written (and tested in .Net Core 2.0):

My model binder:

public class CustomModelBinder : IModelBinder
{
    public Task BindModelAsync(ModelBindingContext bindingContext)
    {
        var valueProviderResult = bindingContext.ValueProvider.GetValue(bindingContext.ModelName);
        var value = valueProviderResult.FirstValue; // get the value as string

        var model = value.Split(",");
        bindingContext.Result = ModelBindingResult.Success(model);

        return Task.CompletedTask;
    }
}

My model (and notice, only one property has my custom model binder annotation):

public class CreatePostViewModel
{
    [Display(Name = nameof(ContentText))]
    [MinLength(10, ErrorMessage = ValidationErrors.MinLength)]
    public string ContentText { get; set; }

    [BindProperty(BinderType = typeof(CustomModelBinder))]
    public IEnumerable<string> Categories { get; set; } // <<<<<< THIS IS WHAT YOU ARE INTERESTER IN

    #region View Data
    public string PageTitle { get; set; }
    public string TitlePlaceHolder { get; set; }
    #endregion
}

What it does is: it receives some text like "aaa,bbb,ccc", and converts it into array, and return it to the ViewModel.

I hope that helps.

DISCLAIMER: I am not an expert in model binders writing, I have learn that 15 minutes ago, and I found your question (with no helpful answer), so I tried to help. This is a very basic model binder, some improvements are surely required. I learned how to write it from the official documentation page.