How to make ASP.NET to stop interpret null as string

Question

I have a Web API method:

public List<Task> GetTasks([FromUri] TaskFilter filter)
{

}

The method has parameter with list of nullable identifiers:

public class TaskFilter
{
  public IList<int?> Assignees { get; set; }
}

When I call it:

GET /tasks?assignees=null

Server returns an error:

{
  "message":"The request is invalid.",
  "modelState": {
    "assignees": [ "The value 'null' is not valid for Nullable`1." ]
  }
}

It works only if I pass empty string:

GET /tasks?assignees=

But standard query string converters (from JQuery, Angular, etc) do not work with nulls in such way.

How to make ASP.NET to interpret 'null' as null?

Upd: The query string can contain several identifiers, e.g.:

GET /tasks?assignees=1&assignees=2&assignees=null

Upd2: JQuery converts nulls in array to empty strings, and ASP.NET interprets them as null. So the question is about calling WebAPI from Angular 1.6 ($HttpParamSerializerProvider)

Upd3: I know about workarounds, but I do not ask for them. I want a solution for specific problem:

• It is a GET method
• Method accepts a list from Uri
• A list can contain null values
• It should be List<int?> because API docs are generated automatically, and I do not want to see text array as parameter type
• By default ASP.NET expects empty strings for null values (JQuery.param works in that way)
• But some client libraries (e.g. Angular) does not convert null array items to empty strings

Answer 1

Finally, I found a solution using custom value provider:

using System;
using System.Collections.Generic;
using System.Web.Http;
using System.Web.Http.Controllers;
using System.Web.Http.ValueProviders;
using System.Web.Http.ValueProviders.Providers;
using System.Globalization;
using System.Net.Http;
using System.Web.Http.ModelBinding;

public sealed class NullableValueProviderAttribute : ModelBinderAttribute
{
    private readonly string[] _nullableColumns;

    public NullableValueProviderAttribute(params string[] nullableColumns)
    {
        _nullableColumns = nullableColumns;
    }

    public override IEnumerable<ValueProviderFactory> GetValueProviderFactories(HttpConfiguration configuration)
    {
        return new ValueProviderFactory[] { new NullableValueProviderFactory(_nullableColumns) };
    }
}

public class NullableValueProviderFactory : ValueProviderFactory, IUriValueProviderFactory
{
    private readonly string[] _nullableColumns;

    public NullableValueProviderFactory(string[] nullableColumns)
    {
        _nullableColumns = nullableColumns;
    }

    public override IValueProvider GetValueProvider(HttpActionContext actionContext)
    {
        return new NullableQueryStringValueProvider(actionContext, CultureInfo.InvariantCulture, _nullableColumns);
    }
}

public class NullableQueryStringValueProvider : NameValuePairsValueProvider
{
    private static readonly string[] _nullValues = new string[] { "null", "undefined" };

    private static IEnumerable<KeyValuePair<string, string>> GetQueryNameValuePairs(HttpRequestMessage request, string[] nullableColumns)
    {
        foreach (var pair in request.GetQueryNameValuePairs())
        {
            var isNull = Array.IndexOf(nullableColumns, pair.Key) >= 0 && Array.IndexOf(_nullValues, pair.Value) >= 0;
            yield return isNull ? new KeyValuePair<string, string>(pair.Key, "") : pair;
        };
    }

    public NullableQueryStringValueProvider(HttpActionContext actionContext, CultureInfo culture, string[] nullableColumns) :
        base(GetQueryNameValuePairs(actionContext.ControllerContext.Request, nullableColumns), culture)
    { }
}

And specify it in Web API action:

public List<Task> GetTasks([NullableValueProvider("assignees")] TaskFilter filter)
{
}

Answer 2

You can create a custom model bind for this specific type, inherithing from DefaultModelBinder, for sample:

using System.ComponentModel.DataAnnotations;
using System.Web.Mvc;
public class TaskFilterBinder : IModelBinder
{
    public object BindModel(ControllerContext controllerContext, System.Web.Mvc.ModelBindingContext bindingContext)
    {
        var request = controllerContext.HttpContext.Request;

        var assignees = request.QueryString["assignees"];

        if (assignees == "null") // check if assignees is null (string) then return NULL
            return null;
        return assignees;
    }

}

Finally we need to inform the controller as to the binding we want it to use. This we can specify using attributes

[ModelBinder(typeof(TaskFilterBinder))]

as below:

public List<Task> GetTasks([FromUri(ModelBinder=typeof(TaskFilterBinder))] TaskFilter filter)
{
// Do your stuff.
}

For more reference check this link on Custom Model Binders. Hope, this solves your problem . Thanks