As I mentioned before the ASP.NET MVC5’s DefaultModelBinder has some quirks. The actual one I met some days ago is the following.

public class MyViewModel
{
    public IEnumerable<int> IntList { get; set; }
}

What happens, when You call Your method with this JSON request?

{ IntList:[] }

I would like to find an empty IEnumerable<int> instance in IntList, but I will found null there.
Why? Because the DefaultModelBinder initializes my empty collection to null.

What You can do to avoid this is to write a custom model binder for this:

public class EmptyEnumerableCapableDefaultModelBinder:DefaultModelBinder
{
    public override object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
    {
       object ret = base.BindModel(controllerContext, bindingContext);

       WorkaroundEmptyEnumerablesInitializedToNullByDefaultModelBinder(ret, controllerContext, bindingContext);

       return ret;
    }
}

private void WorkaroundEmptyEnumerablesInitializedToNullByDefaultModelBinder(object model, ControllerContext controllerContext, ModelBindingContext bindingContext)
{
    if (model != null)
    {
        // workaround case when there is an IEnumerable<> member and there is a "member":[] in request
        // but default binder inits member to null

        var candidateList = bindingContext.PropertyMetadata
            .Where(kvp => bindingContext.PropertyFilter(kvp.Key))
            .Where(kvp => TypeHelper.IsSubclassOf(kvp.Value.ModelType, typeof(IEnumerable<>)))
            .Where(kvp => !bindingContext.ValueProvider.ContainsPrefix(kvp.Key)).ToArray();
        if (candidateList.Any())
        {
            if (!controllerContext.HttpContext.Request.ContentType.StartsWith("application/json"))
            {
                throw new NotImplementedException(controllerContext.HttpContext.Request.ContentType);
            }

            var json = GetJSONRequestBody(controllerContext);

            foreach(var candidate in candidateList)
            {
                var emptyEnumerablePattern = String.Format("\"{0}\":[],", candidate.Key);
                if (json.Contains(emptyEnumerablePattern))
                {
                    var pd = bindingContext.ModelType.GetProperty(candidate.Key);
                    var anEmptyArray = Array.CreateInstance(pd.PropertyType.GetGenericArguments()[0], 0);
                    pd.SetValue(model, anEmptyArray);
                }
            }
        }
    }
}

private string GetJSONRequestBody(ControllerContext controllerContext)
{
    string ret = null;

    var inputStream = controllerContext.HttpContext.Request.InputStream;
    inputStream.Position = 0;

    using (var sr = new StreamReader(inputStream, controllerContext.HttpContext.Request.ContentEncoding, false, 1024, true))
    {
        ret = sr.ReadToEnd();
    }

    return ret;
}

The point is to check the inputs on DefaultModelBinder’s null result whether it missed an empty enumerable on binding.
The valueProviders available in context are useless because they simply dont contain our IntList value. Instead I check our
target viewmodel for possible candidates and check their values directly in request. If an empty value found in request I
replace the binder result with an empty array instance which fits into IEnumerable<T> place.

Once upon a day I created a viewmodel class:

public class MyViewModel
{
    public List<KeyValuePair<string, int>> MyList { get; set; }
}

I wanted to use it as a parameter in my MVC action method. The wonderful model binding feature of MVC allows me to do that and it seemed to be working without error.
I got the exact number of key value pairs in my list property but the Key and Value props were always null and 0. I repeat: without any error!

After checking DefaultModelBinder’s source I realized that it will never work: KeyValuePair<,> is a struct, so assigning to variable means a copy and it’s members are readonly so can be set only during construction. The logic in DefaultModelBinder is different: it creates the model objects, handles them over via variable assignations, evaluates their member values and then assigns those values to members. There is a workaround implemented inside related to Dictionary<,>, but it’s logic isn’t reusable for my situation because the programmer didn’t intended to allow that (private methods) and the logic there is a bit smells for me.

There are solutions on the net, but those I found suffer from one common problem: they evaluate Key and Value on their onnw, which skips some goods of model binding, e.g. validation and model state propagation. Not too good.

Here comes my solution. 🙂

First I created a new default model binder which in case of KeyValuePair<,> model type calls my BindModelViaKeyValuePairSubstitute from BindModel method but leaves all other things handled by original implementation.

public class KeyValuePairCapableDefaultModelBinder:DefaultModelBinder
{
    public override object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
    {
       object ret = null;

       if (TypeHelper.IsSubclassOf(bindingContext.ModelType, typeof(KeyValuePair<,>)))
       {
           ret = BindModelViaKeyValuePairSubstitute(controllerContext, bindingContext);
       }
       else
       {
           ret = base.BindModel(controllerContext, bindingContext);
       }

       return ret;
    }
}

I created a substitute class which overcomes the limitations: not a struct and has writable members.
To make the trick transparent to model binding the substitute class must contain members of same name and type as the KeyValuePair<,> we want to handle.

        /// <summary>
        /// KeyValuePair substitute.
        /// </summary>
        private class KeyValuePairSubstitute<TKey, TValue> : KeyValuePairSubstituteBase
        {
            public TKey Key { get { return (TKey)KeyAsObject; } set { KeyAsObject = value; } }
            public TValue Value { get { return (TValue)ValueAsObject; } set { ValueAsObject = value; } }

            public KeyValuePairSubstitute()
            {
                // set values to defaults to avoid NullReferenceExceptions when trying to get
                // an uninitialized null value from a generic type which cannot stand that (e.g. int).
                this.Key = default(TKey);
                this.Value = default(TValue);
            }
        }
        
        /// <summary>
        /// Base class for KeyValuePair substitute to allow access to generic values in handy way.
        /// </summary>
        private class KeyValuePairSubstituteBase
        {
            public object KeyAsObject { get; set; }
            public object ValueAsObject { get; set; }
        }

Now my BindModelViaKeyValuePairSubstitute is trivial.
The logic here is to let DefaultModelBinder bind our substitute object instead of a KeyValuePair<,> and then instantiate a KeyValuePair<,> from that object’s content.

        /// <summary>
        /// Default BindModel call doesnt handle KeyValuePair well, because it is a struct and has readonly props.
        /// It will return an instance with default values without any error!
        /// </summary>
        private object BindModelViaKeyValuePairSubstitute(ControllerContext controllerContext, ModelBindingContext bindingContext)
        {
            object ret = null;

            var keyValuePairSubstituteGeneric = typeof(KeyValuePairSubstitute<,>).MakeGenericType(bindingContext.ModelType.GetGenericArguments());

            var kvpBindingContext = new ModelBindingContext()
            {
                ModelMetadata = ModelMetadataProviders.Current.GetMetadataForType(null, keyValuePairSubstituteGeneric),
                ModelName = bindingContext.ModelName,
                ModelState = bindingContext.ModelState,
                ValueProvider = bindingContext.ValueProvider
            };

            var keyValuePairSubstitute = (KeyValuePairSubstituteBase)base.BindModel(controllerContext, kvpBindingContext);

            ret = Activator.CreateInstance(bindingContext.ModelType, keyValuePairSubstitute.KeyAsObject, keyValuePairSubstitute.ValueAsObject);
            return ret;
        }

The last step: the new model binder should be registered in Application_Start as usual:

            ModelBinders.Binders.DefaultBinder = new CustomModelBinder();

That’s all. You have bindable and validable KeyValuePair<,>s now!

Today I found a bug. It was so interesting that I decided to write a longer post here about it.
I created a strip down solution with the only classes and methods I need to demonstrate the bug. This is the reason if the story wont seem too realistic.

A long long time ago I need a dictionary to store some integers with a key which was based on a string but has some other features (not shown here). So I created MyKey class for this:

[Serializable]
public class MyKey
{
    private string key = null;

    public MyKey(string key)
    {
        if (key == null)
        {
            throw new ArgumentNullException("key");
        }

        this.key = key;
    }

    private int? hashCode = null;
    public override int GetHashCode()
    {
        int ret = 0;

        if (hashCode == null)
        {
            hashCode = this.key.GetHashCode();
        }

        ret = hashCode.Value;

        return ret;
    }

    public override bool Equals(object obj)
    {
        bool ret = false;

        MyKey other = obj as MyKey;
        if (other != null)
        {
            ret = Equals(other);
        }

        return ret;
    }

    public bool Equals(MyKey other)
    {
        bool ret = false;

        if (other != null)
        {
            if (this.hashCode == other.hashCode)
            {
                if (this.key == other.key)
                {
                    ret = true;
                }
            }
        }

        return ret;
    }

    public override string ToString()
    {
        string ret = String.Concat("\"", key, "\"");
        return ret;
    }
}

It was used happily like this:

// create data
var data = new Dictionary<MyKey, int>();
data[new MyKey("alma")] = 1;

Later I wrote some code to persist these data via serialization.
Everything was working like a charm.

// serialize and save it
var serializedData = Serializer.Serialize(data);
SaveToFile(serializedData);

...

// load and deserialize data
var serializedData = LoadFromFile();
var data = Serializer.Deserialize(serializedData);

There was a usecase when after deserialization some of the values in data must be changed:

// as in deserialized data
var specificKey = new MyKey("alma");
if (data[specificKey] == 1) // a KeyNotFoundException occures here!
{
    data[specificKey] = 2;
}

KeyNotFoundException? I was sure that there should be a value in all of data instances with the given key! Lets see in QuickView:

There is an “alma” key!
Let’s comment out the line causing the exception and check data after the expected value modification to “2”:

Much more interesting isnt it?
I quickly put all the data creation, serialization, deserialization code into one unit test to have a working chunk of code I can use for bug hunting:

[TestMethod]
public void TestMethod1()
{
    var d = new Dictionary<mykey, int="">();
    d[new MyKey("alma")] = 1;

    var serialized = Serializer.Serialize(d);

    var data = Serializer.Deserialize(serialized);

    var specificKey = new MyKey("alma");
    {
        data[specificKey] = 2;
    }
}

But in the unit test everything was working! I simply cant reproduce the bug in such a way.
But when running App1, which was creating and serializing the data and running App2 which was deserializing and modifying it the bug always presents itself.
How can be a duplicate key in a Dictionary<,>? MyKey‘s implemetation, especially the Equals() override is so trivial that it cannot allow two instances created from
same string to be not equal.

But wait a minute!

How can the hashCode’s differ?!?!?!

Yes. A quick search on the net answers everything. MSDN clearly describes in a big “Important” box:

The hash code itself is not guaranteed to be stable. Hash codes for identical strings can differ across versions of the .NET Framework and across platforms (such as 32-bit and 64-bit) for a single version of the .NET Framework. In some cases, they can even differ by application domain.

As a result, hash codes should never be used outside of the application domain in which they were created, they should never be used as key fields in a collection, and they should never be persisted.

App1 was running in x86 and App2 in x64 environment. Thats why the string hashcodes differ.

The fix is really easy. Just turn off hashCode calculation optimalization for serialization:

[Serializable]
public class MyKey
{
   ...

   [NonSerialized]
   private int? hashCode = null;
   ...
}

Now hashCode will be recalculated once in all runtime environments.

I never thought about the possibility of unstable hashcodes.
I hope I am not the only man in the world with such wasteful brain.