Wsrtjtyk

I deserialize object where one of the properties is a foreign key (eg an identity value from a database table). During deserialization I would like to use a JsonConverter to retrieve the corresponding object from a collection.

I know how to write a use custom JsonConverters. I don't know how to pass the collection to the JsonConverter, because the converter is specified at design time (like below), but the collection obviously only exists at runtime:

  <JsonConverter(GetType(JSonCustomConverter))>

  Public Property SomeProperty As SomePropertyClass

So the JSonCustomConverter's ReadJson should look this:

Public Overrides Function ReadJson(reader As JsonReader, objectType As Type, existingValue As Object, serializer As JsonSerializer) As Object

If reader.Value Is Nothing Then Return False

  Dim value As String = reader.Value.ToString().Trim()

  retun MagicallyGetMyCollectionValue(value)

End Function

So the silly function name MagicallyGetMyCollectionValue is just a placeholder to show you where I am stuck. I don't want to access the collection through a global variable, but I don't know how to pass the collection to the ReadJson either.

I would be happy, if someone could point me in the right direction.

Edit: Let me try to give a better example.

Suppose I have the following class:

class ParentObject

  <JssonConverter(GetType(JsonCustomConverter))>

  Property SomeProperty As SomePropertyClass

end class

I would deserialize my json data like this:

 dim result = JsonConvert.DeserializeObject(jsonData, GetType(ParentObject))

Now assume, that the json data doesn't contain the complete representation of an instance of the SomePropertyClass, but only a key value e.g. an key as string. Suppose I have a collection like this:

dim cache as Dictionary(of string, SomePropertyClass)

That cache shall contain all the instances that I need. So my JSonCustomConverter should have a ReadJson Function like this:

Public Overrides Function ReadJson(reader As JsonReader, objectType As Type, existingValue As Object, serializer As JsonSerializer) As Object

If reader.Value Is Nothing Then Return nothing

  Dim value As String = reader.Value.ToString().Trim()

  Dim cachedObject as SomePropertyClass = nothing

  if cache.TryGetValue(value, cachedObject) then return cachedObject

  retun Nothing ' or new SomePropertyClass(value)

End Function

So I want the ReadJson to lookup the instance based on the key value.

How would I pass the cache-Dictionary into the ReadJson function? I could use a singelton class that contains the cache an som getInstance-method to retrieve it, but I wouldn't want to do this.

As requested by @doom87er I will share the code, that worked for me. The solution is based on the comment by @dbc, with some changes. Please treat below code more like a conceptual code: I had to change some of the names and leave out some logik, that isn't required for this proof of concept. So there might be typos in it.

The main solution is to subclass the DefaultContractResolver and add the the cache-dictionary to that class. Something like this:

Public Class CacheContractResolver

    Inherits DefaultContractResolver

    Public Cache As Dictionary(of string, SomePropertyClass)



    Public Sub New(preFilledCache As Dictionary(of string, SomePropertyClass)

        Me.Cache = preFilledCache 

    End Sub

End Class

Then you pass the custom contract resolver using the JsonSerializerSettings like this:

Dim settings = New JsonSerializerSettings

settings.ContractResolver = New SupportControllerContractResolver(prefilledCache)

Dim result = JsonConvert.DeserializeObject(Of ParentObject)(jsonData, settings)

where prefilledCache is an instance of a dictionary containing the SomePropertyClass-objects.

The last step is to retrieve the cache in my JsonConverter's ReadJson function (that I attached to the SomeProperty as shown in the original post's sample code):

Public Overrides Function ReadJson(reader As JsonReader, objectType As Type, existingValue As Object, serializer As JsonSerializer) As Object

  Dim cacheResolver = TryCast(serializer.ContractResolver, CacheContractResolver)

  if cacheResolver is nothing return nothing ' add some better null handling here



  Dim value As String = reader.Value.ToString().Trim()

  Dim cachedObject as SomePropertyClass = nothing

  if cacheResolver.Cache.TryGetValue(value, cachedObject) then return cachedObject

  retun Nothing ' or new SomePropertyClass(value)

End Function

I tried it and it seems to work.

So in a nut shell:

1. Subclass the DefaultContractResolver and include all the additional data, that you need.


2. Pass an instance of you custom contract resolver with the additional data in the JsonSerializerSettings.


3. In your JsonConverter trycast the passed contract resolver back to your custom contract resolver and there you have your additional data.

I would be happy if you comment on any catches that I might miss, but I think this should be solution that I can live with.

Thanks for you coments and help.
Sascha

You can pass additional data to your custom JsonConverter using StreamingContext.Context via JsonSerializer.Context. Using this mechanism it becomes possible to map class instances to names in a generic manner.

First, define the following interfaces and generic converter:

Public Interface ISerializationContext

    Function TryGetNameTable(Of T)(ByRef table as INameTable(Of T)) as Boolean

End Interface



Public Interface INameTable(Of T)

    Function TryGetName(value As T, ByRef name as String) As Boolean    

    Function TryGetValue(name as String, ByRef value as T) As Boolean

End Interface



Public Class NameTable(Of T) :  Implements INameTable(Of T)

    Public Property Dictionary as Dictionary(Of String, T) = New Dictionary(Of String, T)()



    Public Property ReverseDictionary as Dictionary(Of T, String) = New Dictionary(Of T, String)()



    Public Function Add(value as T, name as String) as T

        Dictionary.Add(name, value)

        ReverseDictionary.Add(value, name)              

        Return value

    End Function



    Public Function TryGetName(value As T, ByRef name as String) As Boolean Implements INameTable(Of T).TryGetName

        Return ReverseDictionary.TryGetValue(value, name)

    End Function



    Function TryGetValue(name as String, ByRef value as T) As Boolean Implements INameTable(Of T).TryGetValue

        Return Dictionary.TryGetValue(name, value)

    End Function

End Class



Public Class ObjectToNameConverter(Of T)

    Inherits JsonConverter



    Public Overrides Function CanConvert(objectType As Type) As Boolean

        Return GetType(T) = objectType

    End Function



    Public Overrides Sub WriteJson(writer As JsonWriter, value As Object, serializer As JsonSerializer)

        Dim tValue = CType(value, T)

        Dim context as ISerializationContext = CType(serializer.Context.Context, ISerializationContext)

        If context Is Nothing

            Throw New JsonSerializationException("No ISerializationContext.")

        End If



        Dim nameTable as INameTable(Of T) = Nothing

        If (Not context.TryGetNameTable(Of T)(nameTable))

            Throw New JsonSerializationException("No NameTable.")

        End If



        Dim name as String = Nothing

        if (Not nameTable.TryGetName(tValue, name))

            Throw New JsonSerializationException("No Name.")

        End If



        writer.WriteValue(name)

    End Sub



    Public Overrides Function ReadJson(reader As JsonReader, objectType As Type, existingValue As Object, serializer As JsonSerializer) As Object

        Dim context as ISerializationContext = CType(serializer.Context.Context, ISerializationContext)

        If context Is Nothing

            Throw New JsonSerializationException("No ISerializationContext.")

        End If



        Dim nameTable as INameTable(Of T) = Nothing

        If (Not context.TryGetNameTable(Of T)(nameTable))

            Throw New JsonSerializationException("No NameTable.")

        End If



        Dim name As String = serializer.Deserialize(Of String)(reader)

        If name Is Nothing Then

            Return Nothing

        End If



        dim tValue as T = Nothing

        nameTable.TryGetValue(name, tValue)

        return tValue

    End Function

End Class

Next, define the following concrete implementations:

Public Class RootObject

    <JsonConverter(GetType(ObjectToNameConverter(Of SomePropertyClass)))> _

    Public Property SomeProperty As SomePropertyClass   

End Class



Public Class SomePropertyClass

End Class



Public Class MySerializationContext : Implements ISerializationContext

    Public Function Add(value as SomePropertyClass, name as String) as SomePropertyClass

        Return SomePropertyNameTable.Add(value, name)

    End Function



    Property SomePropertyNameTable as NameTable(Of SomePropertyClass) = New NameTable(Of SomePropertyClass)



    Public Function TryGetNameTable(Of T)(ByRef table as INameTable(Of T)) as Boolean Implements ISerializationContext.TryGetNameTable

        if (GetType(T) Is GetType(SomePropertyClass))

            table = SomePropertyNameTable

            return True

        End If



        table = Nothing

        return False

    End Function

End Class   

Now, you can replace instances of SomePropertyClass with their names during deserialization as follows:

Dim context as MySerializationContext = New MySerializationContext()

Dim someProperty as SomePropertyClass = context.Add(New SomePropertyClass(), "My Name")

Dim root as RootObject = New RootObject With { .SomeProperty = someProperty }

Dim settings = new JsonSerializerSettings With _

{ _

    .Context = New System.Runtime.Serialization.StreamingContext(System.Runtime.Serialization.StreamingContextStates.All, context)

}

Dim json as String = JsonConvert.SerializeObject(root, settings)

Console.WriteLine(json) ' Prints {"SomeProperty":"My Name"}

dim root2 as RootObject = JsonConvert.DeserializeObject(Of RootObject)(json, settings)

' Assert that the same instance of SomeProperty was used during deserialization

Assert.IsTrue(root2.SomeProperty Is root.SomeProperty) 

Assert.IsTrue(json.Equals("{""SomeProperty"":""My Name""}"))

Notes:

• 
  

  ISerializationContext.TryGetNameTable(Of T)(ByRef table as INameTable(Of T)) is generic so that object-to-name replacement can be supported for multiple types of objects simultaneously without the converters interfering with each other.

  
  
  

  The concrete implementation need not be so generic, however. Here MySerializationContext only supports name replacement for instances of SomePropertyClass. Others could be added as needed.

  
  



• As stated in Does Json.NET cache types' serialization information?, Newtonsoft recommends caching instances of DefaultContractResolver and its subtypes for best performance. Thus it may be preferable to pass additional data via StreamingContext.Context rather than via freshly allocated instances of subclasses of DefaultContractResolver.

Sample working .Net fiddle #1 here.

As an alternative, while the design above works, in my opinion it would be simpler to remove <JsonConverter(GetType(ObjectToNameConverter(Of SomePropertyClass)))> from SomeProperty and instead pass an appropriately initialized ObjectToNameConverter(Of SomePropertyClass), containing a local reference to some INameTable(Of SomePropertyClass), in JsonSerializerSettings.Converters.

Define the converter and interfaces like so. Notice that ObjectToNameConverter(Of T) now has a parameterized constructor and that ISerializationContext is no longer required:

Public Interface INameTable(Of T)

    Function TryGetName(value As T, ByRef name as String) As Boolean    

    Function TryGetValue(name as String, ByRef value as T) As Boolean

End Interface



Public Class NameTable(Of T) :  Implements INameTable(Of T)

    Public Property Dictionary as Dictionary(Of String, T) = New Dictionary(Of String, T)()



    Public Property ReverseDictionary as Dictionary(Of T, String) = New Dictionary(Of T, String)()



    Public Function Add(value as T, name as String) as T

        Dictionary.Add(name, value)

        ReverseDictionary.Add(value, name)              

        Return value

    End Function



    Public Function TryGetName(value As T, ByRef name as String) As Boolean Implements INameTable(Of T).TryGetName

        Return ReverseDictionary.TryGetValue(value, name)

    End Function



    Function TryGetValue(name as String, ByRef value as T) As Boolean Implements INameTable(Of T).TryGetValue

        Return Dictionary.TryGetValue(name, value)

    End Function

End Class



Public Class ObjectToNameConverter(Of T)

    Inherits JsonConverter



    Private Property NameTable as INameTable(Of T)



    Public Sub New(nameTable as INameTable(Of T))

        If nameTable Is Nothing 

            Throw new ArgumentNullException("nameTable")

        End If

        Me.NameTable = nameTable

    End Sub



    Public Overrides Function CanConvert(objectType As Type) As Boolean

        Return GetType(T) = objectType

    End Function



    Public Overrides Sub WriteJson(writer As JsonWriter, value As Object, serializer As JsonSerializer)

        Dim tValue = CType(value, T)



        Dim name as String = Nothing

        if (Not NameTable.TryGetName(tValue, name))

            Throw New JsonSerializationException("No Name.")

        End If



        writer.WriteValue(name)

    End Sub



    Public Overrides Function ReadJson(reader As JsonReader, objectType As Type, existingValue As Object, serializer As JsonSerializer) As Object

        Dim name As String = serializer.Deserialize(Of String)(reader)

        If name Is Nothing Then

            Return Nothing

        End If



        dim tValue as T = Nothing

        NameTable.TryGetValue(name, tValue)

        return tValue

    End Function

End Class

Then serialize as follows:

dim nameTable = new NameTable(Of SomePropertyClass)()

Dim someProperty as SomePropertyClass = nameTable.Add(New SomePropertyClass(), "My Name")

Dim root as RootObject = New RootObject With { .SomeProperty = someProperty }



Dim settings = new JsonSerializerSettings()

settings.Converters.Add(new ObjectToNameConverter(Of SomePropertyClass)(nameTable))



Dim json as String = JsonConvert.SerializeObject(root, settings)

Console.WriteLine(json) ' Prints {"SomeProperty":"My Name"}

dim root2 as RootObject = JsonConvert.DeserializeObject(Of RootObject)(json, settings)

' Assert that the same instance of SomeProperty was used during deserialization

Assert.IsTrue(root2.SomeProperty Is root.SomeProperty) 

Assert.IsTrue(json.Equals("{""SomeProperty"":""My Name""}"))

Doing things this way eliminates the dependence of static serialization methods on runtime code that is present in the first solution. Now all name replacement logic is handled in runtime in one location.

Sample fiddle #2 here.