c# - Applying JsonDictionary attribute to dictionary

Question

If I try to add the JsonDictionary attribute to a .net Dictionary(Of Integer, MyClass), the compiler tells me, that the attribute could not be applied. Why is this?

<JsonDictionary()>
Public ReadOnly Property monatswerte As Dictionary(Of Integer, MyClass)

I basically couldn't find any examples of how to use the JsonDictionary online.

Answer 1

<JsonDictionary()> can be applied to a type (a class or interface) to force Json.NET's default contract resolver (and its subclasses) to generate a dictionary contract for the type. It is part of a family of three similar attributes:

• <JsonObject()>. Force a type to be serialized as a JSON object. Useful to force serialization of collection properties instead of items as shown here.
• <JsonArray()>. Force a type to serialized as a JSON array. Possibly useful to, e.g., force a dictionary to be serialized as a key/value pair array.
• <JsonDictionary()>. Force a type to be interpreted as a dictionary and serialized as a JSON object. (It must still implement IDictionary or IDictionary<TKey, TValue> for this to work, of course.)

On the face of it <JsonDictionary()> does not seem that useful because DefaultContractResolver.CreateContract(Type objectType) checks that the incoming type implements IDictionary before checking for any other interface implementations. However, the attribute has several properties that could be useful in customizing how a dictionary is serialized, including:

• NamingStrategyType and NamingStrategyParameters allow control of casing and name-mapping of dictionary keys.

  For instance, the following dictionary will always serialize its keys literally, without renaming, even if CamelCasePropertyNamesContractResolver is in use:

  <JsonDictionary(NamingStrategyType := GetType(LiteralKeyDictionaryNamingStrategy))> _
  Public Class LiteralKeyDictionary(Of TValue)
      Inherits Dictionary(Of String, TValue)
  End Class
  
  Public Class LiteralKeyDictionaryNamingStrategy
      Inherits DefaultNamingStrategy
  
      Public Sub New()
          ProcessDictionaryKeys = False
      End Sub
  End Class
  

• ItemConverterType, ItemConverterParameters, ItemIsReference, ItemReferenceLoopHandlingand ItemTypeNameHandling allow customization of how dictionary values are serialized.

  For instance, in the following dictionary type, the values are always stored with reference preservation enabled:

  <JsonDictionary(ItemIsReference := true)> _
  Public Class ReferenceObjectDictionary(of TKey, TValue As {Class, New})
      Inherits Dictionary(Of TKey, TValue)
  End Class
  

  Or, for a dictionary type containing enum values, you might apply StringEnumConverter as an ItemConverterType to force the values to be serialized as strings.