I ma using Newtonsofts\' Json.Net to serialize some and array of objects to json. The objects have a common set of properties but also have Meta property which is a diction
If your dictionary is a string to object dictionary could can simply use the [JsonExtensionData] attribute:
string
object
[JsonExtensionData]
[JsonExtensionData] public Dictionary Meta { get; set; }
See How to serialize a Dictionary as part of its parent object using Json.Net.