Convert MongoDB BsonDocument to valid JSON in C#

I am working with the MongoDB C# driver. I have a BsonDocument with some data which includes some MongoDB-specific types (like ObjectIDs and ISODates). I want to convert this to a valid general-purpose JSON string. In other words, I can't have something like _id: ObjectId(...) or date: ISODate(...) but would prefer _id: "..." and date: "...". Basically, I want to convert these special types that only MongoDB recognizes to regular strings so they can be parsed more easily. The problem is that a built-in function like .ToJson() (which another StackOverflow answer suggests) doesn't really convert the document to valid JSON at all because it maintains these special types. My document also contains many levels of arrays and sub-documents, so a simple for loop will not suffice. What's the best way to convert a BsonDocument that avoids this problem? I would prefer something built-in rather than manually recursing through the document to fix all the issues.

MongoDB.Bson (2.5+) has support to map between BsonValues and .Net objects. BsonTypeMapper Class

To map a BsonValue (or BsonDocument) to .Net object use

var dotNetObj = BsonTypeMapper.MapToDotNetValue(bsonDoc);

You can then use your choice of serialization library. For example,

JsonConvert.SerializeObject(dotNetObj);

If you have a List of BsonDocument

var dotNetObjList = bsonDocList.ConvertAll(BsonTypeMapper.MapToDotNetValue);

I've ran into the same thing, you can get valid JSON via:

var jsonWriterSettings = new JsonWriterSettings { OutputMode = JsonOutputMode.Strict };
JObject json = JObject.Parse(postBsonDoc.ToJson<MongoDB.Bson.BsonDocument>(jsonWriterSettings));

However it will return something like:

{"_id":{"$oid":"559843798f9e1d0fe895c831"}, "DatePosted":{"$date":1436107641138}}

I'm still trying to find a way to flatten that.