BsonToJson.cs

/**
 * Install-Package mongocsharpdriver
 * Install-Package newtonsoft.json
 */

var document = new BsonDocument
{
	{ "_id", Guid.NewGuid() },
	{ "Name", "Hagbarddenstore" },
	{
		"Message", new BsonDocument
		{
			{ "_id", Guid.NewGuid() },
			{ "CreatedOn", DateTime.UtcNow },
			{ "Value", "Hello, world!" }
		}
	}
};

var dictionary = document.ToDictionary();

Console.WriteLine(JsonConvert.SerializeObject(dictionary, Formatting.Indented));