using System;

using MongoDB.Bson;

using System.Linq;

public class Program

{

    public static void Main()

    {

        BsonDocument doc = new BsonDocument

        (

            "Meeting", new BsonDocument

            (

                "Session", new BsonDocument

                (

                    "Time", "19:00"

                )

            )  

        );

        // Get value for Meeting.Session.Time

        ClientCode(doc, "Meeting.Session.Time");

        Console.WriteLine("---");

        // Get value for Meeting.Session.Time, which is invalid

        ClientCode(doc, "Meeting.Session.Time2");

    }

    public static void ClientCode(BsonDocument doc, string key)

    {

        bool hasKey = doc.TryGetChainedValue(key, out BsonValue @value);

        Console.WriteLine(key);

        Console.WriteLine($"Is Key existed: {hasKey}");

        Console.WriteLine($"Value: {@value}");

    }

}

​

public static class BsonDocumentExtensions

{

    public static bool TryGetChainedValue(this BsonDocument doc, string key, out BsonValue @value)

    {

        @value = default;

        // Get key, value from passed BsonDocument for first time, then refer to BsonValue to get key, value for nested property

        bool first = true;

        try

        {

            if (String.IsNullOrWhiteSpace(key))

                return false;

            string[] chainedKeys = key.Split(".").ToArray();

            bool hasKey = false;

            foreach (var chainKey in chainedKeys)

            {       

                if (first)

                {

                    hasKey = doc.TryGetValue(chainKey, out @value);

                    first = false;

                }

                else

                {

                    hasKey = (@value.ToBsonDocument()).TryGetValue(chainKey, out @value);

                }

​

                // Throw exception if key not existed.

                if (!hasKey)

                    throw new Exception();

            }

            return true;

        }

        catch

        {           

            @value = default;

            return false;   

        }

    }

}