Hi, @Divya_Vyas,

Welcome to the MongoDB Community Forums. I tried your query with the latest driver and it produced reasonable MQL. I had to make some assumptions about your POCOs and updated your Join to be between main and other rather than main and main. Here is the repro that I ran:

using System;
using MongoDB.Bson;
using MongoDB.Driver;
using MongoDB.Driver.Linq;

var client = new MongoClient();
var db = client.GetDatabase("test");
var main = db.GetCollection<Entity>("main");
var other = db.GetCollection<Entity>("other");

var mainCollection = main.AsQueryable();
var otherCollection = other.AsQueryable();

var sharedQuery = mainCollection
    .Join(otherCollection,
        main => main.Id,
        other => other.Id,
        (main, other) => new Entity
        {
            Id = main.Id,
            Model = main.Model,
            Body = main.Body,
            Status = main.Status,
            Version = main.Version,
            WorkFlowInfo = other.Body
        });
Console.WriteLine(sharedQuery);

public class Entity
{
    public ObjectId Id { get; set; }
    public string Model { get; set; }
    public string Body { get; set; }
    public string Status { get; set; }
    public string Version { get; set; }
    public string WorkFlowInfo { get; set; }
}

And the MQL output that it produced:

test.main.Aggregate([{ "$project" : { "_outer" : "$$ROOT", "_id" : 0 } }, { "$lookup" : { "from" : "other", "localField" : "_outer._id", "foreignField" : "_id", "as" : "_inner" } }, { "$unwind" : "$_inner" }, { "$project" : { "_id" : "$_outer._id", "Model" : "$_outer.Model", "Body" : "$_outer.Body", "Status" : "$_outer.Status", "Version" : "$_outer.Version", "WorkFlowInfo" : "$_inner.Body" } }])

Please file a CSHARP ticket with a self-contained repro and stack trace and we will be happy to investigate further with you.

Sincerely,
James