How to aggregate $lookup with MongoDB C# driver?

To avoid running [0] you can run $unwind since there's clearly 1:1 relationship between joined collections (you join them by _id). The in-memory part has to be converted into below $match:

{    $match: {        $expr: {            $or: [                { $ne: [ "$Replicated.lastReplicationStatus", "success" ] },                { $ne: [ "$Replicated.eSeq", "$Replicated.lastReplicationSequence" ] },            ]        }    }}

which would look like below in C#:

var q = _channels.Aggregate()                    .Lookup(                    foreignCollectionName: "JournalInstructionReplication",                     localField: "_id",                     foreignField:"_id",                     @as: "Replicated")    .Unwind("Replicated")    .Match(new BsonDocument()    {        { "$expr", new BsonDocument()        {            { "$or", new BsonArray()            {                new BsonDocument(){{ "$ne", new BsonArray(){ "$Replicated.lastReplicationStatus", "success" } }},                new BsonDocument(){{ "$ne", new BsonArray(){ "$Replicated.eSeq", "$Replicated.lastReplicationSequence" } }            } }        } }    }});var result = q.ToList();

Thanks a lot. It worked as expected.Final working code as below:

 new BsonDocument("$lookup", new BsonDocument()                        .Add("from", "JournalInstructionReplication")                        .Add("localField", "_id")                        .Add("foreignField", "_eid")                        .Add("as", "Replicated")),                 new BsonDocument("$unwind", new BsonDocument()                        .Add("path", "$Replicated")),                   new BsonDocument("$match", new BsonDocument()                        .Add("$expr", new BsonDocument()                                .Add("$or", new BsonArray()                                        .Add(new BsonDocument()                                                .Add("$ne", new BsonArray()                                                        .Add("$Replicated.lastReplicationStatus")                                                        .Add("success")                                                )                                        )                                        .Add(new BsonDocument()                                                .Add("$ne", new BsonArray()                                                        .Add("$Replicated.eSeq")                                                        .Add("$Replicated.lastReplicationSequence")                                                )                                        )                                )                        ))            };            var cursor = await collection.AggregateAsync(pipeline, options);            List<BsonDocument> list = cursor.ToList();