using MongoDB.Driver;
using MongoDB.Driver.Linq;

var clientSettings = MongoClientSettings.FromConnectionString("SomeConnectionString");
clientSettings.LinqProvider = LinqProvider.V2;
var clientV2 = new MongoClient(clientSettings);
clientSettings.LinqProvider = LinqProvider.V3;
var clientV3 = new MongoClient(clientSettings);
var dbV2 = clientV2.GetDatabase("Test");
var dbV3 = clientV3.GetDatabase("Test");

var collectionV2 = dbV2.GetCollection<Data>("Test");
var collectionV3 = dbV3.GetCollection<Data>("Test");

var now = DateTime.UtcNow;
await collectionV3.InsertOneAsync(new Data() { StartDate = now, SpawnPeriod = SpawnPeriod.LIVE });

// Works.
var competitionSetV2 = (await collectionV2.Find("{}").Project(x => new SpawnData(x.StartDate, x.SpawnPeriod)).ToListAsync()).ToHashSet();
// Worked all the way up until 2.18.0. Fails on 2.19.0 in NewExpressionToAggregationExpressionTranslator.GetMatchingPropertyName().
var competitionSetV3 = (await collectionV3.Find("{}").Project(x => new SpawnData(x.StartDate, x.SpawnPeriod)).ToListAsync()).ToHashSet();

;

public class Data
{
    public DateTime StartDate;
    public SpawnPeriod SpawnPeriod;
}

public enum SpawnPeriod { LIVE, MIDNIGHT, MORNING, EVENING }

public struct SpawnData
{
    public readonly DateTime Date;
    public readonly SpawnPeriod Period;

    public SpawnData(DateTime date, SpawnPeriod period)
    {
        // Normally there is more complex handling here, value-type semantics are important, there are custom comparison operators, etc. hence the point of this struct.
        Date = date;
        Period = period;
    }

    public bool Equals(SpawnData other) => Date == other.Date && Period == other.Period;
}