new Data(x.Value) { Optional = 42 }

new Data(x.Value)

[Fact]
public void Projection_using_MemberInit_should_work()
{
    RequireServer.Check().Supports(Feature.FindProjectionExpressions);
    var collection = CreateCollection(LinqProvider.V3);
    var find = collection.Find("{}").Project(x => new SpawnDataWithOptionalExtraInfo(x.StartDate, x.SpawnPeriod) { ExtraInfo = 42 });
                                                                                                                                                                        
    var results = find.ToList();
                                                                                                                                                                        
    var projection = find.Options.Projection;
    var serializerRegistry = BsonSerializer.SerializerRegistry;
    var documentSerializer = serializerRegistry.GetSerializer<MyData>();
    var renderedProjection = projection.Render(documentSerializer, serializerRegistry, LinqProvider.V3);
    renderedProjection.Document.Should().Be("{ Date : '$StartDate', Period : '$SpawnPeriod', ExtraInfo: 42, _id : 0 }");
                                                                                                                                                                        
    results.Should().HaveCount(1);
    results[0].Date.Should().Be(new DateTime(2023, 1, 2, 3, 4, 5, DateTimeKind.Utc));
    results[0].Period.Should().Be(SpawnPeriod.LIVE);
    results[0].ExtraInfo.Should().Be(42);
}
                                                                                                                                                                        
private struct SpawnDataWithOptionalExtraInfo
{
    public readonly DateTime Date;
    public readonly SpawnPeriod Period;
    public int ExtraInfo;
                                                                                                                                                                        
    public SpawnDataWithOptionalExtraInfo(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;
        ExtraInfo = 42;
    }
                                                                                                                                                                        
    public bool Equals(SpawnDataWithOptionalExtraInfo other) => Date == other.Date && Period == other.Period && ExtraInfo == other.ExtraInfo;
}