var projectionTypedCamel = Builders<CamelDocument>.Projection.Expression(x => new CamelDocument { Id = x.Id, Name = x.Name });
var queryTypedCamel = camelColl.Aggregate().Project(projectionTypedCamel).Limit(1);
var resultTypedCamel = queryTypedCamel.FirstOrDefault();

{ "_id" : ObjectId("000000000000000000000000"), "name" : null, "activeSince" : ISODate("0001-01-01T00:00:00Z"), "isActive" : false }

{ "_id" : ObjectId("000000000000000000000000"), "Name" : "Test638544783272212590", "ActiveSince" : ISODate("0001-01-01T00:00:00Z"), "IsActive" : false }

{ "_id" : ObjectId("66746157f75c0df9cadda90e"), "Name" : "Test638544783270965320" }

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

ConventionRegistry.Register("camelCaseConvention", new ConventionPack { new CamelCaseElementNameConvention() }, t => t == typeof(CamelDocument));
ConventionRegistry.Register("extraElementsConvention", new ConventionPack { new IgnoreExtraElementsConvention(true) }, _ => true);

var mongoSettings = MongoClientSettings.FromUrl(new MongoUrl("mongodb://localhost:27017"));
mongoSettings.LinqProvider = LinqProvider.V2;
var client = new MongoClient(mongoSettings);
var db = client.GetDatabase("test");
var pascalColl = db.GetCollection<PascalDocument>("pascal");
var camelColl = db.GetCollection<CamelDocument>("camel");

pascalColl.DeleteMany("{}");
camelColl.DeleteMany("{}");

var camel = new CamelDocument
{
    Id = ObjectId.GenerateNewId(),
    Name = $"Test{DateTime.Now.Ticks}",
    ActiveSince = DateTime.UtcNow,
    IsActive = true
};
camelColl.InsertOne(camel);
var pascal = new PascalDocument
{
    Id = ObjectId.GenerateNewId(),
    Name = $"Test{DateTime.Now.Ticks}",
    ActiveSince = DateTime.UtcNow,
    IsActive = true
};
pascalColl.InsertOne(pascal);

var projectionTypedCamel = Builders<CamelDocument>.Projection.Expression(x => new CamelDocument { Id = x.Id, Name = x.Name });
var queryTypedCamel = camelColl.Aggregate().Project(projectionTypedCamel).Limit(1);
var resultTypedCamel = queryTypedCamel.FirstOrDefault();

var projectionTypedPascal = Builders<PascalDocument>.Projection.Expression(x => new PascalDocument { Id = x.Id, Name = x.Name });
var queryTypedPascal = pascalColl.Aggregate().Project(projectionTypedPascal).Limit(1);
var resultTypedPascal = queryTypedPascal.FirstOrDefault();

var projectionAnonymousCamel = Builders<CamelDocument>.Projection.Expression(x => new { x.Id, x.Name });
var queryAnonymousCamel = camelColl.Aggregate().Project(projectionAnonymousCamel).Limit(1);
var resultAnonymousCamel = queryAnonymousCamel.FirstOrDefault();

var projectionAnonymousPascal = Builders<PascalDocument>.Projection.Expression(x => new { x.Id, x.Name });
var queryAnonymousPascal = pascalColl.Aggregate().Project(projectionAnonymousPascal).Limit(1);
var resultAnonymousPascal = queryAnonymousPascal.FirstOrDefault();

Console.WriteLine("Generated MQL is identical across queries (modulo casing of field names)...");
Console.WriteLine("Query with projection to typed (camel):      " + queryTypedCamel);
Console.WriteLine("Query with projection to typed (pascal):     " + queryTypedPascal);
Console.WriteLine("Query with projection to anonymous (camel):  " + queryAnonymousCamel);
Console.WriteLine("Query with projection to anonymous (pascal): " + queryAnonymousPascal);
Console.WriteLine("Deserialized results are incorrect for typed queries...");
Console.WriteLine($"Result with projection (camel) to CamelDocument:   {resultTypedCamel.ToJson()}");
Console.WriteLine($"Result with projection (pascal) to PascalDocument: {resultTypedPascal.ToJson()}");
Console.WriteLine($"Result with projection (camel) to anonymous:       {resultAnonymousCamel.ToJson()}");
Console.WriteLine($"Result with projection (pascal) to anonymous:      {resultAnonymousPascal.ToJson()}");

record CamelDocument
{
    public ObjectId Id { get; set; }
    public string Name { get; set; }
    public DateTime ActiveSince { get; set; }
    public bool IsActive { get; set; }
}

record PascalDocument
{
    public ObjectId Id { get; set; }
    public string Name { get; set; }
    public DateTime ActiveSince { get; set; }
    public bool IsActive { get; set; }
}