MongoDb c# driver find item in array by field value

There is ElemMatch

var filter = Builders<Post>.Filter.ElemMatch(x => x.Tags, x => x.Name == "test");var res = await collection.Find(filter).ToListAsync()

Here's an example that returns a single complex item from an array (using MongoDB.Driver v2.5.0):

Simple Data Model

public class Zoo{    public List<Animal> Animals { get; set; }}public class Animal{    public string Name { get; set; }}

Option 1 (Aggregation)

public Animal FindAnimalInZoo(string animalName){    var zooWithAnimalFilter = Builders<Zoo>.Filter        .ElemMatch(z => z.Animals, a => a.Name == animalName);    return _db.GetCollection<Zoo>("zoos").Aggregate()        .Match(zooWithAnimalFilter)        .Project<Animal>(            Builders<Zoo>.Projection.Expression<Animal>(z =>                 z.Animals.FirstOrDefault(a => a.Name == animalName)))        .FirstOrDefault(); // or .ToList() to return multiple}

Option 2 (Filter & Linq) This was about 5x slower for me

public Animal FindAnimalInZoo(string animalName){    // Same as above    var zooWithAnimalFilter = Builders<Zoo>.Filter        .ElemMatch(z => z.Animals, a => a.Name == animalName);    var zooWithAnimal = _db.GetCollection<Zoo>("zoos")        .Find(zooWithAnimalFilter)        .FirstOrDefault();    return zooWithAnimal.Animals.FirstOrDefault(a => a.Name == animalName);}

You need the $elemMatch operator. You could use Builders<T>.Filter.ElemMatch or an Any expression:

Find(x => x.Tags.Any(t => t.Name == "test")).ToListAsync()

http://mongodb.github.io/mongo-csharp-driver/2.0/reference/driver/expressions/#elemmatch