Get only a specified field in MongoDB with C# - mongodb

Question

first time i'm using MongoDB.
I have read this example:
SELECT a,b FROM users WHERE age=33
db.users.find({age:33}, {a:1,b:1})
But I can't translate it into C#. Can anyone help me?

Answer 1

I have translated your query below using the new C# driver (2.2)
var mongoClient = new MongoClient(""mongodb://127.0.0.1:27017"");
var database = mongoClient.GetDatabase("databaseName");
IMongoCollection<Users> _collection = database.GetCollection<Users>("Users");
var condition = Builders<Users>.Filter.Eq(p => p.age, 33);
var fields = Builders<Users>.Projection.Include(p => p.a).Include(p => p.b);
var results= _collection.Find(condition).Project<Users>(fields).ToList().AsQueryable();

Answer 2

You can do it using SetFields method of MongoCursor class, below full example:
var server = MongoServer.Create(connectionString);
var db = _server.GetDatabase("dbName");
var users = db.GetCollection("users");
var cursor = users.FindAs<DocType>(Query.EQ("age", 33));
cursor.SetFields(Fields.Include("a", "b"));
var items = cursor.ToList();

Answer 3

you can use anonymous class
public class User
{
public int age;
public string a;
public string b;
}
var collection = db.GetCollection<User>("Users");
var results = collection.Find(Builders<User>.Filter.Eq(user => user.age, 33))
.Project(u => new { u.a, u.b }).ToList();

Answer 4

//create user class
//(not sure how your class looks like)
public class User
{
public int age;
public string a;
public string b;
}
//then you can use LINQ easily
var server = MongoServer.Create(connectionString);
var db = server.GetDatabase("dbName");
var usersCollection = db.GetCollection<User>("users");
var filteredCollection = usersCollection.AsQueryable().Where(x=> x.age < 33).Where(x=> x.a != null).Contains(x=> x.b != null);