Filtro Linq para MongoDB

public class Test
{
    public ObjectId _id;
    public string ticker;
    public string exchange;
    public string localtick;
    public string compname;
    public string currency;
    public DateTime insertedtime;
}

var query = db.GetCollection<Test>("test")
    .AsQueryable()
    .Where(c => c.ticker.Contains("=CA"));