//3a. Lấy List Collection
public string ListCollections()
{
    // Replace the uri string with your MongoDB deployment's connection string.
    var client = new MongoClient(
        "mongodb://localhost:27017/?readPreference=primary&appname=MongoDB%20Compass&directConnection=true&ssl=false"
    );

    string re = "List Collections : <br/>";

    //lấy database
    var database = client.GetDatabase("HvDadabase");
    //Lấy List Collection
    var collectionList = database.ListCollections().ToList();

    foreach (var row in collectionList)
    {
        re += row + "<br/>";
    }

    return re;
}