I used to use this command FindOne to query the result from mongoDB with C# in the past and it can be used at that time. But now I use the same code but it doesn't work. What should I use instead of `FindOne`?

My code is like this:

```
        var connectionString = "mongodb://localhost";
        var client = new MongoClient(connectionString);
        var database = client.GetDatabase("extend");
        var collection = database.GetCollection<Entity>("user");

        var query = Query<Entity>.EQ(e => e.user_id, int.Parse(targetUser.CurrentUser));
        var entity_TargetUser = collection.FindOne(query);

```

When I try to run it, I got this error 

```
Error CS1061: 'IMongoCollection<Entity>' does not contain a definition for 'FindOne' and no extension method 'FindOne' accepting a first argument of type 'IMongoCollection<Entity>' could be found (are you missing a using directive or an assembly reference?) (CS1061)

```

What command that I can use instead of `FindOne`?