Converting object property values to dictionary

Question

Lets say, I have a list of objects like this

public class Room{
    public string Name {get; set;}
    public int[] UserId {get; set;}
}

What could be an efficient way of converting this list to dictionary like the following

Dictionary<int, List<string>> 

where the key is the UserId and String is the name of the room. Thanks in Advance.

Answer 1

I would use Linq's Aggregate method as shown below. ( Note I took liberties with the original object to be a List vs an array for the demo, but you can change that ).

var names = new List<Room>()
{
    new Room() { Name = "Alpha", UserId = new List<int> { 10, 40 }},
    new Room() { Name = "Omega", UserId = new List<int> { 10, 20, 30 }},
};

// Aggregate needs an item to pass around, we will
// seed it with the dictionary which will be the ultimate returned
// value. The following lambda takes a dictionary object (`dict`)
// and the current room ('current') to add to the dictionary. 
names.Aggregate (new Dictionary<int, List<string>>(), (dict, current) =>
            {
               current.UserId.ForEach(id => {
                                               if (dict.ContainsKey(id) == false)
                                                  dict.Add(id, new List<string>());

                                               dict[id].Add(current.Name);
                                            });
                return dict;
            });

Result:

Answer 2

Concept

• Iterate through the List of Rooms.
• If the dictionary contains the UserID add the name to the associated list.
• Otherwise make a new dictionary item.

Implementation

foreach (Room r in RoomsList) {
    foreach (int id in r.UserID) {
        if (RoomDictionary.Contains(id))
            RoomDictionary[id].Add(r.Name);
        else
            RoomDicationary.Add(id, new List<string>() { r.Name });
    }
}

Or something like that I just typed that into the web browser so it may need some adjusting but something like that.

Answer 3

Just some clarification, since UserId is declared as an array from your class Room. Assuming it is not an array, and 1 id corresponds to 1 room name. A RoomCollection implementing IDictionary would give you better control as shown below:

 public class RoomCollection : IDictionary<int, string>
{
    private Dictionary<int, string> roomCollection = new Dictionary<int, string>();

    //Add modified version of Add()
    public void Add(Room room)
    {
        //Do something to efficiently check whether room already exists
        this.Add(room.UserId, room.Name);
    }

    public void Add(int key, string value)
    {
        //Checking can be done here
        if (this.roomCollection.ContainsKey(key))
        {
            this.roomCollection[key] = value; //Overwrite values
        }
        else
        {
            this.roomCollection.Add(key, value); //Create new item
        }
    }

    //Modify other functionalities to your own liking
    public bool ContainsKey(int key)
    {
        return this.roomCollection.ContainsKey(key);
    }

    public ICollection<int> Keys
    {
        get { throw new NotImplementedException(); }
    }

    public bool Remove(int key)
    {
        throw new NotImplementedException();
    }

    public bool TryGetValue(int key, out string value)
    {
        throw new NotImplementedException();
    }

    public ICollection<string> Values
    {
        get { throw new NotImplementedException(); }
    }

    public string this[int key]
    {
        get
        {
            throw new NotImplementedException();
        }
        set
        {
            throw new NotImplementedException();
        }
    }

    public void Add(KeyValuePair<int, string> item)
    {
        throw new NotImplementedException();
    }

    public void Clear()
    {
        throw new NotImplementedException();
    }

    public bool Contains(KeyValuePair<int, string> item)
    {
        throw new NotImplementedException();
    }

    public void CopyTo(KeyValuePair<int, string>[] array, int arrayIndex)
    {
        throw new NotImplementedException();
    }

    public int Count
    {
        get { throw new NotImplementedException(); }
    }

    public bool IsReadOnly
    {
        get { throw new NotImplementedException(); }
    }

    public bool Remove(KeyValuePair<int, string> item)
    {
        throw new NotImplementedException();
    }

    public IEnumerator<KeyValuePair<int, string>> GetEnumerator()
    {
        throw new NotImplementedException();
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        throw new NotImplementedException();
    }
}

public class Room
{
    public string Name { get; set; }
    public int UserId { get; set; }
}

public class TestCollection
{
    public void Test()
    {
        Room r1 = new Room();
        r1.UserId = 1;
        r1.Name = "Room One";

        Room r2 = new Room();
        r2.UserId = 2;
        r2.Name = "Room Two";

        RoomCollection roomCollection = new RoomCollection();
        roomCollection.Add(r1);
        roomCollection.Add(r2);

        foreach (int roomId in roomCollection.Keys)
        {
            Console.WriteLine("Room number: {0} - Room name: {1}", roomId, roomCollection[roomId]);
        }
    }
}

Answer 4

This seems quite straight forward:

var rooms = new List<Room>()
{
    new Room() { Name = "Alpha", UserId = new List<int>() { 10, 40 }.ToArray() },
    new Room() { Name = "Omega", UserId = new List<int>() { 10, 20, 30 }.ToArray() },
};

var query =
    from r in rooms
    from u in r.UserId 
    group r.Name by u;

var result = query.ToDictionary(x => x.Key, x => x.ToList());

The result I get is: