CODEWITHSUNDEEP
c#.netc#-4.0
Oli
Oli

Reputation: 3136

Only Add Unique Item To List

I'm adding remote devices to a list as they announce themselves across the network. I only want to add the device to the list if it hasn't previously been added.

The announcements are coming across an async socket listener so the code to add a device can be run on multiple threads. I'm not sure what I'm doing wrong but no mater what I try I end up with duplications. Here is what I currently have.....

lock (_remoteDevicesLock)
{
    RemoteDevice rDevice = (from d in _remoteDevices
                            where d.UUID.Trim().Equals(notifyMessage.UUID.Trim(), StringComparison.OrdinalIgnoreCase)
                            select d).FirstOrDefault();
     if (rDevice != null)
     {
         //Update Device.....
     }
     else
     {
         //Create A New Remote Device
         rDevice = new RemoteDevice(notifyMessage.UUID);
         _remoteDevices.Add(rDevice);
     }
}

Upvotes: 129

Views: 196116

Answers (3)

Luke Eckley
Luke Eckley

Reputation: 360

Just like Austin Salonen's answer says, a HashSet doesn't have an order. If order is important you can continue to use a List and check if it contains the item before you add it.

if (_remoteDevices.Contains(rDevice))
    _remoteDevices.Add(rDevice);

Performing List<T>.Contains() on a custom class/object requires implementing IEquatable<T> on the custom class or overriding the Equals. It's a good idea to also implement GetHashCode in the class as well. This is per the documentation.

public class RemoteDevice: IEquatable<RemoteDevice>
{
    private readonly int id;
    public RemoteDevice(int uuid)
    {
        id = id
    }
    public int GetId
    {
        get { return id; }
    }

    // ...

    public bool Equals(RemoteDevice other)
    {
        if (this.GetId == other.GetId)
            return true;
        else
            return false;
    }
    public override int GetHashCode()
    {
        return id;
    }
}

Upvotes: 21

Austin Salonen
Austin Salonen

Reputation: 50215

If your requirements are to have no duplicates, you should be using a HashSet.

HashSet.Add will return false when the item already exists (if that even matters to you).

You can use the constructor that pstrjd mentioned below (or here) to define the equality operator or you'll need to implement the equality methods in RemoteDevice (GetHashCode & Equals).

Upvotes: 201

RavinderReddy Seelam
RavinderReddy Seelam

Reputation: 917

//HashSet allows only the unique values to the list
HashSet<int> uniqueList = new HashSet<int>();

var a = uniqueList.Add(1);
var b = uniqueList.Add(2);
var c = uniqueList.Add(3);
var d = uniqueList.Add(2); // should not be added to the list but will not crash the app

//Dictionary allows only the unique Keys to the list, Values can be repeated
Dictionary<int, string> dict = new Dictionary<int, string>();

dict.Add(1,"Happy");
dict.Add(2, "Smile");
dict.Add(3, "Happy");
dict.Add(2, "Sad"); // should be failed // Run time error "An item with the same key has already been added." App will crash

//Dictionary allows only the unique Keys to the list, Values can be repeated
Dictionary<string, int> dictRev = new Dictionary<string, int>();

dictRev.Add("Happy", 1);
dictRev.Add("Smile", 2);
dictRev.Add("Happy", 3); // should be failed // Run time error "An item with the same key has already been added." App will crash
dictRev.Add("Sad", 2);

Upvotes: 46

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