Only Add Unique Item To List

Question

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

User · Answer

HashSet allows only the unique values to the list HashSet lt int gt  uniqueList   new HashSet lt int gt      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 lt int  string gt  dict   new Dictionary lt int  string gt      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 lt string  int gt  dictRev   new Dictionary lt string  int gt      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

User · Answer

Just like the accepted 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 Contains   on a custom class object requires implementing IEquatable lt T gt  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 at https   msdn microsoft com en-us library ms224763 aspx  public class RemoteDevice  IEquatable lt RemoteDevice gt        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

User · Answer

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  pstrjds links to below  or here  to define the equality operator or you ll need to implement the equality methods in RemoteDevice  GetHashCode  amp  Equals

[c#] Only Add Unique Item To List

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