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<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 at https://msdn.microsoft.com/en-us/library/ms224763.aspx
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;
}
}