How can I mock object which is created with the action in the runtime? In the following code I want to mock the Connection class for the unit test.

private readonly Dictionary<IPEndPoint, IDeviceConnection> _connections = new Dictionary<IPEndPoint, IDeviceConnection>();

private async Task DiscoveryTask(CancellationToken stoppingToken)
{
    try
    {
        while (!stoppingToken.IsCancellationRequested)
        {
                // Action which allows new Connection to be created and added to dict
                _connections.Add(
                    ipEndPoint,
                    new DeviceConnection(
                        ipEndPoint,
                        new TcpClient(),
                        stoppingToken));

                _connections[ipEndPoint].StartConnectionTask();
            }
        }
    }
    catch (Exception ex)
    {

    }
}

Advices on modification the code is acceptable.