var deviceId = "myDeviceId";
var connectionString = ConfigurationManager.AppSettings["myIoTHubConnectionString"];
if (string.IsNullOrEmpty(connectionString))
{
   connectionString = ConfigurationManager.ConnectionStrings["myIoTHubConnectionString"]?.ConnectionString;
}
if (string.IsNullOrEmpty(connectionString))
{
   throw new InvalidOperationException("Could not find connection string in appsettings or connectionStrings for IoT Hub.");
}
var deviceClient = DeviceClient.CreateFromConnectionString(connectionString, deviceId);