# Azure Functions (Isolated Worker) Upgrade Version 4 to 5


## Default serializer has changed

The default serializer has changed from [Json.NET](/nservicebus/serialization/newtonsoft.md) to [System.Text.Json](/nservicebus/serialization/system-json.md).

The Json.NET serializer can continue to be used by adding a package reference to the `NServiceBus.Newtonsoft.Json` package and setting `NewtonsoftJsonSerializer` as the serializer:

<!-- snippet: ASBFunctionsWorker-4to5-serializer -->

```cs
var host = new HostBuilder()
.ConfigureFunctionsWorkerDefaults()
.UseNServiceBus(configuration =>
{
    configuration.AdvancedConfiguration.UseSerialization<NewtonsoftJsonSerializer>();
})
.Build();
```

<!-- endsnippet -->