# Upgrade Version 1 to 2



## Enable callbacks

Callbacks have been made opt-in. In order to make callbacks work they have to be enabled explicitly. 

```csharp
var instanceDiscriminator = ConfigurationManager.AppSettings["InstanceId"];
endpointConfiguration.MakeInstanceUniquelyAddressable(instanceDiscriminator);
endpointConfiguration.EnableCallbacks();
```

Enabling callbacks in default mode requires the endpoint to be made [uniquely addressable](/nservicebus/messaging/callbacks.md#message-routing).

## Enable callbacks without requests

Endpoints only replying to callbacks with object messages like the following:

```csharp
public class Handler :
    IHandleMessages<Message>
{
    public Task Handle(Message message, IMessageHandlerContext context)
    {
        var responseMessage = new ResponseMessage
        {
            Property = "PropertyValue"
        };
        return context.Reply(responseMessage);
    }
}
```

don't require a reference to the callbacks package. 

For endpoints replying with int or enum results, the callbacks package can be enabled in response-only mode:

```csharp
endpointConfiguration.EnableCallbacks(makesRequests: false);
```