Callbacks should only be used in exceptional situations, for example, to introduce messaging behind a synchronous API in a legacy component that can't be changed. They allow gradually transitioning applications towards messaging. See the when to use callbacks section for more information.
Callbacks allow a sender to receive a response value directly from a message handler, turning a one-way send into a request/reply interaction. This sample demonstrates callbacks returning an enum, an integer, and an object.
Running the sample
- Run the solution. Two console applications will start.
- In the Sender console application, press various keys to trigger each scenario when prompted.
Shared project
A class library containing the messages and shared code.
Status enum
This is used for the enum callback scenario.
public enum Status
{
OK,
Error
}
Sender project
A console application responsible for sending messages and handling the callback from the reply.
Send and callback for an enum
var message = new EnumMessage();
var sendOptions = new SendOptions();
sendOptions.SetDestination("Samples.Callbacks.Receiver");
var status = await messageSession.Request<Status>(message, sendOptions);
Console.WriteLine($"Callback received with status:{status}");
Send and callback for an int
var message = new IntMessage();
var sendOptions = new SendOptions();
sendOptions.SetDestination("Samples.Callbacks.Receiver");
var response = await messageSession.Request<int>(message, sendOptions);
Console.WriteLine($"Callback received with response:{response}");
Send and callback for an object
var message = new ObjectMessage();
var sendOptions = new SendOptions();
sendOptions.SetDestination("Samples.Callbacks.Receiver");
var response = await messageSession.Request<ObjectResponseMessage>(message, sendOptions);
Console.WriteLine($"Callback received with response property value:{response.Property}");
Receiver project
A console application responsible for replying to messages from the Sender.
Return an enum
public class EnumMessageHandler(ILogger<EnumMessageHandler> logger) :
IHandleMessages<EnumMessage>
{
public Task Handle(EnumMessage message, IMessageHandlerContext context)
{
logger.LogInformation("Message received, Returning");
return context.Reply(Status.OK);
}
}
Return an int
public class IntMessageHandler(ILogger<IntMessageHandler> logger) :
IHandleMessages<IntMessage>
{
public Task Handle(IntMessage message, IMessageHandlerContext context)
{
logger.LogInformation("Message received, Returning");
return context.Reply(10);
}
}
Return an object
Note that this scenario requires a Reply with a real message.
public class ObjectMessageHandler(ILogger<ObjectMessageHandler> logger) :
IHandleMessages<ObjectMessage>
{
public Task Handle(ObjectMessage message, IMessageHandlerContext context)
{
logger.LogInformation("Message received, Returning");
var objectResponseMessage = new ObjectResponseMessage
{
Property = "PropertyValue"
};
return context.Reply(objectResponseMessage);
}
}