Using NServiceBus in an ASP.NET Core Web Application

Component: NServiceBus
NuGet Package: NServiceBus (8-pre)
This page targets a pre-release version. Pre-releases are subject to change and samples are not guaranteed to be fully functional.

Run the solution. A new browser window/tab opens, as well as a console application.

Enter the number "1" into the text box in the browser and click "Go". Notice the result "None" appears, as shown:

AsyncPages sample running

Changing the number in the text box from even to odd numbers changes the result in the Server console.

The web page renders synchronously; from the user's perspective, the interaction is synchronous and blocking, even though behind the scenes NServiceBus is doing asynchronous messaging.

This sample has three projects: Shared, Server, and WebApplication. WebApplication is a web application that sends messages (found in the Shared project) to the Server project, which is hosted as a console application.

Initializing NServiceBus

In WebApplication, open Program.cs and look at the code in the UseNServiceBus method:

    .ConfigureWebHostDefaults(builder => builder.UseStartup<Startup>())
    .UseNServiceBus(context =>
        var endpointConfiguration = new EndpointConfiguration("Samples.AsyncPages.WebApplication");
        endpointConfiguration.UseTransport(new LearningTransport());
        return endpointConfiguration;

Sending a message

Open Index.cshtml.cs in WebApplication to see the OnPostAsync method:

if (!int.TryParse(textField, out var number))
    return Page();
var command = new Command
    Id = number

var sendOptions = new SendOptions();

var code = await messageSession.Request<ErrorCodes>(command, sendOptions);
ResponseText = Enum.GetName(typeof(ErrorCodes), code);

return Page();

The first line of code parses the text passed in by the user. The second line creates a new NServiceBus message of the type Command, and initializes its Id property with the value from the text box.

Open the class definition for the Command type in the Shared project:

public class Command :
    public int Id { get; set; }

Return to Index.cshtml.cs and look at the code messageSession.Request. The message session offers methods to send messages via NServiceBus. Skip the rest of the code and see what happens to the message just sent.

Handling the message

In the Server project, find this code in the CommandMessageHandler class:

public class CommandMessageHandler :
    static ILog log = LogManager.GetLogger<CommandMessageHandler>();

    public Task Handle(Command message, IMessageHandlerContext context)
        log.Info("Hello from CommandMessageHandler");
        Task reply;
        if (message.Id%2 == 0)
            reply = context.Reply(ErrorCodes.Fail);
            reply = context.Reply(ErrorCodes.None);
        return reply;

This class implements the NServiceBus interface IHandleMessages<T> where T is the specific message type being handled; in this case, the Command message. NServiceBus manages classes that implement this interface. When a message arrives in the input queue, it is deserialized and then, based on its type, NServiceBus instantiates the relevant classes and calls their Handle method, passing in the message object.

In the method body notice the response being returned to the originating endpoint. This will result in a message being added to the input queue for MyWebClient.

Handling the response

When the response arrives back at WebApplication, the bus invokes the callback that was registered when the request was sent.

The messageSession.Request method takes the callback code and tells the bus to invoke it when the response is received. There are several overloads to this method; the code above accepts a generic Enum parameter, effectively casting the return code from the server to the given enum.

Finally, the code updates the Text property of a label on the web page, setting it to the string that represents the enum value: sometimes None, sometimes Fail.

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