Using NServiceBus in a ASP.NET Web Application

Component: NServiceBus | Nuget: NServiceBus (Version: 6.x)

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" appear, 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 the bus

In WebApplication, open Global.asax.cs and look at the code in the ApplicationStart method:

var endpointConfiguration = new EndpointConfiguration("Samples.AsyncPages.WebApplication");
endpointConfiguration.MakeInstanceUniquelyAddressable("1");
endpointConfiguration.UseSerialization<JsonSerializer>();
endpointConfiguration.EnableInstallers();
endpointConfiguration.UsePersistence<InMemoryPersistence>();
endpointConfiguration.SendFailedMessagesTo("error");

Endpoint = NServiceBus.Endpoint.Start(endpointConfiguration).GetAwaiter().GetResult();

Sending a message

Open Default.aspx.cs in WebApplication to see the Button1Click method:

int number;
if (!int.TryParse(TextBox1.Text, out number))
{
    return;
}
var command = new Command
{
    Id = number
};

var sendOptions = new SendOptions();
sendOptions.SetDestination("Samples.AsyncPages.Server");

var code = await Global.Endpoint.Request<ErrorCodes>(command, sendOptions)
    .ConfigureAwait(false);
Label1.Text = Enum.GetName(typeof(ErrorCodes), code);

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 :
    IMessage
{
    public int Id { get; set; }
}

Return to Default.aspx.cs and look at the code Global.Bus.Send(command). Global.Bus references the Bus property of the Global class in Global.asax.cs. Then the code calls the Send method, passing in the newly created command object.

The "bus" isn't anything special in code; it is just an object for calling methods.

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 :
    IHandleMessages<Command>
{
    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);
        }
        else
        {
            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 Register 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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