无法在silverlight客户端打开信号器连接

Question

I am trying to implement SignalR in Silverlight based on this blog: SignalR and Silverlight .

When I try to .Invoke() I get a runtime error "System.InvalidOperationException: The Start method must be called before data can be sent. at Microsoft.AspNet.SignalR.Client.Connection.Send(String data) at Microsoft.AspNet.SignalR.Client.Hubs.HubProxy.Invoke[T](String method, Object[] args)..."

I have _conn.Start() on my connection. If I try to Start() it a second time, right before invoke, it throws an exception. Here is my code:

    private IHubProxy _hub;
    private HubConnection _conn;

    public AddProductView()
    {
        InitializeComponent();
        var url = Application.Current.Host.Source.GetComponents(UriComponents.Scheme | UriComponents.HostAndPort,
                                                                UriFormat.Unescaped);
        _conn = new HubConnection(url);
        _hub = _conn.CreateHubProxy("SilverlightPrism.Mvc.Services.ProductHub");
        _hub.On<string>("NewMessage", message => Deployment.Current.Dispatcher.BeginInvoke(() => DoAddItem(message) ));
        _conn.Start();
    }

    private void DoAddItem(string item)
    {
        var product = DeserializeToProduct(item);
        ProductData.Products.Add(product);
    }

    private void buttonAdd_Click(object sender, RoutedEventArgs e)
    {
        Random random = new Random();
        var id = Guid.NewGuid();
        var product = new Product
            {
                Price = random.Next(1000,100000),
                ProdId = id,
                ProdName = "New prod."
            };
        var jsonMessage = SerializeToJsonString(product);
        _hub.Invoke("SendMessage", jsonMessage);
    }

It is throwing the exception on _hub.Invoke();

How do I correctly get the hub connect and send a message?

Answer 1

HubConnection.Start is asynchronous. You cannot call Start right before Invoke because your HubConnection is probably still in the Connecting state.

You need to wait for the Task returned from Start to complete before you can call IHubProxy.Invoke . You can use await (or Task.ContinueWith if you aren't running .NET 4.5) to ensure Start finishes before you enable buttonAdd .

You could also create your AddProductView object asynchronously in a factory method. Instead of calling HubConnection.Start in the constructor, you could do it in a static Task<AddProductView> CreateAddProductView() method.

Alternatively, if you don't care about the creation of your AddProductView being asynchronous, you can just call Start synchronously:

_conn.Start().Wait();