Question

I am currently developing a WCF Publish Subscribe service. My Service has the following code,

public void PublishPost(string postSampleData)
{
    PostChangeEventArgs e = new PostChangeEventArgs();
    e.PostData = postSampleData;
    PostChangeEvent(this, e);
}

and the code for the postChangeEvent is

public class PostChangeEventArgs : EventArgs
{
    public string PostData;
}

and in my client file, i wrote this code in the main method,

class Program : IPostingContractCallback
{
static void Main()
{
        InstanceContext site = new InstanceContext(null, new Program());
        PostingContractClient client = new PostingContractClient(site);

        WSDualHttpBinding binding = (WSDualHttpBinding)client.Endpoint.Binding;
        String clientcallbackaddress = binding.ClientBaseAddress.AbsoluteUri;
        clientcallbackaddress += Guid.NewGuid().ToString();
        binding.ClientBaseAddress = new Uri(clientcallbackaddress);

        client.Subscribe();
}

public void PostReceived(string postSampleData)
{
    MessageBox.Show("PostChange(item {0})", postSampleData);
}
}

and for the code for my data source…

class Program : IPostingContractCallback
{
    static void Main(string[] args)
    {
        InstanceContext site = new InstanceContext(new Program());
        PostingContractClient client = new PostingContractClient(site);

        client.PublishPost("testing");

        Console.WriteLine();
        Console.WriteLine("Press ENTER to shut down data source");
        Console.ReadLine();

        //Closing the client gracefully closes the connection and cleans up resources
        client.Close();
    }

    public void PostReceived(string postSampleData)
    {
        Console.WriteLine("PostChange(item {0})",postSampleData);
    }
}

After running the service, followed by the client, followed by the datasource, I’m suppose to receive a popup messagebox from my client. However there gives an error on the line

PostChangeEvent(this, e);

Object reference not set to an instance of an object.

Anyone know how to solve this?

Answer 1

It sounds like there’s nothing subscribed to the event. To check for this, you should use:

var handler = PostChangeEvent;
if (handler != null)
{
    handler(this, e);
}

That will stop the NullReferenceException, but of course it won’t address why there were no subscribers… you haven’t shown anything which subscribes to the event – what were you expecting to be subscribed?