Silverlight 4 callback is accumulating with each button press - silverlight

Question

I'm writing a Silverlight 4 application using "Silverlight 4 Unleashed" as a foundation.
I have a ChildWindow for loggin in with Username, Password, and Remember Me. The OK button is tied to my AuthUserViewModel SignIn using RelayCommand.
Since I'm just starting, I don't have any data validation yet and noticed something weird.
If I click "OK" on my Login child window, my Action callback tells me I have invalid credentials in a MessageBox...which is perfect. I'm using my own Authentication service for various reasons.
However, if I click "OK" again, my service gets called once, but the Action callback is fired twice, telling me I have invalid credentials. If I press "OK" again, the service is called once, but the Action callback is fired three times, and so on and so on.
Why would it be doing that?
Here is the offending code:
public void SignIn(AuthUserDataModel authUser, Action<ErrorContainer> callback)
{
EnsureClient();
client.SignInCompleted += (sender, result) =>
callback(new ErrorContainer
{
AsyncException = result.Error,
CustomError = result.Result.CustomMessage //holds "Invalid credentials"
});
client.SignInAsync(authUser);
}
Like I said, the button event is fired once, the web service is called once, but the callback is fired an amount equaling the number of times I press OK.
Thank you.

Answer 1

Ah! your client object is a field, and you ensured that it is shared across calls. It prevent it from being initialized on each SignIn call, but each time you add an handler to the SignInCompleted vent before executing the SignInAsyncFunction.
Therefore it's normal that the handler gets executed one time by subsequent SignIn.
To prevent this, you have 2 approaches:
1) create a new client in each SignIn call (it will be garbage collected later)
2) attach your handler when you initialize the client.
ie in your EnsureClient, you should have something like:
if(client == null)
{
client = new MyClient();
client.SignInCompleted +=(sender,result) =>{...};
}
and in the SignIn function:
EnsureClient();
client.SignInAsync(authUser);