Callback method in RIA Services in Silverlight 4

I have a Silverlight 4 appliction. This appliation is using RIA Services for operations. I have an operat开发者_运维百科ion that I want to do something when it has completed. I know that I need to wireup a callback, but I'm not sure how. Currently, I have the following:

DomainContext.CalculateTotal(param1, param2, OnCalculateTotalCompleted);

...

private void OnCalculateTotalCompleted(InvokeOperation response)
{
}

When I compile this, I get an error that says: No overload for method 'CalculateTotal' takes 3 arguments.

What am I doing wrong? How do I wireup a callback for this specific operation?

Thank you!

there's another parameter that you have to add at the end (UserState), just use "null".

DomainContext.CalculateTotal(param1, param2, OnCalculateTotalCompleted, null);

The DomainContext.CalculateTotal needs a 4th argument "Object state", so give it a null, and you are OK.