Help with web services in C#

I have trouble with calling a web method dynamically from a combobox which contains the different operations of a web service. My program can work with different web services navigated by a URL that is specified at runtime. The problem is that, the items of the operation's comboBox are not constant and they change if the web service in use changes, so when writing the code, I don't know the web service method which I want to call, and I want it to be retrieved from the combobox at runtime. If we assume that the web reference to the service is called wrg, I don't know how to call the desired method. I want it to be something like this, wrg."comboBox.Text", but this doesn't work...
What should I do?

I'll be thankful with your help