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Metadata exchange is not universally supported. In practice, contract discovery can happen in all sorts of ways, including (and we re not making this up) being faxed a printout showing samples of the messages the service expects to send and receive. If you re getting the contract through that kind of informal channel, you ll need to write an interface (by hand) in your client program to represent the service contract.

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The process of metadata import also highlights an important point about service evolution. You might modify the ChatService after the ChatClient has added its reference. If these modifications involve changing the contract, it s clear that there s a problem: the client s copy of the contract is out of date. You might think that sharing the interface directly through a common DLL would be a good way to avoid this problem, but it might only make the problem harder to see: what if you ve already deployed a version of the client If you then modify the contract the modified code might run fine on your machine, but if you deploy an update to the service with this changed contract any copies of the old client out there will now be in trouble because they re still working with an old copy of the contract. Explicitly going through the metadata exchange

In fact, it has revealed a small problem: the tempuri.org that appears in the URL indicates something temporary that we re supposed to fill in the ServiceContract attribute on the original service definition has a Namespace attribute, and we re supposed to pick a URI that is unique to our service. It s not mandatory in this particular scenario because everything works with the default, but a temporary-looking URI doesn t look entirely professional. It could be worse. See http://www.neopoleon.com/home/blogs/neo/archive/2003/09/29/5458 .aspx.

Summary

doesn t make this problem any easier to solve, of course, but it makes it less likely for changes to creep in by accident and go undetected. A complete solution to the problem of service evolution is beyond the scope of this book, so for now, just be aware that changing a contract should not be undertaken lightly.

Looking further through the Reference.cs file generated by adding the service reference, the next most interesting feature after the contract is a class called ChatServiceClient. This implements IChatService, because it acts as a proxy for the service. If we want to communicate with the service, all we need to do is create an instance of this proxy and invoke the method representing the operation we d like to perform. So if we add a using ChatClient.ChatService; directive to the top of Program.cs in ChatClient, we can then modify its Main method as shown in Example 13-5.

static void Main(string[] args) { using (ChatServiceClient chatProxy = new ChatServiceClient()) { } chatProxy.PostNote("Ian", "Hello again, world");

This graph will be a simple line chart that uses only one pane, which is the one at the zero index of this collection You refer to the pane with this line: GraphPane pane = mPane[0]; The subsequent graphical operations will then be performed on this pane object To draw a line curve, it is a good idea to use the PointPairList collection that the ZedGraph library gives you This allows you to create a single collection of data items that correspond to the X and Y values of a chart The PointPairList supports many data types, including dates, so is perfect for the example s needs Once the input parameters (ticker and days) have been read in and sanitized, the DataTier service is called to return a DataTable containing the results of the query for that stock and the number of days of price history you want for it.

Providing help is about much more than just responding to the F1 key; it s about providing an intuitive user interface and adding support when the user needs it. The support must be made available through the channels that the user knows so the help is intuitive. By providing tooltips and What s this help for most widgets, many questions can be avoided. When tooltips no longer help, a wizard can be used, or you can attempt to redesign the user interface to avoid problems. The latter must always be an option, but sometimes a wizard is the best alternative. To make information available, you can use the status bar to give the user the same information regardless of what the user is doing. But don t count on the user seeing the status bar at all times if the working mode is changed by accident, users usually don t go for the status bar; instead they go to wherever they were when the change took place. The final piece of a help system is online documentation. The Qt Assistant can help you by providing a nice interface to your documentation. Simply compile your documentation into a set of HTML documents, create a documentation profile, and use the Assistant as your help client.

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