For good or bad partial classes in C# 2.0 allows breaking of encapsulation as this example will show.
In a consulting job I recently ran into an interesting case involving a webservice with several different service methods f1, f2, fn (sample names, not actual names) all taking the same string argument and all returning a string. The user would select an operation name after which my code had to call the named operation on a web service using a standard parameter. Trivial really, if one would do accept bad code like this below, but I don’t:
String operationName = …
String arg = …
Webserviceproxy webserviceproxy = …
// Warning: Badly coupled code begins here (need to update each time we add/rename/delete operations).
switch (operationName) {
case “f1″: return webserviceproxy.f1(arg); break;
case “f2″: return webserviceproxy.f2(arg); break;
}
What is really needed is a method to invoke a webservice method by name, while still using the generated .NET proxy to do the hard soap/http stuff (no time to reinvent a better wheel here). Reflection is one way to do this, but let’s try another type-safe method for this posting, because the technique shown here is quite powerful for all sorts of related problems:
Let’s look at an extract of the generated proxy:
public partial class Webserviceproxy : System.Web.Services.Protocols.SoapHttpClientProtocol
{
…
public string f1(string arg) {
object[] results = this.Invoke(”f1″, new object[] {arg});
return ((string)(results[0]));
}
…
}
and at the inherited SoapHttpClientProtocol:
public class SoapHttpClientProtocol : HttpWebClientProtocol {
…
protected object[] Invoke(string methodName, object[] parameters) {
…
}
…
}
It seems the method “invoke” would fulfil our needs if it was only public (which it isn’t). So what do we do? Certainly we do not want to modify the generated file (and lose our changes each time it is regenerated).
The good news is that Generatedwebserviceproxy is a partial class so we can extend it with the following code. We will place the code in a file safely outside the generated proxy class file:
public partial class Webserviceproxy : System.Web.Services.Protocols.SoapHttpClientProtocol
{
///
/// Dynamic operation that allows us to call an operation by name.
///
public string InvokeAny(String operationName, string arg)
{
object[] results = this.Invoke(operationName, new object[] {arg});
return ((string)(results[0]));
}
}
The compiler will merge the two class definitions effectively adding a new public InvokeAny method to the generated class. And now we can call our web service calls dynamicly from using InvokeAny:
String operationName = … String arg = … Webserviceproxy webserviceproxy = … return webserviceproxy.InvokeAny(operationName, arg);
Clearly, easy to do and with better overall code than a “switch” – even though it is not without drawbacks as it breaks encapsulation of the generated proxy.
Post scriptum:
Used the same partial classes trick today to add a common custom interface to two differently generated proxies. I now officially miss this feature in Java (yes, AspectJ can do the thing but it is not a official part of the language).
Recent update and notice:
This post an almost identical copy from my old blog at “http://www.mortench.net/blog” which I will shortly retire for good.
Filed under: .Net
