Generic webservice creator using generics and delegates

· November 9, 2006

This now contains an update Be sure to look it up for a more elegant solution. ====================================================

Yesterday was spent with my nose deep down the documentation of .NET 2.0, pondering poses and headscratching.

But the reward was a quite cool little hack that solved a tricky problem in our solution.

In my current project we’re are using some webservices and these all need to be configured in the same way. These configurations has to do with setting the URL in runtime, using other credentials, caching and maybe other things that we want to be able to do for all webservices.

So what i wanted is a WebServiceCreator with metods that creates the webservices and does the requested configurations. So my first thought was to use generics - and that was right for quite a while.

I created a class WebServiceCreator that has one (and so many overloads we need:-)) method:

public static TWebService GetWebService(string WebServiceURL) where TWebService : SoapHttpClientProtocol

This means that when this metod is called TWebService needs to be replaced with the type that GetWebService should return - and that type needs to be a SoapHttpClientProtocol - which all the webservice proxies inhierties. So the call would look something like this:

DemoWebService.Service wss = WebServiceCreator.GetWebService([WebServiceURL]);

So far just my thoughts and hopes on how this would work…. but the inner workings of the GetWebService-method was a bit trickier.

Somewhere someone would have to instanciate the WebService by calling a constructor. Since the GetWebService-method only manages generic types (TWebService) this would be very tricky… The following code would of course not compile:

TWebService wss = new TWebService();

The solution to this problem was to use a delegate that “points” to a method that creates the webservice of the right type. This delegate is then passed as a parameter to the GetWebService-method. The delegate looks like this:

public delegate TWebService WebServiceConstructorDelegate();

GetWebService is then updated to look like this (sorry for the weird indentention):

public static TWebService GetWebService ( WebServiceConstructorDelegate ConstructorForWebService, string WebServiceURL) where TWebService : SoapHttpClientProtocol

And when you call GetWebService you supply a method that creates the webservice like this (really sorry for the indentention):

DemoWebService.Service wss = WebServiceCreator.GetWebService(

new WebServiceCreator.WebServiceConstructorDelegate(pCreateDemoWebService) ,[WebServiceURL]);

And then of course the small pCreateDemoWebService that is sent-in as delegate above is very simple:

private DemoWebService.Service pCreateDemoWebService() { return new Epas.Shared.Tests.DemoWebService.Service(); }

But hey - what’s this? Now we have a WebService creator that creates webservices of any type in a uniform way. So there is only one place to handle of code like that.

This is an excellent example on the power of generics and the first time i found a real business use for it with C#. If you want a complete code example please email me about it.

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