Dynamically Create a generic type for template

问题:

I'm programming WCF using the ChannelFactory which expects a type in order to call the CreateChannel method. For example:

IProxy proxy = ChannelFactory<IProxy>.CreateChannel(...);

In my case I'm doing routing so I don't know what type my channel factory will be using. I can parse a message header to determine the type but I hit a brick wall there because even if I have an instance of Type I can't pass that where ChannelFactory expects a generic type.

Another way of restating this problem in very simple terms would be that I'm attempting to do something like this:

string listtype = Console.ReadLine(); // say "System.Int32"
Type t = Type.GetType( listtype);
List<t> myIntegers = new List<>(); // does not compile, expects a "type"
List<typeof(t)> myIntegers = new List<typeof(t)>(); // interesting - type must resolve at compile time?

Is there an approach to this I can leverage within C#?

回答1:

What you are looking for is MakeGenericType

string elementTypeName = Console.ReadLine();
Type elementType = Type.GetType(elementTypeName);
Type[] types = new Type[] { elementType };

Type listType = typeof(List<>);
Type genericType = listType.MakeGenericType(types);
IProxy  proxy = (IProxy)Activator.CreateInstance(genericType);

So what you are doing is getting the type-definition of the generic "template" class, then building a specialization of the type using your runtime-driving types.

回答2:

You should look at this post from Ayende: WCF, Mocking and IoC: Oh MY!. Somewhere near the bottom is a method called GetCreationDelegate which should help. It basically does this:

string typeName = ...;
Type proxyType = Type.GetType(typeName);

Type type = typeof (ChannelFactory<>).MakeGenericType(proxyType);

object target = Activator.CreateInstance(type);

MethodInfo methodInfo = type.GetMethod("CreateChannel", new Type[] {});

return methodInfo.Invoke(target, new object[0]);

回答3:

Here's a question: Do you really need to create a channel with the exact contract type in your specific case?

Since you're doing routing, there's a very good chance you could simply deal with the generic channel shapes. For example, if you're routing a one-way only message, then you could create a channel to send the message out like this:

ChannelFactory<IOutputChannel> factory = new ChannelFactory<IOutputChannel>(binding, endpoint);
IOutputChannel channel = factory.CreateChannel();
...
channel.SendMessage(myRawMessage);

If you needed to send to a two-way service, just use IRequestChannel instead.

If you're doing routing, it is, in general, a lot easier to just deal with generic channel shapes (with a generic catch-all service contract to the outside) and just make sure the message you're sending has all the right headers and properties.