How to decide at runtime which type to pass as a generic parameter?

Question

I have several message queues that have specific messages on them. I've created classes for these messages using xsd.exe. I can receive a message syncronously and deseriazlise it with this method:

public oneOfMyTypes DeserializeMessage(XDocument message)
{
 var serializer = new XmlSerializer(typeof(oneOfMyTypes));
 var entity = (oneOfMyTypes)serializer.Deserialize(message.CreateReader());
 return entity;
}

I then persist the entity via Fluent NHibernate.

So I've got about five message queues that all have their own type of message. I would like to keep this little processor app maintainable, so that adding more message queues and message types doesn't become a pain.

So I have a list of queue names in my app.config that I use to create the message queues on start up and then I want to wire up a single method to the .ReceiveCompleted event of all queues:

void queue_ReceiveCompleted(object sender, ReceiveCompletedEventArgs e)
{
 var queue = (MessageQueue)sender;
 var message = queue.EndReceive(e.AsyncResult);
 var body = message.Body.ToString();
 var xml = XDocument.Parse(body);
 var queueName = queue.QueueName;
 Type entityType = GetTypeFromQueueName(queueName);
 entityType entity = DeserializeMessage<entityType>(xml);

 var repository = new LogRepository();
 repository.AddEntity<entityType>(entity);
}

private T DeserializeMessage<T>(XDocument message)
{
 var serializer = new XmlSerializer(typeof(T));
 var entity = (T)serializer.Deserialize(message.CreateReader());
 return entity;
}

public Type GetTypeFromQueueName(string queueName)
{
 switch (queueName)
 {
  case "some-message-queue-name":
   return typeof (oneOfMyTypes);
 }
}

But when I try to pass entityType to the generic methods I get "Type or namespace name expected". I'm probably doing something really silly, but I can't figure out how this should work.

I've tried using the dynamic keyword and also .MakeGenericType but no luck. I've also looked at:

• http://stackoverflow.com/questions/1516385/dynamic-generic-declaration-of-type-t
• http://stackoverflow.com/questions/2282009/function-returning-a-generic-type-whose-value-is-known-only-at-runtime
• http://stackoverflow.com/questions/3482219/determining-a-generic-type-at-runtime-in-non-generic-class
• http://stackoverflow.com/questions/2247553/how-to-pass-variable-of-type-type-to-generic-parameter

But I'm still not getting it ... help?

Answer 1

This link should help you

http://todotnet.com/post/2006/11/13/Instantiating-Generic-Types-at-runtime.aspx

You should be able to go from the link i posted to call the methode through reflection.

See the following link:

http://www.victorchen.info/call-static-method-with-a-string-name/

So basically when you've created your generic Type, you then grab the MethodInfo of your static method and then call the Invoke method on the MethodInfo object.

(i haven't actually tried this but in theory i believe it should work)

Answer 2

Unfortunately, generics are not meant to function dynamically like this. Generic parameters must be defined at design time. It's been a while since I've used NHibernate, but isn't there a way to insert entities with a syntax like:

repository.AddEntity(entity, typeof(myEntityType));

EDIT:

or

 Session.SaveOrUpdate(object);