FYI: the verbose preamble is to help explain why I am using Activator.CreateInstance. I have a number of entities (objects corresponding to database column information) that are "contained" in multiple databases, each of which has a different table/column setup. So I am able to retrieve an entity from each database, but the way I retrieve it is different per database. The database type is not known till runtime and could vary throughout execution. I have created the following setup:
First define the query operations each entity should support and each entity reader should support these operations.
public abstract class Operations<T> {
public delegate T findDelegate(int id);
public findDelegate find;
}
// there are many of these N=1,2,..., but here is one
// use abstract class since implementation of queries should be done by handlers
public class EntityNReader : Operations<EntityN> {
public Reader();
}
Define an interface for "Handler" classes, i.e. these classes implement the query operations listed above.
public interface IHandler<T> {
public string find(int id);
}
// there are many of these N,M=1,2..., but here is one
// use of interface is to force handlers to implement all query types
public class EntityNHandlerForDbTypeM : IHandler<EntityN> {
public string find(int id) {/*whatever*/}
}
This allows the developers to create a single class for handling EntityN query operations for DbTypeM. Now, create a Database class that contains the reader objects and binds the handler methods to the reader delegates.
public class Database {
// there are many of these, but here is one
public EntityNReader EntitiesN;
public Database(string dbType) {
// this is called for each EntityNReader
bindHandlers<Reader, TypeN>(MyReader, dbType);
// ...
// nullreferenceexception
EntitiesN.find(0);
}
// this is a factory that also performs late binding
private void bindHandlers<T,K>(T reader, string dbTypeM)
where T: Operations<K>, new()
{
// create instance of EntityNReader
r = (T)Activator.CreateInstance(typeof(T));
// r != null
// create instance of handler
IHandler<K> h = (IHandler<K>)(Activator.CreateInstance(
Type.GetType("namespace.to.EntityNHandlerForDbTypeM"),
new object[] { this }
));
// bind delegates
r.find = h.find;
}
}
As you can see in Databases constructor, the way the code is written now, I get a NullReferenceException even though instance r of EntityNReader is created and (verified to be) not null.
However, if I instantiate EntitiesN where it is declared in Database instead of within bindHandlers, the code compiles and everything works. The reason I don't just do this is that (subsequently) I would like to conditionally create readers/handlers inside of bindHandlers at the time the Database class is instantiated.
What is happening here? Link to actual code if necessary. P.S. I am relatively new to programming, so I am open to hearing how an experience developer might design this component (especially if I am heading down the wrong path).
