Generics has certainly opened up a few design opportunities for class library developers that weren’t there before. My gut feeling however is that there is more we could do so I’m going to expand upon my post on indexes in .NET BCL 3.0 collections and introduce the concept of member parameters.
The idea is that members such as properties, fields and even methods become valid parameters for generic types. It would work something like this.
1 Customer customer = new Customer();
2 customer.ID = "MDENN";
3 customer.FirstName = "Mitch";
4 customer.LastName = "Denny";
5
6 StringIndex<Customer, Customer.ID> customerIndex = new StringIndex<Customer, Customer.ID>();
7 QueryResults<Customer> results = customerIndex.Query("MDENN");
You can see in the declaration for “customerIndex” that the second parameter for the type is actually a property and support syntax like the following in the Add method (where things are actually inserted into the index).
1 public class StringIndex<EntityType, IndexProperty>
2 {
3 public void Add(Customer entity)
4 {
5 string indexValue = IndexProperty(entity);
6 // Add entity to index with indexValue;
7 }
8 }
You could achieve something similar by using generic delegate types but I think this syntax would be clearer in most situations – maybe its just a shortcut syntax. Obviously you would need to be able to apply constraints to the member parameters just like you can to type parameters, for example you’d probably constrain the StringIndex’s member parameter to being a string.