Question

I have the following class and a factory (omitted unnecessary code). I have 3 separate implementations of the IManageableEntryDao, and a string/type map that gets accessed in the createDao method.

I get the following compilation error: “ManageableEntry.IManageableEntryDao’ requires ‘1’ type arguments”. What’s the best practice in terms of solving this problem? Would I want to somehow determine what is? Or is there an alternative solution?

public interface IManageableEntryDao<T> where T : IManageableEntry {
    T findById(long id);
    T findByName(string name);

    int findUnapprovedCount();
    List<T> findUnapproved(ManageableEntryCriteria criteria);

    long insert(T manageableEntry);
    bool update(T manageableEntry);
    bool delete(T manageableEntry);
}

public class ManageableEntryDaoFactory {
    public IManageableEntryDao createDao(string manageableEntryType) {
            manageableEntryType = manageableEntryType.ToLower();
            Type type = daoTypes[manageableEntryType];
            if (type != null) {
                object dao = Activator.CreateInstance(type);                    
                return dao as IManageableEntryDao;
            }
            throw new NotImplementedException("Failed to find DAO for type: " + manageableEntryType);
        }
}

Answer 1

You need to specify a type in the method call. This does mean you can probably avoid the need for the string:

public IManageableEntryDao<T> CreateDao<T>() where T : IManageableEntry
{
        Type manageableEntryType = typeof(T);

        // You'll need to modify daoTypes to be a HashSet<Type> (or List<Type>) of allowable types, or something similar, instead of using a dictionary lookup
        if (daoTypes.Contains(manageableEntryType) {
            object dao = Activator.CreateInstance(type);                    
            return dao as IManageableEntryDao<T>;
        }
        throw new NotImplementedException("Failed to find DAO for type: " + manageableEntryType);
    }