.net architecture: Creating a IRepository<T> generically

Question

I am using a generic repository interface which allows me to add, find, update and delete objects of different types. I have then implemented two concrete repositories and can switch them around without changing the application. Everything is wonderfully disconnected. But I have hit a snag. The code inside my repository methods just feels wrong and I cannot figure out how to do it better. Here is a snippet of my code:

public class OracleRepository<T> : IRepository<T> where T : new()
{
    public IQueryable<T> GetAll()
    {
        if (typeof(T) == typeof(Object1))
        {
            return (IQueryable<T>)DataAccess.GetAllObject1().AsQueryable();

        }

        if (typeof(T) == typeof(Object2))
        {
            return (IQueryable<T>)DataAccess.GetAllObject2().AsQueryable();
        }

        throw new NotImplementedException();
    }
}

The problem is that my DataAccess (creates objects from a datareader) is not generic and has specific methods for each type of object that it supports.

Can I rewrite the above so that I avoid the list of if-then-elseif using generics or otherwise?

Answer 1

One common pattern is to use a dictionary from type to a function of the relevant type. For instance:

private static readonly IDictionary<Type, Func<object>> Fetchers =
    new Dictionary<Type, Func<object>>();
    {
        (typeof(Object1), () => DataAccess.GetAllObject1().AsQueryable()),
        (typeof(Object2), () => DataAccess.GetAllObject2().AsQueryable()),
        // etc
    };

public IQueryable<T> GetAll()
{
    Func<object> func = Fetchers[typeof(T)];
    if (func == null)
    {
        throw new NotImplementedException();
    }
    return (IQueryable<T>) func();
}

The cast is there because you can't really express the type relationship in the dictionary, but at least it's slightly easier to add new types etc.

You may be able to move the call to AsQueryable() to GetAll() depending on what GetAllObject1() etc return.