具有泛型类型的函数中使用的Ordeby（），Where（）等

Question

I am currently fiddling with generic repositories. My test application is in C#. I have made a function that prints out all items in particular repository. A repository is an input parameter. The whole block is at the bottom of this post and currently the function itself looks like this:

static void PrintCollection<T, TKey>(IRepository<T, TKey> repo)
{
    // Error since compiler does not recognizes p.LastName
    //foreach (Person p in repo.GetItems().OrderBy(p => p.LastName)) Console.WriteLine(Convert.ToString(p));
    foreach (T p in repo.GetItems())
        Console.WriteLine(Convert.ToString(p));
}

It's working fine but it's missing things like ordering by particular property in a class, such as LastName in Person or ProductName in Product . I was wondering if it's possible to send an extra parameter to PrintCollection<T, TKey>(...) and test whether the particular field exists and then order by it. For instance:

static void PrintCollection<T, TKey, TOrder>(IRepository<T, TKey> repo, TOrder o = default(TOrder))
{
    if (o == default(TOrder))
        foreach (T p in repo.GetItems()) Console.WriteLine(Convert.ToString(p));
    else
        // Order by o
}

At the moment, I don't know what I should use to test the fields and properties against their existance in a class. And then, how to use them in LINQ and Lambdsa Expression to eg order, sort, filter collections.

Thanks

---

Below is a snippet of my code. I cut out unimportant parts.

Main program

class PeopleRepository : IRepository<Person, string>
{
    private static List<Person> _proxy;

    public PeopleRepository(List<Person> people = null)
    {
        if(people == null) _proxy = Person.GetAllPeople();
        else _proxy = people;
    }

    public IEnumerable<Person> GetItems()
    {
        return _proxy;
    }

    public Person GetItem(string key)
    {
        return _proxy.SingleOrDefault(p => p.ID == key);
    }
}

Interface IRepository

class ProductsRepository : IRepository<Product, int>
{
    private static List<Product> _proxy;

    public ProductsRepository(List<Product> products = null)
    {
        if (products == null) _proxy = Product.GetAllProducts();
        else _proxy = products;
    }

    public IEnumerable<Product> GetItems()
    {
        return _proxy;
    }

    public Product GetItem(int key)
    {
        return _proxy.SingleOrDefault(p => p.ID == key);
    }
}

Repository for People

 class PeopleRepository : IRepository<Person, string> { private static List<Person> _proxy; public PeopleRepository(List<Person> people = null) { if(people == null) _proxy = Person.GetAllPeople(); else _proxy = people; } public IEnumerable<Person> GetItems() { return _proxy; } public Person GetItem(string key) { return _proxy.SingleOrDefault(p => p.ID == key); } } 

Repository for Products

 class ProductsRepository : IRepository<Product, int> { private static List<Product> _proxy; public ProductsRepository(List<Product> products = null) { if (products == null) _proxy = Product.GetAllProducts(); else _proxy = products; } public IEnumerable<Product> GetItems() { return _proxy; } public Product GetItem(int key) { return _proxy.SingleOrDefault(p => p.ID == key); } } 

Answer 1

Yes, you can, using delegate for selecting order by clause:

static void PrintCollection<T, TKey, TOrder>(IRepository<T, TKey> repo, 
       Func<T,TOrder> orderBy)
{
    var items = repo.GetItems();
    if (orderBy != null)
        items = items.OrderBy(orderBy);
    foreach (T p in items)
        Console.WriteLine(Convert.ToString(p));
}

you can call it like:

PrintCollection(repo, x=>x.Name);

For Where you need argument of type Func<T,bool> and the process is the same as above.

Note: If you use ORM like EF, the ORMs need entire expression tree in order to build the SQL statement for the query. As we pass a delegate, so the ordering or filtering does not translate to SQL. The ORM loads the data then applies ordering or filtering on it.

If you want to make sure they translate to SQL, the return type must be of type that implements IQueryable and the parameters for the function should change to Expression<Func<T,TOrder>> for orderBy and Expression<Func<T,bool>> for while .

Suggestion

Define this method as extension for IEnumerable :

public static class IEnumerableExtenstions
{
    static void PrintCollection<T>(this IEnumerable<T> collection)
    {
        foreach (T p in collection)
            Console.WriteLine(Convert.ToString(p));
    }
}

and then you can call it like:

repo.GetItems().Where(x=>x.Age>15).OrderBy(x=>x.Name).PrintCollection();

which is more flexible and natural than defining parameters for each functionality in the function.