IPagedList OrderBy

Question

I want to create a generic repository, and have a method that returns an IPagedList like the following:

    public IPagedList<TEntity> GetPage(int? page, int? size)
    {
        IQueryable<TEntity> query = _dbSet; // DbSet<TEntity> _dbSet;
        return query.ToPagedList(page ?? 1, size ?? 10);
    }

However, IPagedList requires to add an OrderBy() or it throws the following message:

The method 'Skip' is only supported for sorted input in LINQ to Entities. The method 'OrderBy' must be called before the method 'Skip'.

How can i achieve this, how can i add ordering for TEntity type? I can have the method return IQueryable and do the paging on controller / caller and it works, but wanted to do this on the generic repository.

Answer 1

Make your TEntity more specific as in implementing either an abstract base class or an interface then update you method signature to reflect this.

To illustrate I have a mode class called Blog which implements interface IEntity

public interface IEntity
{
    Int32 Id { get; set; }
}
public class Blog : IEntity
{
    public int Id { get; set; }
    public String Name { get; set; }
}

Then you could update your Method signature like so:

public IPagedList<TEntity> GetPage<TEntity>(int? page, int? size) where TEntity : IEntity
{
    IQueryable<TEntity> query = _dbSet; // DbSet<TEntity> _dbSet;
    return query.OrderBy(x => x.Id).ToPagedList(page ?? 1, size ?? 10);
}

Alternatively you could use the handy Extension method from here Dynamic Linq OrderBy.

Then you could update your method signature to be

public IPagedList<TEntity> GetPage<TEntity>(int? page, int? size, string orderBy) where TEntity : IEntity
{
    IQueryable<TEntity> query = _dbSet; // DbSet<TEntity> _dbSet;
    return query.OrderBy(orderBy).ToPagedList(page ?? 1, size ?? 10);
}

You could easily extend this to also take sort direction into consideration