Paging with LINQ for objects

Question

How would you implement paging in a LINQ query? Actually for the time being, I would be satisfied if the sql TOP function could be imitated. However, I am sure that the need for full paging support comes up sooner later anyway.

var queryResult = from o in objects
                  where ...
                  select new
                      {
                         A = o.a,
                         B = o.b
                      }
                   ????????? TOP 10????????

Answer 1

You're looking for the Skip and Take extension methods. Skip moves past the first N elements in the result, returning the remainder; Take returns the first N elements in the result, dropping any remaining elements.

See MSDN for more information on how to use these methods: http://msdn.microsoft.com/en-us/library/bb386988.aspx

For example:

int numberOfObjectsPerPage = 10;
var queryResultPage = queryResult
  .Skip(numberOfObjectsPerPage * pageNumber)
  .Take(numberOfObjectsPerPage);

Answer 2

EDIT - Removed Skip(0) as it's not necessary

var queryResult = from o in objects
                      where ...
                      select new
                          {
                             A = o.a,
                             B = o.b
                          }.Take(10);

Answer 3

   ( for o in objects
    where ...
    select new
   {
     A=o.a,
     B=o.b
   })
.Skip((page-1)*pageSize)
.Take(pageSize)

Answer 4

Using Skip and Take is definitely the way to go. If I were implementing this, I would probably write my own extension method to handle paging (to make the code more readable). The implementation can of course use Skip and Take:

static class PagingUtils {
  IEnumerable<T> Page(this IEnumerable<T> en, int pageSize, int page) {
    return en.Skip(page * pageSize).Take(pageSize);
  }
  IQueryable<T> Page(this IQueryable<T> en, int pageSize, int page) {
    return en.Skip(page * pageSize).Take(pageSize);
  }
}

The class defines two extension methods - one for IEnumerable and one for IQueryable, which means that you can use it with both LINQ to Objects and LINQ to SQL (when writing database query, the compiler will pick the IQueryable version).

Depending on your paging requirements, you could also add some additional behavior (for example to handle negative pageSize or page value). Here is an example how you would use this extension method in your query:

var q = (from p in products
         where p.Show == true
         select new { p.Name }).Page(10, pageIndex);