实体框架和IEnumerable

Question

I have written a paging function that I had hoped would do an EF safe paging. It seems though that this functions executes the EF query BEFORE paging, where as what I would like it to do is defer the paging to the database. I thought IEnumerable would be safe but it appears it is not. What is happening here?

private IEnumerable<T> PageList<T>(IEnumerable<T> list, PaginationOptions options)
{
    return list.Skip((options.Page - 1) * options.ResultsPerPage).Take(options.ResultsPerPage);
}

If I test this as a function vs the actual function called one generates SQL that contains the paging information and the other (the function) doesnt.

var pagedStuff = this.PageList(activities, options); // doesnt create correct query
var pagedStuff = activities.Skip((options.Page - 1) * options.ResultsPerPage).Take(options.ResultsPerPage);

Ps I use IEnumerable as sometimes this function is used for normal Lists.

Answer 1

I use IEnumerable as sometimes this function is used for normal List s

You may want to consider making an IQueryable overload as well:

private IQueryable<T> PageList<T>(IQueryable<T> list, PaginationOptions options)
{
    return list.Skip((options.Page - 1) * options.ResultsPerPage).Take(options.ResultsPerPage);
}

The problem is that since the input is an IEnumerable , the query is being executed before the paging is applied, so you get all records back and the paging is done in Linq-to-Objects.

Using IQueryable delays the execution of the query until after the paging is applied.

Answer 2

You're invoking IEnumerable.Skip/Take instead of IQueryable.Skip/Take .

Try to use the AsQueryable method:

return list.AsQueryable()
           .Skip((options.Page - 1) * options.ResultsPerPage)
           .Take(options.ResultsPerPage);