Using Linq to get groups from a collection

I've a collection of 55 string values in my C# program. I want to get group each having 20 (or less) items from this co开发者_运维百科llection.

So, that means, I'll have 3 groups with 20, 20 and 15 items. Then for each group, I need to call a method.

How do i achieve this using Linq?

Thanks.

List<string> strings = ...
List<List<string>> groups = strings
    .Select((s, idx) => new { Item = s, Group = idx / 20 })
    .GroupBy(s => s.Group)
    .Select(grp => grp.Select(g => g.Item).ToList())
    .ToList();

You can build an extension method like so. Note that I haven't error handled null/empty source.

static class EnumerableExtensions {
    public static IEnumerable<IEnumerable<T>> GetPages<T>(
        this IEnumerable<T> source,
        int pageSize
    ) {
        var e = source.GetEnumerator();
        while(e.MoveNext()) {
            int count = pageSize - 1;
            List<T> list = new List<T>(pageSize) { e.Current };
            while (count > 0 && e.MoveNext()) {
                list.Add(e.Current);
                count--;
            }
            yield return list;
        } 
    }
}

Then:

IEnumerable<string> strings;
var pages = strings.GetPages(20)

Example:

var list = Enumerable.Range(0, 55).Select(x => x.ToString()).ToList();
foreach (var page in list.GetPages(20)) {
    Console.WriteLine(page.Count());
}

Output:

20
20
15