Grouping lists into groups of X items per group

Question

I'm having a problem knowing the best way to make a method to group a list of items into groups of (for example) no more than 3 items. I've created the method below, but without doing a ToList on the group before I return it, I have a problem with it if the list is enumerated multiple times.

The first time it's enumerated is correct, but any additional enumeration is thrown off because the two variables (i and groupKey) appear to be remembered between the iterations.

So the questions are:

• Is there a better way to do what I'm trying to achieve?

• Is simply ToListing the resulting group before it leaves this method really such a bad idea?

  public static IEnumerable<IGrouping<int, TSource>> GroupBy<TSource>
                (this IEnumerable<TSource> source, int itemsPerGroup)
  {
      const int initial = 1;
      int i = initial;
      int groupKey = 0;
  
      var groups = source.GroupBy(x =>
      {
          if (i == initial)
          {
              groupKey = 0;
          }
  
          if (i > initial)
          {
              //Increase the group key if we've counted past the items per group
              if (itemsPerGroup == initial || i % itemsPerGroup == 1)
              {
                  groupKey++;
              }
          }
  
          i++;
  
          return groupKey;
      });
  
      return groups;
  }
  

Answer 1

Here's one way to do this using LINQ...

public static IEnumerable<IGrouping<int, TSource>> GroupBy<TSource>
    (this IEnumerable<TSource> source, int itemsPerGroup)
{
    return source.Zip(Enumerable.Range(0, source.Count()),
                      (s, r) => new { Group = r / itemsPerGroup, Item = s })
                 .GroupBy(i => i.Group, g => g.Item)
                 .ToList();
}

Live Demo