GroupBy With Maximum Size

public void Splits_Group_When_GroupSize_Greater_Than_MaxSize()
{
    var items = new[] { "A1", "A2", "A3", "B4", "B5" };
 
    var result = items.GroupByWithMaxSize(i => i[0], maxSize: 2);
 
    Assert.True(result.ElementAt(0).SequenceEqual(new[] { "A1", "A2" }));
    Assert.True(result.ElementAt(1).SequenceEqual(new[] { "A3" }));
    Assert.True(result.ElementAt(2).SequenceEqual(new[] { "B4", "B5" }));
}

The following code is not the fastest or cleverest solution, but it does make all the tests turn green.  

public static IEnumerable<IEnumerable<T>> GroupByWithMaxSize<T, TKey>(
    this IEnumerable<T> source, Func<T, TKey> keySelector, int maxSize)
{
    var originalGroups = source.GroupBy(keySelector);

    foreach (var group in originalGroups)
    {
        if (group.Count() <= maxSize)
        {
            yield return group;
        }
        else
        {
            var regroups = group.Select((item, index) => new { item, index })
                                .GroupBy(g => g.index / maxSize);
            foreach (var regroup in regroups)
            {
                yield return regroup.Select(g => g.item);
            }
        }
    }
}

In this case I don’t need the Key property provided by IGrouping, so the return type is a generically beautiful IEnumerable<IEnumerable<T>>.