Process a list with a loop, taking 100 elements each time and automatically less than 100 at the end of the list

You can make use of LINQ Skip and Take and your code will be cleaner.

for (int i = 0; i < listLength; i=i+100)
{
    var items = bigList.Skip(i).Take(100); 
    // Do something with 100 or remaining items
}

Note: If the items are less than 100 Take would give you the remaining ones.

I didn't like any of the answers listed, so I made my own extension:

public static class IEnumerableExtensions
{
    public static IEnumerable<IEnumerable<T>> MakeGroupsOf<T>(this IEnumerable<T> source, int count)
    {
        var grouping = new List<T>();
        foreach (var item in source)
        {
            grouping.Add(item);
            if(grouping.Count == count)
            {
                yield return grouping;
                grouping = new List<T>();
            }
        }

        if (grouping.Count != 0)
        {
            yield return grouping;
        }
    }
}

Then you can use it:

foreach(var group in allItems.MakeGroupsOf(100))
{
    // Do something
}

You can keep an explicit variable for the end point:

for (int i = 0, j; i < listLength; i = j)
{
    j = Math.min(listLength, i + 100);
    // do your thing with bigList.GetRange(i, j)
}