C# method for getting all possible combinations of elements

CollectionExtensions.cs

/// <summary>
/// Gets all possible combinations of the specified list, based on the specified length of each resulting element.
/// </summary>
/// <typeparam name="T"></typeparam>
/// <param name="list"></param>
/// <param name="length"></param>
/// <param name="check">Additional check before adding an element to the final result. (t1 = IEnumerable of T, t2 = T)</param>
/// <returns></returns>
static IEnumerable<IEnumerable<T>> GetCombinations<T>(IEnumerable<T> list, int length, 
    Func<IEnumerable<T>, T, bool> check = null)
{
    if (length == 1) return list.Select(t => new T[] { t });

    return GetCombinations(list, length - 1, check).ToArray()
        .SelectMany(t => list, (t1, t2) => {
            if (check == null || (t1 != null && check(t1, t2)))
                return t1.Concat(new T[] { t2 }).ToArray();
            else
                return null;
            })
            .Where(result => result != null)
            .ToArray();
        // Simple way (no check):
        //.SelectMany(t => list, (t1, t2) => t1.Concat(new T[] { t2 }));
        }