LinqExtensions.cs

static class LinqExtensions
{
    public static IQueryable<TSource> WhereIf<TSource>(
        this IQueryable<TSource> source,
        bool condition,
        Expression<Func<TSource, bool>> predicate)
    {
        return condition ? source.Where(predicate) : source;
    }
    public static List<T> Append<T>(this List<T> list, params T[] values)
    {
        list.AddRange(values);
        return list;
    }

    public static List<T> AppendIf<T>(this List<T> list, bool condition, params T[] values)
    {
        return condition ? list.Append(values) : list;
    }

}