Справочник по LINQ — Важные сигнатуры

IEnumerable<TSource> Where<TSource>(
    this IEnumerable<TSource> source,
    Func<TSource, bool> predicate)

IQueryable<TSource> Where<TSource>(
    this IQueryable<TSource> source,
    Expression<Func<TSource, bool>> predicate)

IEnumerable<TResult> Select<TSource, TResult>(
    this IEnumerable<TSource> source,
    Func<TSource, TResult> selector)

IQueryable<TResult> Select<TSource, TResult>(
    this IQueryable<TSource> source,
    Expression<Func<TSource, TResult>> selector)