public static int Count<T>(this IEnumerable<T> source) { if (source == null) throw new ArgumentNullException(nameof(source)); if (source is ICollection<T>) { return ((ICollection<T>)source).Count; } if (source is ICollection) { return ((ICollection)source).Count; } ... } public static int Count<T>(this IEnumerable<T> source) { if (source is null) throw new ArgumentNullException(nameof(source)); if
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