Fairly elegant & performant way to find if object is contained in set without using Linq

is-element-of-linq.cs

    using System;
    using System.Collections.Generic;
    using System.Linq;

    public static class GenericExtensions
    {
        public static bool IsElementOf<T>(this T item, params T[] list)
        {
            return Array.IndexOf(list, item) != -1;
        }

        public static bool IsElementOf<T>(this T item, IEnumerable<T> list)
        {
            return IsElementOf(item, list.ToArray());
        }
    }

is-element-of.cs

using System;

public static class GenericExtensions
{
    public static bool IsElementOf<T>(this T item, params T[] list)
    {
        return Array.IndexOf(list, item) != -1;
    }
}