Where和.Contors的解具有O(N平方)的复杂性。简单.join应该有更好的性能(由于散列而接近O(N))。正确的代码是：

_dataContext.UserProfile.Join(idList, up => up.ID, id => id, (up, id) => up);

请看下面的测试示例：

    private static void Main(string[] args)
    {
        var userProfiles = GenerateUserProfiles();
        var idList = GenerateIds();
        var stopWatch = new Stopwatch();
        stopWatch.Start();
        userProfiles.Join(idList, up => up.ID, id => id, (up, id) => up).ToArray();
        Console.WriteLine("Elapsed .Join time: {0}", stopWatch.Elapsed);
        stopWatch.Restart();
        userProfiles.Where(up => idList.Contains(up.ID)).ToArray();
        Console.WriteLine("Elapsed .Where .Contains time: {0}", stopWatch.Elapsed);
        Console.ReadLine();
    }

    private static IEnumerable<int> GenerateIds()
    {
       // var result = new List<int>();
        for (int i = 100000; i > 0; i--)
        {
            yield return i;
        }
    }

    private static IEnumerable<UserProfile> GenerateUserProfiles()
    {
        for (int i = 0; i < 100000; i++)
        {
            yield return new UserProfile {ID = i};
        }
    }

控制台输出：

Elapsed .Join time: 00:00:00.0322546

Elapsed .Where .Contains time: 00:02:19.4072107