c# - Enumerable.Count()==n 的替代品

Question

我正在为 Enumerable.Count() == n 寻找更好的替代方法。我能想到的最好的是:

static class EnumerableExtensions
{
    public static bool CountEquals<T>(this IEnumerable<T> items, int n)
    {
        if (n <= 0) throw new ArgumentOutOfRangeException("n"); // use Any()

        var iCollection = items as System.Collections.ICollection;
        if (iCollection != null)
            return iCollection.Count == n;

        int count = 0;
        bool? retval = null;
        foreach (var item in items)
        {
            count++;

            if (retval.HasValue)
                return false;

            if (count == n)
                retval = true;
        }

        if (retval.HasValue)
            return retval.Value;

        return false;
    }
}

class Program
{
    static void Main(string[] args)
    {
        var items0 = new List<int>();
        var items1 = new List<int>() { 314 };
        var items3 = new List<int>() { 1, 2, 3 };
        var items5 = new List<int>() { 1, 2, 3, 4, 5 };
        var items10 = Enumerable.Range(0, 10);
        var itemsLarge = Enumerable.Range(0, Int32.MaxValue);

        Console.WriteLine(items0.CountEquals(3));
        Console.WriteLine(items1.CountEquals(3));
        Console.WriteLine(items3.CountEquals(3));
        Console.WriteLine(items5.CountEquals(3));
        Console.WriteLine(itemsLarge.CountEquals(3));
    }
}

我可以做得更好吗？有没有一种方法可以进一步概括这一点——传递比较？

Answer 1

您可以结合使用 Take 和 Count 来完全摆脱循环:

public static bool CountEquals<T>(this IEnumerable<T> items, int n)
{
  var iCollection = items as System.Collections.ICollection;
  if (iCollection != null)
    return iCollection.Count == n;
  return items.Take(n + 1).Count() == n;
}