c# - 如何递归遍历嵌套泛型集合？

Question

我想计算嵌套集合中类的数量，例如[[[1,2],[3,4,5]],[[1,2],[3,4,5]]]。这里引用的类是“int”，预期答案是 10。

我将此列表生成为:

        List<int> list1 = new List<int>(2);
        list1.Add(1);
        list1.Add(2);
        List<int> list2 = new List<int>(3);
        list2.Add(3);
        list2.Add(4);
        list2.Add(5);

        List<List<int>> listlist = new List<List<int>>(2);
        listlist.Add(list1);
        listlist.Add(list2);

        List<List<List<int>>> listlistlist = new List<List<List<int>>>(2);
        listlistlist.Add(listlist);
        listlistlist.Add(listlist);

我在编程的时候，比较喜欢为泛型类写这样的方法，我的代码是:

    public static int CountT<T>(ICollection<T> pTCol, int intLevel = 1)
    {
        int intCount = 0;
        if (intLevel > 0)
        {
            intLevel--;
            foreach (T pT in pTCol)
            {
                ICollection<T> subTCol = pT as ICollection<T>;   //PROBLEM: I got "null" for subTCol when the program was running
                intCount += CountT(subTCol, intLevel);
            }
        }
        else if (intLevel == 0)
        {
            intCount = pTCol.Count;
        }
        return intCount;
    }

我测试了上面的代码

        int intCount = CountT(listlistlist, 2);

然后我遇到了问题

        ICollection<T> subTCol = pT as ICollection<T>;   //PROBLEM: I got "null" for subTCol when the program was running

我也试过代码:

    public static int CountT2<T, T2>(ICollection<T> pTCol, int intLevel = 1)
    {
        int intCount = 0;
        if (intLevel > 0)
        {
            intLevel--;
            foreach (T pT in pTCol)
            {
                ICollection<T2> subTCol = pT as ICollection<T2>;
                intCount += CountT2(subTCol, intLevel);   //PROBLEM: The type arguments for method cannot be inferred from the usage. Try specifying the type arguments explicitly. (I could not pass the compiling)
            }
        }
        else if (intLevel == 0)
        {
            intCount = pTCol.Count;
        }
        return intCount;
    }

编译不通过

        intCount += CountT2(subTCol, intLevel);   //PROBLEM: The type arguments for method cannot be inferred from the usage. Try specifying the type arguments explicitly. (I could not pass the compiling)

我该怎么做？

Answer 1

ICollection subTCol = pT as ICollection//问题:我的 subTCol 为“null”

这是 null 的原因是 pT不是 ICollection<T>因为T是int和 listlistlist是 List<List<List<T>>> .因此pT是 List<List<T>>这就是为什么如果您尝试将其转换为 ICollection<T>你会得到空值。

我想计算嵌套集合中的项目数。

使用 linq 可以更轻松地做到这一点的 Sum方法。如果您知道嵌套级别，例如:

Assert.AreEqual(10, listlistlist.Sum(x => x.Sum(y => y.Count)));

如果您不知道嵌套级别或者您想要一个更通用的方法，您可以创建一个扩展方法，例如:

public static class RecursiveSumExtension
{
    public static int RecursiveSum(this IEnumerable items)
    {
        if (null == items)
            return 0;

        var total = 0;

        foreach (var item in items)
        {
            if (item is IEnumerable)
                total += (item as IEnumerable).RecursiveSum();

            else
                total++;
        }

        return total;
    }
}

和测试:

 Assert.AreEqual(10, listlistlist.RecursiveSum());

列表生成

顺便说一句，您可以使用 .net的集合初始化器语法使代码在创建集合时更具可读性:

        var listlist = new List<List<int>>
        {
            new List<int> {1, 2},
            new List<int> {3, 4, 5}
        };

        var listlistlist = new List<List<List<int>>>
        {
            listlist,
            listlist
        };