c# - Mongodb 在使用 Take(1) 时返回多个结果

Question

我只想使用 C# LINQ 获取一行:

collection.AsQueryable().Take(1).OrderByDescending(x => x.Id).Single();

我收到以下错误:

序列包含多个元素

这不是 Take(1) 的用途吗？
更新1 这是只获取没有单个/第一个的结果...

Answer 1

在我的情况下它有效

        int[] grades = { 1 , 2, 3 };
        Console.WriteLine(grades.Take(1).Single());

        //the result is 
        // 1

更广泛的例子

        var a = new List<int>{10, 20, 30};
        var b = new List<int>{100, 200, 300};
        var grades = new List<List<int>>{a, b};
        Console.WriteLine(grades.Take(1).Single());

产生的结果

List<Int32> (3 items)
10 
20 
30 

LINQ 语句的顺序很重要

检查这两个例子

        int[] grades = { 1, 2, 3, 4, 5 };

        IEnumerable<int> topThreeGrades =
            grades.Take(3).OrderByDescending(grade => grade);

        Console.WriteLine("The top three grades are:");
        foreach (int grade in topThreeGrades)
        {
            Console.WriteLine(grade);
        }

产生

The top three grades are:
3
2
1

但是

        int[] grades = { 1, 2, 3, 4, 5 };

        IEnumerable<int> topThreeGrades =
            grades.OrderByDescending(grade => grade).Take(3);

        Console.WriteLine("The top three grades are:");
        foreach (int grade in topThreeGrades)
        {
            Console.WriteLine(grade);
        }

会产生

The top three grades are:
5
4
3