c# - 默认后不能出现linq过滤器？

Question

我一直试图在 LINQ 中重写一些命令式代码，直到我意识到我遗漏了什么。但我不知道为什么这是一个问题。考虑以下。使用

运行 Test.Test2(testList) 时测试失败

Sequence contains no matching elements

过去，我交替处理这些形式，通过将我的谓词推送到 First、Single 等...子句来编写更小的代码。显然，当涉及到 DeafultIfEmpty 时，我不能这样做。这是因为 Where 和 First、Single 等不能互换吗？或者，是因为 DefaultIfEmpty 子句引入了复杂性吗？

编辑 1我添加了一个测试来表明 FirstOrDefault 不起作用。它因“未找到”不等于 (null) 而失败。

public static class Test
{
    public static string Test1(params string[] input)
    {
        return input
            .Where(x => x == "apples")
            .DefaultIfEmpty("bannanas")
            .First();
    }

    public static string Test2(params string[] input)
    {
        return input
            .DefaultIfEmpty("bannanas")
            .First(x => x == "apples");
    }

    public static string Test3(params string[] input)
    {
        return input
            .DefaultIfEmpty("bannanas")
            .FirstOrDefault(x => x == "apples");
    }
}

public class TestStuff
{
    [Fact]
    public static void TestOneAndTwo()
    {
        var testList = new string[] { "oranges", "pears", "pineapples" };
        var one = Test.Test1(testList);
        var two = Test.Test2(testList);
        Assert.Equal(one, two);
    }

    [Fact]
    public static void TestOneAndThree()
    {
        var testList = new string[] { "oranges", "pears", "pineapples" };
        var one = Test.Test1(testList);
        var three = Test.Test3(testList);
        Assert.Equal(one, three);
    }
}

Answer 1

LINQ 方法的顺序很重要，考虑每个方法对输入可枚举的作用({ "oranges", "pears", "pineapples"}):

public static string Test1(params string[] input)
{
    return input
        .Where(x => x == "apples") // empty enumerable, because no item matches "apples"
        .DefaultIfEmpty("not found") // {"not found"}, since the enumerable is empty
        .First(); //"not found", since we have this item
} 

public static string Test2(params string[] input)
{
    return input
        .DefaultIfEmpty("not found") // { "oranges", "pears", "pineapples" }
                                     //i.e., nothing changes, because input is not empty
        .First(x => x == "apples"); //Exception because there is no
                                    //item that is equal to "apples"
}

如果将最后一个方法从 First 更改为 FirstOrDefault，那将产生 null 因为 default(string) 为 null。