用于声明显式类型而不是 var 的 Linq 语法

Question

我正在尝试编写一个 LINQ 查询来获取List<List<testobject>> results = ...

我使用 var 得到正确的结果，但想声明显式类型而不是使用 var .执行此操作的正确语法是什么？

简单的例子如下

class Program
{
    static void Main(string[] args)
    {
        List<testobject> testobjectList = new List<testobject>()
        {
            new testobject(){field1 = 1, field2 = "1",field3 = "1",field4 = "1", field5 = "1"},
            new testobject(){field1 = 1, field2 = "1",field3 = "1a",field4 = "1a", field5 = "1a"},
            new testobject(){field1 = 1, field2 = "1",field3 = "1b",field4 = "1b", field5 = "1b"},
            new testobject(){field1 = 2, field2 = "2",field3 = "2",field4 = "2", field5 = "2"},
            new testobject(){field1 = 3, field2 = "3",field3 = "3",field4 = "3", field5 = "3"},
            new testobject(){field1 = 4, field2 = "4",field3 = "4",field4 = "4", field5 = "4"},
            new testobject(){field1 = 4, field2 = "4",field3 = "4a",field4 = "4a", field5 = "4a"},
            new testobject(){field1 = 5, field2 = "5",field3 = "5",field4 = "5", field5 = "5"},
            new testobject(){field1 = 6, field2 = "6",field3 = "6",field4 = "6", field5 = "6"},
            new testobject(){field1 = 6, field2 = "6",field3 = "6a",field4 = "6a", field5 = "6a"},
            new testobject(){field1 = 6, field2 = "6",field3 = "6b",field4 = "6b", field5 = "6b"},
            new testobject(){field1 = 7, field2 = "7",field3 = "7",field4 = "7", field5 = "7"}
        };
        // Correct output
        var results1 = testobjectList.Where(x => x.field1 >= 2)
                                     .GroupBy(x => x.field2).ToList();
        // But how do I do the same but explicitly state type?
        List<List<testobject>> results2 = testobjectList.Where(x => x.field1 >= 2)
                                                        .GroupBy(x => x.field2).ToList();
    }
}
class testobject
{
    public int field1 { get; set; }
    public string field2 { get; set; }
    public string field3 { get; set; }
    public string field4 { get; set; }
    public string field5 { get; set; }
}

Answer 1

首先，当您使用GroupBy函数时，var被编译为：

List<IGrouping<string,testobject>>

如果您确实想要List<List<testobject>>可以使用此查询：

testobjectList.Where(x => x.field1 >= 2).GroupBy(x => x.field2).Select(x => x.ToList()).ToList();

如果你想拥有List<testobject>你可以使用：

testobjectList.Where(x => x.field1 >= 2).GroupBy(x => x.field2).Select(x => x.First()).ToList();

Answer 2

List<List<testobject>>不是正确的类型。如果在声明results1之后的行上添加了断点并检查其类型，则可能会看到类似于List<IGrouping<string, testobject>>的内容。如果要声明显式类型，请使用一些方法来找出实际类型，例如调试器，IDE或某些插件。ReSharper可以给你一个重构选项来声明显式类型而不是var。

Answer 3

当您将鼠标悬停在 VisualStudio 中的ToList()调用上时，可以看到返回类型为 List<IGrouping<string, testobject>>

尝试：

List<IGrouping<string, testobject>> results2 = testobjectList.Where(x => x.field1 >= 2)
                                                             .GroupBy(x => x.field2).ToList();

如果你有ReSharper，你可以使用"显式指定类型"重构将var语句转换为显式类型。

Answer 4

您需要通过添加 .选择多(( 以强类型化结果作为列表。

您还将列表声明为列表<列表结果 2，当它应该是列表时

工作示例：

List<testobject> results2 = testobjectList .Where(x => x.field1 >= 2)
                                                    .GroupBy(x=>x.field2)
                                                    .SelectMany(x=>x)
                                                    .ToList();