Question

我尝试编写LINQ查询以获取List<List<testobject>> results = ...

我使用var得到了正确的结果但是想要声明显式类型而不是使用var。这样做的正确语法是什么？

简单示例如下

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将编译为：

List<IGrouping<string,testobject>>

如果您真的想拥有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

当您将鼠标悬停在VisualStudio中的ToList()调用上时，您会看到返回类型为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“明确指定类型”重构将var语句转换为显式类型。

Answer 3

List<List<testobject>>不是正确的类型。如果您在一行之后添加断点并且声明了results1并检查了它的类型，您可能会看到与List<IGrouping<string, testobject>>类似的内容。如果要声明显式类型，请使用一些方法来确定实际类型，如调试器，IDE或某些插件。 ReSharper可以为您提供重构选项来声明显式类型而不是var。

Answer 4

您需要通过添加.SelectMany（）来强化您的结果，以便将结果强类型为List。

您还将列表声明为列表＆lt; 列表＆gt;结果2应该是列表

工作样本：

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

Linq语法指定显式类型而不是var

4 个答案: