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GroupBy KeySelector函数是什么样子的?

linq c#

user875234 · 技术社区 · 7 年前

GroupBy 以自己的方式。键选择程序代码不正确。一个主要问题是 Widget.foo

小工具.cs

public class Widget
{
    public string foo { get; set; }
    public double bar { get; set; }
    public bool fee { get; set; }
}

static void Main(string[] args)
{
    var widgets = new List<Widget>()
    {
        new Widget() { foo = "red"  , bar = 1.0, fee = true },
        new Widget() { foo = "green", bar = 2.0, fee = true },
        new Widget() { foo = "green", bar = 2.0, fee = false },
        new Widget() { foo = "green", bar = 3.0, fee = false },
        new Widget() { foo = "blue" , bar = 4.0, fee = true }
    };

    var gb = widgets.GroupBy(
        w => GenerateGroupByKey(),
        w => w,
        (prop, groupedWidgets) => new
        {
            GroupedWidgets = groupedWidgets
        }
    ).ToList();
}

按键选择器

static Func<Widget, object> GenerateGroupByKey() 
{
    Func<Widget, object> s = delegate(Widget widget)
    {
        return new { widget.foo };
    };

    return s;
}

3 回复 | 直到 7 年前

Django 7 年前

必须将委托传递给参数,所以只需调用函数并返回委托。

var gb = widgets.GroupBy(
    GenerateGroupByKey(),
    w => w,
    (prop, groupedWidgets) => new
    {
        GroupedWidgets = groupedWidgets
    }
).ToList();

Alexander I. 7 年前

您的分组不起作用,因为 GenerateGroupByKey()

Equals here .

new { widget.foo } ?

您可以只返回字符串而不是对象。

试着像这样使用委派:

static Func<Widget, object> GenerateGroupByKey() 
{
    Func<Widget, object> s = delegate(Widget widget)
    {
        return widget.foo;
    };

    return s;
}

ManishM 7 年前

使用如下参数分组

Func<TSource, TKey> keySelector,
Func<TSource, TElement> elementSelector,
Func<TKey, IEnumerable<TElement>, TResult> resultSelector

编写时w=>GenerateGroupByKey() 您正在创建一个输入参数为GenerateGroupByKey()的新FUNC

static Func<Widget, object> GenerateGroupByKey() 
{
    Func<Widget, object> s = delegate(Widget widget)
    {
        return new { widget.foo };
    };

    return s;
}

你不能调用这个函数

解决方案:

static void Main(string[] args)
{
    var widgets = new List<Widget>()
    {
        new Widget() { foo = "red"  , bar = 1.0, fee = true },
        new Widget() { foo = "green", bar = 2.0, fee = true },
        new Widget() { foo = "green", bar = 2.0, fee = false },
        new Widget() { foo = "green", bar = 3.0, fee = false },
        new Widget() { foo = "blue" , bar = 4.0, fee = true }
    };

    var gb = widgets.GroupBy(
        GenerateGroupByKey(), // Your FUNC method
        w => w,
        (prop, groupedWidgets) => new
        {
            GroupedWidgets = groupedWidgets
        }
    ).ToList();
}

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