GroupBy使用已知类作为分组键[重复]

This question already has an answer here:

这个问题在这里已有答案:

Why does using anonymous type work and using an explicit type not in a GroupBy? 1 answer

为什么使用匿名类型工作并使用不在GroupBy中的显式类型? 1个答案

We wanted to GroupBy with a known class as the grouping key. It doesn't seem to work - LINQ appears to need to group with an anonymous class.

我们希望GroupBy使用已知类作为分组键。它似乎不起作用 - LINQ似乎需要与匿名类组合。

Is it possible to use GroupBy with a known class as the grouping key?

是否可以将GroupBy与已知类一起用作分组键?

If not, why not?

如果没有,为什么不呢?

If so, how?

如果是这样,怎么样?

This question is different than Why does using anonymous type work and using an explicit type not in a GroupBy? because it asks how to use GroupBy with a known class.

这个问题与为什么使用匿名类型工作和使用不在GroupBy中的显式类型不同?因为它询问如何将GroupBy与已知类一起使用。

Here is a fiddle that shows what works and what fails. The one that fails doesn't create a group.

这是一个小提琴,显示哪些有效,哪些无效。失败的那个不会创建一个组。

public static void Works(EnumerableRowCollection<DataRow> dataRows)
{
    var grouped = dataRows.GroupBy(
        row => new {
            Name = row["Name"]
        }, 
        (k, g) => new {
            Key = k,
            Group = g.CopyToDataTable()
        });
}

public static void Fails(EnumerableRowCollection<DataRow> dataRows)
{
    var grouped = dataRows.GroupBy(
        row => new GroupingKey {
            Name = row["Name"]
        }, 
        (k, g) => new {
            Key = k,
            Group = g.CopyToDataTable()
        });
}

1 个解决方案

#1

The difference is in the way that comparison is performed on anonymous types vs. concrete types.

区别在于对匿名类型与具体类型进行比较的方式。

According to the GroupBy documentation:

根据GroupBy文档:

The default equality comparer Default is used to compare keys.

默认的相等比较器Default用于比较键。

For anonymous types, the default equality comparer uses a property-by-property comparison, so any two anonymous types with the same members and values will be considered equivalent.

对于匿名类型,默认的相等比较器使用逐个属性的比较,因此具有相同成员和值的任何两个匿名类型将被视为等效。

For concrete types, the default equality comparer uses reference comparison, so any two instances of the object are always considered inequivalent, regardless of their values.

对于具体类型,默认的相等比较器使用引用比较,因此对象的任何两个实例始终被视为不等价,而不管它们的值如何。

To make GroupBy work with your GroupingKey class, you can either use the overload that takes an IEqualityComparer<T> to define comparisons, or override Object.Equals in GroupingKey to perform the desired comparison.

要使GroupBy与GroupingKey类一起使用,您可以使用带有IEqualityComparer 的重载来定义比较,或者覆盖GroupingKey中的Object.Equals以执行所需的比较。

#1