通过动态linq分组后的访问密钥 [英] Access key after grouping by with dynamic linq

问题描述

使用 System.Dynamic.Linq 我有一个类似的group by语句:

Using System.Dynamic.Linq I have a group by statement that looks like this:

var rowGrouped = data.GroupBy(rGroup, string.Format("new({0})", c));

c只是字段名称的字符串数组，我需要在分组中选择它们.在这种情况下，GroupBy返回IEnumerable(注意:不是IEnumerable<T>，因为我们不知道T是什么).常规 GroupBy 将返回

Where c is just a string array of field names I need to have selected in my grouping. GroupBy in this case returns a IEnumerable (note: not an IEnumerable<T> because we don't know what T is). The regular GroupBy would return a System.Collections.Generic.IEnumerable<IGrouping<TKey, TElement>>

现在我的问题是，如何遍历各个组，以便可以访问密钥(IGrouping<TKey, TElement>在IGrouping<TKey, TElement>中定义，但我不知道TKey和TElement a先验)?

Now my question is, how do I iterate through the groups such that I have access to the key (Key is defined in IGrouping<TKey, TElement> but I don't know TKey and TElement a priori)?

我最初尝试过:

foreach (var row in rowGrouped)

进行迭代，但是我无法访问row.Key(在这种情况下，row是类型object)

Which iterates, but I can't access row.Key (row in this case is type object)

所以我做到了:

foreach (IGrouping<object,dynamic> row in rowGrouped)

令人惊讶的是，只要键是一个字符串，它就可以工作.但是，如果键碰巧是数字键(例如short)，则会出现此错误:

Which, surprisingly, worked...as long as the key was a string. If, however, the key happened to be numeric (for example short), then I get this error:

Unable to cast object of type 'Grouping`2[System.Int16,DynamicClass2]' to type 
'System.Linq.IGrouping`2[System.Object,System.Object]'.

我需要能够遍历rowGrouped并为每个row获取Key，然后遍历每个组中的集合.

I need to be able to iterate through rowGrouped and get the Key for every row and then iterate through the collection in each group.

推荐答案

问题是您的密钥是short，并且您试图投射到密钥为object的接口.尽管IGrouping<>具有协变类型参数，但它只是行不通，因为仅变化适用于引用类型.由于short是值类型，因此将不起作用.

The problem is that your key is a short and you're trying to cast to an interface where the key is object. While IGrouping<> has covariant type parameters, it just simply won't work because variance only applies to reference types. Since short is a value type, it will not work.

如果知道密钥将始终是short，则应将其强制转换为这样.

If you know the key will always be a short, you should cast it as such.

foreach (IGrouping<short, dynamic> row in rowGrouped)
    ...

否则，如果它可以是值类型或引用类型，则将整个行保留为dynamic可能会更容易.

Otherwise if it's possible that it can be a value type or a reference type, it might just be easier to keep the entire row as dynamic.

foreach (dynamic row in rowGrouped)
    ...

尽管我个人对此有麻烦.在这种情况下，也许是一个错误，但是运行时无法确定该行具有Key属性.我无法告诉您确切原因，但请记住这一点.

Though I personally have had troubles with this. Perhaps a bug but the runtime cannot figure out that the row has a Key property in this case. I can't tell you why exactly but keep that in mind.

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