C# 通过 EF / Linq 投射到 KeyValuePair
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Projecting into KeyValuePair via EF / Linq
提问by GrandMasterFlush
I'm trying to load a list of KeyValuePairs from an EF / Linq query like this:
我正在尝试从 EF/Linq 查询中加载 KeyValuePairs 列表，如下所示：
return (from o in context.myTable
select new KeyValuePair<int, string>(o.columnA, o.columnB)).ToList();
My problem is that this results in the error
我的问题是这会导致错误
"Only parameterless constructors and initializers are supported in LINQ to Entities."
“LINQ to Entities 中仅支持无参数构造函数和初始值设定项。”
Is there an easy way around this? I know I could create a custom class for this instead of using KeyValuePair but that does seem like re-inventing the wheel.
有没有简单的方法来解决这个问题？我知道我可以为此创建一个自定义类而不是使用 KeyValuePair 但这似乎是在重新发明轮子。
采纳答案by Sergey Berezovskiy
Select only columnA and columnB from your table, and move further processing in memory:
从表中仅选择 columnA 和 columnB，并在内存中进行进一步处理：
return context.myTable
.Select(o => new { o.columnA, o.columnB }) // only two fields
.AsEnumerable() // to clients memory
.Select(o => new KeyValuePair<int, string>(o.columnA, o.columnB))
.ToList();
Consider also to create dictionary which contains KeyValuePairs:
还考虑创建包含 KeyValuePairs 的字典：
return context.myTable.ToDictionary(o => o.columnA, o => o.columnB).ToList();
回答by cuongle
Since LINQ to Entities does not support KeyValuePair, you should turns to LINQ to Object by using AsEnumerablefirst:
由于 LINQ to Entities 不支持KeyValuePair，您应该AsEnumerable首先使用 LINQ to Object ：
return context.myTable
.AsEnumerable()
.Select(new KeyValuePair<int, string>(o.columnA, o.columnB))
.ToList();
回答by Bartosz
There is also alternative, when you want to store multiple values for one key exists something what is called Lookup.
还有另一种选择，当您想为一个键存储多个值时，存在称为Lookup 的东西。
Represents a collection of keys each mapped to one or more values.
代表一组键，每个键都映射到一个或多个值。
Here you have some official documentations.
这里有一些官方文档。
More over lookupseems to be much faster than Dictionary < TKey, List < TValue > >.
更多的查找似乎比Dictionary < TKey, List < TValue >>快得多。
