延期执行 - 字典输出 [英] Deferred Execution - Dictionary output

问题描述

我有一个返回约50万行的Linq Query。但是，由于我没有调用ToList（），执行仍然延迟。

I have a Linq Query that returns about half a million rows. However, since I have not called ToList(), the execution is still deferred. So far so good.

我需要一个字典< int，List< DateTime> 这将在Dictionary中返回大约100,000个条目（Distinct integer column values）。

I need a Dictionary<int, List<DateTime>> output from this query which would return about 100,000 entries (Distinct integer column values) in the Dictionary.

我尝试过这样的东西...

I tried something like this ...

var myDictionary = new Dictionary<int, List<DateTime>>();

var myDictionaryOutput = MyQuery.Select(p => AddOrUpdateDictionary(myDictionary, p));

MyQuery是我的LINQ查询，AddOrUpdateDictionary是我写的一个自定义函数。

Where MyQuery is my linq query and AddOrUpdateDictionary is a custom function I wrote.

这个项目将每个条目都设置为不需要我的字典。

This projects each entry to a Dictionary which is not what I want.

我只想输入字典。

样本数据如下所示...

The sample data looks as follows ...

1234 | 2015-08-24 | 2015-08-25 | null       |
1234 | null       | 2015-08-26 | null       |
2345 | null       | null       | 2015-08-23 |

我希望输出是一个具有两个键的字典

I want the output to be a Dictionary with two keys

（来自500,000行的3行的2个键=> 100,000个键），

(2 Keys from 3 rows <=> 100,000 keys from 500,000 rows),

1234 -> List<DateTime>{2015-08-24, 2015-08-25, 2015-08-26}
and 
2345 -> List<DateTime>{2015-08-23}

我的想法是评估MyQuery.ToList（）和将它传递给我的自定义函数将不会非常有效，因为我将不得不使用ForEach循环五十万行。

My thoughts are evaluating MyQuery.ToList() and passing it to my Custom function will not be very efficient as I would then have to loop through half a million rows using ForEach for example.

我错了，还是有一个

推荐答案

看起来你想做的是一个组过滤 null s，然后转换为字典：

It looks like what you want to do is a group by with filtering nulls, and subsequent conversion to dictionary:

var dict = MyQuery
    .Select(row => new {row.Id, Dates = new[] {row.Date1, row.Date2, row.Date3}})
    .SelectMany(row => row.Dates.Select(d => new {row.Id, Date=d}))
    .GroupBy(row => row.Id)
    .ToDictionary(g => g.Key, g => g.Select(r => r.Date).ToList());

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