如果值在先前的记录中，则在linq查询中选择null [英] Select null in a linq query if the value is in a previous record

问题描述

考虑以下查询:

var ds = (from t1 in list1
          from t2 in list2
          select new {t1.Name, t2.Name}
         ).ToList();

这将返回类似的内容:(粗略表示)

This returns something like: (rough representation)

Name1, InnerName1
Name1, InnerName2
Name2, InnerName1
Name2, InnerName2

我想得到的是:

Name1, InnerName1
Null, InnerName2
Name2, InnerName1
Null, InnerName2.

意思是，如果我已经在列表中包含t1.Name，我想在t1的其余结果中使用null或空字符串.

我已经知道我可以遍历结果，但是我将其用作数据源，并且希望基于集合进行操作.

I already know that I could loop through the results, but I'm using this as a datasource and would like to do something set based.

有没有办法在单个查询中完成此操作?

Is there a way to accomplish this in a single query?

推荐答案

假设您使用Linq-to-Objects，则可以执行以下操作:

Assuming your using Linq-to-Objects, you could do something like this:

string previous = null;
var results = 
    (from t1 in list1
     from t2 in list2
     select new {
         Name1 = (previous == t1.Name) ? null : (previous = t1.Name), 
         Name2 = t2.Name 
     })
    .ToList();

但是这依赖于副作用，而且不是特别优雅.您可能会喜欢这样的东西:

But that relies on side effects and isn't particularly elegant. You might prefer something like this:

var result = 
    (from t1 in list1
     select 
       list2.Take(1)
         .Select(t2 => new { Name1 = t1.Name,  Name2 = t2.Name })
         .Concat(list2.Skip(1)
           .Select(t2 => new { Name1 = (string)null, Name2 = t2.Name }))
    .SelectMany(x => x)
    .ToList();

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