有条件转让C＃VAR：优雅，因为它得到？ [英] Conditionally assign C# var: As elegant as it gets?

问题描述

据我所知，C＃关键字的 VAR 的意味在编译时的类型，因此需要在同一个地方申报和分配，使得下面的构造非法的：

  VAR的东西; 
 
如果（情况）：
 {
的东西= 1; 
} 
，否则
 {
的东西= 0; 
} 
  

尽管C＃编译器可能主要确定在当前范围内产生的所有作业在同类型的，它（目前）没有，因此产生一个编译时错误。

在使用的 VAR 的使用LINQ，我有时会想初始化我的结果变量略有不同取决于程序的条件。我已经找到了一种方法是使用？：运算符，如：

  VAR参与者=（电子邮件== NULL 
（从部分db.Participant 
，其中part.SequenceNumber == seqNr 
选择部分）
：
（从部分db.Participant 
，其中一部分.EmailAddress ==电子邮件
选择部分））; 
  

这工作得很好，如果我只是有两个可能的任务，但如果有多个可能很快就会变得不实用分配。

是否有一个更优雅的方式呢？

解决方案

 <$ C：>您可以使用。凡（...） $ C> VAR参与者=电子邮件== NULL 
？ db.Participant.Where（P => p.SequenceNumber == seqNr）
：db.Participant.Where（P => p.EmailAddress ==电子邮件）; 
  

但我个人根本不会使用 VAR 在这里 - 它不会扩展到超过2，但是这是一个典型的搜索的格局。我可能是：

 的IQueryable<部分>查询= db.Participant; 
如果（电子邮件！= NULL）查询= query.Where（P => p.EmailAddress ==电子邮件）; 
如果（seqNr！= NULL）查询= query.Where（P => p.SequenceNumber == seqNr）; 
 ... 
 //消耗查询
  

这允许多个条件为（在这种情况下，与），用于更具体的查询组成。你也可以做OR，但它是稍微复杂一些（除非你使用的毗连 / 联盟，这是凌乱）。

I understand that the C# keyword var implies the type at compile time and therefore requires the declaration and assignment in the same place, making the following construct illegal:

var something;

if (condition)
{
    something=1;
}
else
{
    something = 0;
}

Even though the C# compiler could principally determine that all assignments in the current scope result in the same type, it (presently) does not and therefore generates a compile time error.

In using var with LINQ, I sometimes want to initialize my result variable slightly differently depending on program conditions. The approach I have found is to use the ?: operator, as in:

var participants = (email == null ?
                   (from part in db.Participant
                    where part.SequenceNumber == seqNr
                    select part)
                   :
                   (from part in db.Participant
                    where part.EmailAddress == email
                    select part));

That works well enough if I just have two possible assignments, but quickly becomes unwieldy if there are multiple possible assignments.

Is there a more elegant approach to this?

解决方案

You could use .Where(...) rather than query syntax:

var participants = email == null
    ? db.Participant.Where(p => p.SequenceNumber == seqNr)
    : db.Participant.Where(p => p.EmailAddress == email);

but personally I simply wouldn't use var here - it doesn't scale beyond 2, but this is a classic "search" pattern. I might use:

IQueryable<Part> query = db.Participant;
if(email != null) query = query.Where(p => p.EmailAddress == email);
if(seqNr != null) query = query.Where(p => p.SequenceNumber == seqNr);
...
// consume query

This allows multiple conditions to be composed (in this case "AND") for a more specific query. You can also do "OR" but it is slightly more complex (unless you use Concat/Union, which is messy).

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