在Devel Machine上同时使用Scala 2.10-M1和2.9？(Scala 2.10-M1 and 2.9 same time on Devel Machine?)

 在版本3中， select x返回与您的标准匹配的字符串序列 ; 它恰好是一个有一个项目的序列。 
 
 Console.WriteLine内部调用.ToString() ，无论你传入它。 由于IEnumerable<T>没有有意义的字符串表示，因此.NET中的默认值是打印该类型的字符串名称。 
 
 根据你的措词，我认为你的困惑的一部分确实来自误解为什么版本2的工作方式。 在版本2中， select x.First()实际上是一个怪癖/巧合，因为一个字符串也是一个IEnumerable<char> ，因此您可以对该字符串执行LINQ操作。 .First()返回该char序列的第一个元素，用于符合条件的每个结果。 所以你说： 
 
 
 
 “对于符合我的标准的每个元素，请选择第一个字符，然后返回匹配的所有第一个字符的序列。” 
 
 
 因此，实际上，版本2中的item是一个IEnumerable<char> ，其中包含一个元素。 在IEnumerable<char>上调用Console.WriteLine()将按顺序打印字符。 所以你得到“我”。 
 
 （注意：在我回答这个问题后，我看到问题被编辑为在投影内部和结果中调用.First() ，所以关于将IEnumerable<char>传递给Console.WriteLine一点不再完全相关） 
 
 请记住，LINQ几乎是关于使用集合，直到你明确地减少它们。 例如， Select只是一个投影或转换。 它返回传递给它的相同数量的项目，进行转换。 Where减少了集合，但它仍然是一个集合。 

In version 3, select x is returning a sequence of strings that match your critera; it just happens to be a sequence with one item in it.
 
Console.WriteLine internally calls .ToString() on whatever you pass into it. Since there is no meaningful string representation for IEnumerable<T>, the default in .NET is to print the string name of the type.
 
Based on your wording, I think part of your confusion does come from a misunderstanding of why version 2 works the way it does. In version 2, select x.First() is actually a bit of a quirk/coincidence, because a string is also an IEnumerable<char>, so you can do LINQ operations on the string. .First() returns the first element of that char sequence, for each result that matches your criteria. So you're saying:
 
 
 
"For each element which matches my criteria, select the first character, and then return the sequence of all the first characters for the matches."
 
 
So in fact, item in version 2 is an IEnumerable<char> with one element in it. Calling Console.WriteLine() on an IEnumerable<char> will just print the chars in order. So you get "i".
 
(note: I see after I answered this, the question was edited to call .First() both inside the projection and on the result, so the bit about passing IEnumerable<char> to Console.WriteLine isn't totally relevant anymore)
 
Keep in mind LINQ is pretty much about working with sets until you explicitly reduce them down. For example, Select is simply a projection or transformation. It returns the same number of items that were passed to it, transformed. Where reduces down the set, but it's still a set.