枚举期间的LINQ铸造 [英] LINQ casting during enumeration

问题描述

我有一个列表< string>

List<string> students;
students.Add("Rob");
students.Add("Schulz");

和字典< string，string>

Dictionary<string, string> classes= new Dictionary<string, string>();
classes.Add("Rob", "Chemistry");  
classes.Add("Bob", "Math"); 
classes.Add("Holly", "Physics"); 
classes.Add("Schulz", "Botany"); 

我现在的目标是获取一个带有值的列表 - code>和植物学 - 我正在使用此

My objective now is to get a List with the values - Chemistry and Botany - for which I am using this

var filteredList = students.Where(k => classes.ContainsKey(k))
                                         .Select(k => new { tag = students[k] });

尝试枚举值时 - 我可以获取 - tag =化学& tag = Botany ...而我只想要化学和 Botany 。

While trying to enumerate the values - I am able to obtain - tag=Chemistry & tag=Botany...while I want just Chemistry and Botany.

要应用的相应投影是什么？是否有更好的方法来获得这些值？

What is the appropriate casting to be applied? Is there a better way to get to these values?

推荐答案

你只需要写：

var filteredList = students.Where(student => classes.ContainsKey(student));

这里，学生是 string ，因为学生是 List< string> 必须应用 Where（）。结果将是 IEnumerable< string> 。

Here, student is a string, since students is a List<string>, so you only have to apply Where(). The result will be an IEnumerable<string>.

您可以应用 ToList（）如果你想将枚举排除到另一个 List< string> ：

You can apply ToList() if you want to exhaust the enumerable into another List<string>:

var filteredList = students.Where(student => classes.ContainsKey(student)).ToList();

如果你想要一个类列表（从你的问题的代码不清楚）必须选择（）才能向学生投射课程：

If you want a list of classes (it's not clear from the code in your question), then you have to apply Select() to project classes from students:

var filteredList = students.Where(student => classes.ContainsKey(student))
                           .Select(student => classes[student]);

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