LINQ casting during enumeration

问题

I have a List<string>

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

and a Dictionary<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");

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] });

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

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

回答1:

You only have to write:

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

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>.

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

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

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]);

回答2:

try:

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

回答3:

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

来源：https://stackoverflow.com/questions/12146296/linq-casting-during-enumeration

标签

linq

list

dictionary

casting