Question

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?

Answer 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]);