如何迭代键值对的每个值<string,list<string> > 词典</string,list<string>

Question

I've got a exercise, which askes me to make dictionary with next Tasks: Methods which Checks two dictionarys with dictionary<string,List<string>> key refers to teacher object,string list refers to subject values. I'm using List because it can have multiple values

    Dictionary<string, List<string>> subjectsOfTeachers = new();
    Dictionary<string, List<string>> subjectsOfPupils = new();

Next task was to check, if any values in subjectOfPupils equals to teachers list values.so i did next:

    public List<string> GetTeachers(string pupil)
    {
        var a = new List<string>();
        foreach(KeyValuePair<string,List<string>>kvp in subjectsOfTeachers)
        {
            if(subjectsOfPupils[pupil] == kvp.Value)
            {
                a.Add(kvp.Key);
            }
        }
        return a;

The logic is that it does checks for same values, but it checks for whole list value similarity. Is there any way to access values lists and check it manualy? sorry for the broken english.

Answer 1

 subjectsOfPupils[pupil] == kvp.Value

Will compare two lists. Lists are reference types , so by default they use reference equality , and that is probably not what you want. If I understand the task correctly you want the intersection between the two lists. Luckily we have a method just for that: Intersect . That will return all the items that are present in both lists. If we are only concerned if there was any items or not we can use the .Any method.

If the task is to check if any of the teachers have any subject shared with any pupil we can use SelectMany to flatten the dictionaries to to plain lists.

var anySharedSubject = subjectsOfTeachers.SelectMany(kvp => kvp.Value)
    .Intersect(subjectsOfPupils.SelectMany(kvp => kvp.Value))
    .Any();

Note that this uses Linq methods, and while they are very convenient, you should probably think about how you would write the intersect-method using just loops.