Question

I have this dictionary mappings declared as a Dictionary<string, HashSet<string>>.
I also have this method to do stuff on a hashset in the dictionary:

public void DoStuff(string key, int iClassId){
    foreach (var classEntry in
             from c in mappings[key]
             where c.StartsWith(iClassId + "(")
             select c)
    {
        DoStuffWithEntry(classEntry);
    }
}

private void DoStuffWithEntry(string classEntry){
    // Do stuff with classEntry here
}

In one case, I need to do this on a number of keys in the mappings dictionary, and I was thinking it was better to rewrite and filter on a list of keys instead of calling DoStuff for each key to optimise the execution.

Currently I do this:

DoStuff("key1", 123);
DoStuff("key2", 123);
DoStuff("key4", 123);
DoStuff("key7", 123);
DoStuff("key11", 123);

Logically something like this instead of calling DoStuff for each (FilterOnKeys is not a method – just what I want…):

    foreach (var classEntry in
             from c in mappings.FilterOnKeys("key1", "key2", "key4", "key7", "key11")
             where c.StartsWith(iClassId + "(")
             select c)
    {
        DoStuffWithEntry(classEntry);
    }

Answer 1

It sounds like you want:

string[] keys = { "key1", "key2", ... }
var query = from key in keys
            from c in mappings[key]
            ...;

foreach (var entry in query)
{
    ...
}

(I would personally use a separate variable for the query just for readability – I’m not too keen on the declaration bit of a foreach loop getting huge.)