Question

I have code that looks like this:

class A {
public string Name {get;set;}
public IList<string> Keywords {get;set;}
}

…

IList<A> a = new List<A>{
        new A{Name = "a1", Keywords = new List<string>{"k1", "k2"}},
        new A{Name = "a2", Keywords = new List<string>{"k1", "k3"}},
        new A{Name = "a3", Keywords = new List<string>{"k3", "k4"}},
        new A{Name = "a4", Keywords = new List<string>{"k1", "k3", "k4"}}
    };

Is there any way I can use LINQ to create a dictionary in the form of:

{"k1", List<A> {a1, a2, a4}} 
{"k2", List<A> {a1}}
{"k3", List<A> {a2, a3, a4}}
{"k4", List<A> {a3, a4}}

Basically mapping the objects to the keywords.

Thanks

Answer 1

Something like

var dict = a
    .SelectMany(myA =>
        myA.Keywords.Select(kw => new { Name = myA.Name, Keyword = kw }))
    .GroupBy(pair => pair.Keyword)
    .ToDictionary(g => g.Key, g => g.Select(pair => pair.Name).ToList());

should do it.