I use LINQ on a `Dictionary<string, IList<ID>>` like this: 

```
var searchCategories = new List {"A", "B", "C"};
Result = CategoryMapper.Mapping.Where(
         x => searchCategories.Contains(x.Key)).
         Select(x => new Tuple<string, IList<ID>>(x.Key, x.Value)).ToList();

```

This returns all ids that are either in Category A, B or C. However what I would like to do is retrieve ids that are in Category A, B and C.

I'm having difficulties figuring out how to do this with Linq. 

**UPDATE**

I'm sorry but I should have added some more information in my initial post. The lists in my dictionary look somewhat like this (I only use numbers here to make it simple):

A : {1, 2, 3}  

B : {1,3}  

C : {3}

So what I would like as a result of my query would be '3' in this case because it is the only number that has all categories.