Question

I have

IDictionary<string,object> d1;
Dictionary<string, object> d2;

I need to remove from d1 all entries that are not in d2.

I know I can do this with a for loop etc but that’s so last century; I want to do it right.

I got to

   d1.Where(x => {return d2.ContainsKey(x.key);});

but dont know what to do next

Answer 1

LINQ isn’t designed to modify existing elements – but you could always create a new dictionary. For example:

d1 = d1.Where(x => d2.ContainsKey(x.Key))
       .ToDictionary(x => x.Key, x => x.Value);

Or:

d1 = d1.Keys.Intersect(d2.Keys)
       .ToDictionary(key => x.Key, key => d1[key]);

As others have said, if you’re more keen on doing a Remove operation, I’d just loop. For example:

foreach (var key in d1.Keys.Except(d2.Keys).ToList())
{
    d1.Remove(key);
}

(I’m not sure why you used a statement lambda in your sample code, by the way.)