Better way of comparing two lists with LINQ?

Question

I have the 2 collections:

IEnumerable<Element> allElements
List<ElementId> someElements, 

What is a concise way of doing the following together:

[1] Verifying that all elements in someElements exist in allElements , return quickly when condition fails.

and

[2] Obtain a list of Element objects that List<ElementId> someElements maps to.

Every Element object has an ElementId

Thank you.

Answer 1

I would do this:

var map = allElements.ToDictionary(x => x.Id);    
if (!someElements.All(id => map.ContainsKey(id))
{
    // Return early
}
var list = someElements.Select(x => map[x])
                       .ToList();

Note that the first line will throw an exception if there are any duplicates in allElements .

Answer 2

1. someElements.All(e => allElements.Contains(e));
2. allElements.Where(e => someElements.Contains(e.ElementId));

Answer 3

Not as efficient as the Skeet answer, but good enough for reasonable-sized collections:

IEnumerable<Element> allElements = new List<Element>
    { new Element { Id = 1 }, new Element { Id = 2 } };
List<int> someElements = new List<int> { 1, 2 };

var query =
    (from element in allElements
    join id in someElements on element.Id equals id
    select element)
    .ToList();

if (query.Count != someElements.Count)
{
    Console.WriteLine("Not all items found.");
}

foreach (var element in query)
{
    Console.WriteLine ("Found: " + element.Id);
}