Find items from a list which exist in another list

var commonNumbers = first.Intersect(second); 

This will give you the common values between two lists, a much faster and cleaner approach than join or other Lambda expressions.

Just try it.

Source : MSDN

ListA.Where(a => ListX.Any(x => x.b == a.b))

What you want to do is Join the two sequences. LINQ has a Join operator that does exactly that:

List<PropX> first;
List<PropA> second;

var query = from firstItem in first
    join secondItem in second
    on firstItem.b equals secondItem.b
    select firstItem;

Note that the Join operator in LINQ is also written to perform this operation quite a bit more efficiently than the naive implementations that would do a linear search through the second collection for each item.

Well all above will not work if you have multiple parameters, So I think this is the best way to do it.

For example: Find not matched items from pets and pets2 .

var notMatchedpets = pets
    .Where(p2 => !pets2
    .Any(p1 => p1.Name == p2.Name && p1.age == p2.age))
    .ToList();