Check if a list contains all of another lists items when comparing on one property

Question

I am learning Linq and I have two object lists. I want to compare one of these lists against the other to see if all of one of the properties of the objects within it can be matched to those in the other list. So, I provide code for that but I want to change it to Linq expressions.

var list1 = new List<Product>
{
    new Product{SupplierId = 1,ProductName = "Name1"},
    new Product{SupplierId = 2,ProductName = "Name2"},
    new Product{SupplierId = 3,ProductName = "Name3"},
    new Product{SupplierId = 4,ProductName = "Name4"}
};

var list2 = new List<Product>
{
    new Product {SupplierId = 1,ProductName = "Name5"},
    new Product {SupplierId = 4,ProductName = "Name6"}
};

private static bool CheckLists(List<Product> list1, List<Product> list2)
{
    foreach (var product2 in list2)
    {
        bool result = false;
        foreach (var product in list1)
        {
            if (product.SupplierId == product2.SupplierId)
            {
                result = true;
                break;
            }
        }
        if (!result)
        {
            return false;
        }
    }
    return true;
}

How can I do it using LINQ?

For extra clarity, for anyone reading this subsequently, the check being done is whether all the items in list2 can be found in list1. — d219, Nov 22 '17 at 11:23

score 35 · Answer 1 · edited Mar 31 '20 at 13:40

35

bool existsCheck = list1.All(x => list2.Any(y => x.SupplierId == y.SupplierId));

would tell you if all of list1's items are in list2.

edited Mar 31 '20 at 13:40

granadaCoder

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answered Nov 21 '17 at 19:32

Steve

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4

The most readable solution. Why does everyone complicate things? – NotADeveloper Nov 21 '17 at 21:36
1

This solution really makes sense. I'd recommend this one as my answer. – Terry Wei Oct 09 '18 at 07:30

score 8 · Answer 2 · answered Nov 21 '17 at 19:28

8

You want to check whether there are any IDs in list1 that aren't in list2:

if (list1.Select(p => p.SupplierId).Except(list2.Select(p => p.SupplierId)).Any())

answered Nov 21 '17 at 19:28

SLaks

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I think you have `list1` and `list2` backwards. – NetMage Nov 21 '17 at 19:40
@SLaks I'm not sure this is correct; if you change '4' in List2 to '5' won't this will still return true whereas the original code sample would return false? – d219 Nov 21 '17 at 19:59
Like NetMage says he's checking if there are any IDs in List2 which aren't in List1, not the other way round, so I think these need reversing and a '!' adding – d219 Nov 21 '17 at 20:07

NetMage · Answer 3 · 2017-11-21T19:49:31.330

To see that list1 contains all of list2, check if Any of list1 matches All of list2:

private static bool CheckLists(List<Product> list1, List<Product> list2) => list2.All(l2p => list1.Any(l1p => l1p.SupplierId == l2p.SupplierId));

However, I would probably write an generic extension method:

public static class Ext {
    static public bool ContainsAll<T, TKey>(this List<T> containingList, List<T> containee, Func<T, TKey> key) {
        var HSContainingList = new HashSet<TKey>(containingList.Select(key));
        return containee.All(l2p => HSContainingList.Contains(key(l2p)));
    }
    static public bool ContainsAll<T>(this List<T> containingList, List<T> containee) => containingList.ContainsAll(containee, item => item);
}

Then you can call it like:

var ans = list1.ContainsAll(list2, p => p.SupplierId);

Check if a list contains all of another lists items when comparing on one property

3 Answers3