如果我不能直接修改类，如何使用元素类型的字段值从另一个列表中删除列表？

Question

Suppose I have a List<Product> named big of some object Product which has 2 fields, ID and Name . The list is fully populated, meaning that each element has both fields not null , although they can be.

I have another List<Product> , smaller than the first, named small , but this time the field Name is null for some elements, while ID is always present.

I want to remove small from big where ID is the same.

Example:

List<Product> big = { {1,A},{2,B},{3,C},{4,D} };
List<Product> small = { {1,null},{3,C} };
List<Product> result = { {2,B},{4,D}};

I can't modify the Product object, ie I can't implement IEquatable<Product> , and such interface isn't implemented, meaning that big.Except(small) or big.RemoveAll(small) or big.Contains(anElementOfSmall) won't work (for the sake of the question, I've tried them).

I want to avoid the double loop with iterator removal or similar, I'm searching for built-in functions with specific predicates.

Answer 1

Using a simple predicate you can easily achieve it:

big.Where(p => !small.Any(o => o.id == p.id)).ToList();

which translates in: select from big where the element ( p ) doesn't suffice the case where an element o in small has the same ID .

Answer 2

Implement an IEqualityComparer for your Product that compares two Product based on ID . Then just use Except and pass the comparer:

var result = big.Except(small, new ProductComparer()).ToList();

Answer 3

You need to tell Except how to compare instances of Product .

public class ProductEqualityComparer : IEqualityComparer<Product>
{
    public bool Equals(Product x, Product y)
    {
        //they're both the same instance or they're both null
        if(ReferanceEquals(x, y))
            return true;

        //only one of them is null
        if(x == null || y == null)
            return false;

        return x.Id == y.Id;
    }

    public int GetHashCode(Product prod)
    {
        return prod == null? 0 : prod.Id.GetHashCode();
    }
}


big.Except(small, new ProductEqualityComparer())