Using Linq not equals

Question

I've 2 list collections in my C# app..A and B.

Both the collections have customer object which has Id and Name attributes.Typically, A has more items than B.

Using Linq,I want to return only those customers whose Id is in A but not in B.

How do I do this?

Answer 1

There are multiple approaches to take. The cleanest approach is to use the Except extension method if you have overriden Equals and GetHashCode . If you have not, there are other options.

// have you overriden Equals/GetHashCode?
IEnumerable<Customer> resultsA = listA.Except(listB);

// no override of Equals/GetHashCode? Can you provide an IEqualityComparer<Customer>?
IEnumerable<Customer> resultsB = listA.Except(listB, new CustomerComparer()); // Comparer shown below

// no override of Equals/GetHashCode + no IEqualityComparer<Customer> implementation?
IEnumerable<Customer> resultsC = listA.Where(a => !listB.Any(b => b.Id == a.Id));

// are the lists particularly large? perhaps try a hashset approach 
HashSet<int> customerIds = new HashSet<int>(listB.Select(b => b.Id).Distinct());
IEnumerable<Customer> resultsD = listA.Where(a => !customerIds.Contains(a.Id));

...

class CustomerComparer : IEqualityComparer<Customer>
{
    public bool Equals(Customer x, Customer y)
    {
        return x.Id.Equals(y.Id);
    }

    public int GetHashCode(Customer obj)
    {
        return obj.Id.GetHashCode();
    }
}

Answer 2

如果您为客户对象重写等于，则只需使用

A.Except(B);

Answer 3

Expanding on Except, providing your own equality so you don't need to change your Equals behavior. I got this from here:

http://www.codeproject.com/KB/dotnet/LINQ.aspx#distinct

List<Customer> customersA = new List<Customer> { new Customer { Id = 1, Name = "A" }, new Customer { Id = 2, Name = "B" } };
List<Customer> customersB = new List<Customer> { new Customer { Id = 1, Name = "A" }, new Customer { Id = 3, Name = "C" } };

var c = (from custA in customersA
        select custA.Id).Distinct()
             .Except((from custB in customersB
            select custB.Id).Distinct());