Question

Using Linq, trying to select all records not in my local set fails:

var localset = new List<Category>();
localset.Add( new Category { pk1 = 1, pk2 = 1, name = "one" } );
localset.Add( new Category { pk1 = 1, pk2 = 2, name = "two" } );

var dbCategories = dc.Categories;

var diff = dbCategories.Except(localset);

I need to do basic CRUD: delete from the db set when not in my local set, update where exists, and add new when not there.

I would normally do:

delete from Category C where not exists
    ( select null from LocalSet L where C.pk1 = L.pk1 and C.pk2 = L.pk2 )

update Category set name = L.name
from LocalSet L
where L.pk1 = Category.pk1 and L.pk2 = Category.pk2

insert into Category (pk1, pk2, name)
select pk1, pk2, name
from LocalSet L
where not exists (
    select null from Category C where L.pk1 = C.pk1 and L.pk2 = C.pk2 )

Easy enough I thought. However, .Contains seems to be the only method that works with local sets, and it only seems to compare a single field. The database table has composite keys.

Without changing the composite keys, is there any way to accomplish these tasks?

Answer 1

You’ll have to implement a CategoryComparer like the following:

public class CategoryComparer : IEqualityComparer<Category>
{
    public bool Equals(Category a, Category b)
    {
        bool result = false;
        if( a.pk1 == b.pk1 && a.pk2 == b.pk2 && a.name == b.name)
            result = true;
        return result;    
    }

    public int GetHashCode(Category category)
    {        
        if (Object.ReferenceEquals(category, null)) return 0;        
        return category.pk1.GetHashCode() ^ category.pk2.GetHashCode();
    }
}

Then call Enumerable.Except:

var localset = new List<Category>();
localset.Add( new Category { pk1 = 1, pk2 = 1, name = "one" } );
localset.Add( new Category { pk1 = 1, pk2 = 2, name = "two" } );

var diff = dc.Categories
             .AsEnumerable()
             .Except(localset, new CategoryComparer());

Let me know if this gets you what you need.