How to use LINQ Distinct() with multiple fields

Question

I have the following EF class derived from a database (simplified)

class Product
{ 
     public string ProductId;
     public string Product         


        

Answer 1

public List<ItemCustom2> GetBrandListByCat(int id)
    {

        var OBJ = (from a in db.Items
                   join b in db.Brands on a.BrandId equals b.Id into abc1
                   where (a.ItemCategoryId == id)
                   from b in abc1.DefaultIfEmpty()
                   select new
                   {
                       ItemCategoryId = a.ItemCategoryId,
                       Brand_Name = b.Name,
                       Brand_Id = b.Id,
                       Brand_Pic = b.Pic,

                   }).Distinct();


        List<ItemCustom2> ob = new List<ItemCustom2>();
        foreach (var item in OBJ)
        {
            ItemCustom2 abc = new ItemCustom2();
            abc.CategoryId = item.ItemCategoryId;
            abc.BrandId = item.Brand_Id;
            abc.BrandName = item.Brand_Name;
            abc.BrandPic = item.Brand_Pic;
            ob.Add(abc);
        }
        return ob;

    }

Answer 2

The Distinct() guarantees that there are no duplicates pair (CategoryId, CategoryName).

- exactly that

Anonymous types 'magically' implement Equals and GetHashcode

I assume another error somewhere. Case sensitivity? Mutable classes? Non-comparable fields?

Answer 3

the solution to your problem looks like this:

public class Category {
  public long CategoryId { get; set; }
  public string CategoryName { get; set; }
} 

...

public class CategoryEqualityComparer : IEqualityComparer<Category>
{
   public bool Equals(Category x, Category y)
     => x.CategoryId.Equals(y.CategoryId)
          && x.CategoryName .Equals(y.CategoryName, 
 StringComparison.OrdinalIgnoreCase);

   public int GetHashCode(Mapping obj)
     => obj == null 
         ? 0
         : obj.CategoryId.GetHashCode()
           ^ obj.CategoryName.GetHashCode();
}

...

 var distinctCategories = product
     .Select(_ => 
        new Category {
           CategoryId = _.CategoryId, 
           CategoryName = _.CategoryName
        })
     .Distinct(new CategoryEqualityComparer())
     .ToList();