Flag all but one duplicates in a list

Question

I have the list of objects of the following class,

class Invoice
{
   public int InvoiceNumber;
   public string CustomerName;
   public bool IsDupe;
}

The invoice numbers can be duplicate or there can even be a case of 4 invoices , all with the same number.

I need to set the IsDupe flag on all except one of the invoice object. One approach is the brute force method of having a list of invoice numbers and comparing each one to flag. I have also tried this question . Is there any better syntactical way to do it ? TIA

Answer 1

This works.

var invs = new List<Invoice> { new Invoice { InvoiceNumber = 1 }, 
                               new Invoice { InvoiceNumber = 1 }, 
                               new Invoice { InvoiceNumber = 1 },
                               new Invoice { InvoiceNumber = 2 }, 
                               new Invoice { InvoiceNumber = 3 }, 
                               new Invoice { InvoiceNumber = 3 } 
                             };

invs.ForEach(i => i.IsDupe = true);
invs.GroupBy (i => i.InvoiceNumber)
    .Select(g => g.First())
    .ToList()
    .ForEach(i => i.IsDupe = false);

Produces

1 null False 
1 null True 
1 null True 
2 null False 
3 null False 
3 null True 

Alternatively, you could call the property IsNotDupe and take advantage of the fact that the boolean defaults to false (you can remove the first ForEach)