How do I get Distinct() to work with a collection of custom objects

Question

I have followed the suggestions from this post to try and get Distinct() working in my code but I am still having issues. Here are the two objects I am working with:

    public class InvoiceItem : IEqualityComparer<InvoiceItem>
    {
        public InvoiceItem(string userName, string invoiceNumber, string invoiceAmount)
        {
            this.UserName = userName;
            this.InvoiceNumber= invoiceNumber;
            this.InvoiceAmount= invoiceAmount;
        }
        public string UserName { get; set; }
        public string InvoiceNumber { get; set; }
        public double InvoiceAmount { get; set; }

        public bool Equals(InvoiceItem left, InvoiceItem right)
        {
            if ((object)left.InvoiceNumber == null && (object)right.InvoiceNumber == null) { return true; }
            if ((object)left.InvoiceNumber == null || (object)right.InvoiceNumber == null) { return false; }
            return left.InvoiceNumber == right.InvoiceNumber;
        }


        public int GetHashCode(InvoiceItem item)
        {
            return item.InvoiceNumber == null ? 0 : item.InvoiceNumber.GetHashCode();
        }
    }


    public class InvoiceItems : List<InvoiceItem>{ }

My goal is to populate an InvoiceItems object (we will call it aBunchOfInvoiceItems) with a couple thousand InvoiceItem objects and then do:

InvoiceItems distinctItems = aBunchOfInvoiceItems.Distinct();  

When I set this code up and run it, I get an error that says

Cannot implicitly convert type 'System.Collections.Generic.IEnumerable' to 'InvoiceReader.Form1.InvoiceItems'. An explicit conversion exists (are you missing a cast?)

I don't understand how to fix this. Should I be taking a different approach? Any suggestions are greatly appreciated.

Answer 1

Distinct returns a generic IEnumerable<T>. It does not return an InvoiceItems instance. In fact, behind the curtains it returns a proxy object that implements an iterator that is only accessed on demand (i.e. as you iterate over it).

You can explicitly coerce it into a List<> by calling .ToList(). You still need to convert it to your custom list type, though. The easiest way is probably to have an appropriate constructor, and calling that:

public class InvoiceItems : List<InvoiceItem> {
    public InvoiceItems() { }

    // Copy constructor
    public InvoiceItems(IEnumerable<InvoiceItems> other) : base(other) { }
}

// …

InvoiceItems distinctItems = new InvoiceItems(aBunchOfInvoiceItems.Distinct());