C# LINQ flatten array based on int property

Question

I have a list of order items with the given structure :

OrderItem { Id = 1, Name = "First Item", Quantity = 2 }
OrderItem { Id = 2, Name = "Second Item", Quantity = 2 }
OrderItem { Id = 3, Name = "Third Item", Quantity = 1 }

I want to flatten it to the following structure :

DBItem{ Id = 1, Name = "First Item" }
DBItem{ Id = 2, Name = "First Item" }
DBItem{ Id = 3, Name = "Second Item" }
DBItem{ Id = 4, Name = "Second Item" }
DBItem{ Id = 5, Name = "Third Item" }

Is there a way using LINQ SelectMany?

Answer 1

You can use Enumerable.Range to generate aditional records:

var dbItems = orderItems
    .OrderBy(oi => oi.Id) // ensure correct order
    .SelectMany(oi => Enumerable.Range(0, oi.Quantity), (oi, n) => oi) // records duplication
    .Select((oi, idx) => new DBItem // Select overload with index
    {
        Id = idx + 1,
        Name = oi.Name,
    })
    .ToList();