C# List grouping and assigning a value

Question

I have a list of Orders. This list contains multiple orders for the same item, see the table below.

I then want to assign each item that is the same (i.e. ABC) the same

Answer 1

You can do this that way, it will assign same blockid for same orderid

var ordered = listOrder.GroupBy(x => x.OrderId).ToList();
for (int i = 0; i < ordered.Count(); i++)
{
    ordered[i].ForEach(x=>x.BlockId=i+1);
}   

it will group orders by orderid then assign each group next blockid. Note that it won't be done fully in linq, because linq is for querying not changing data.