Accepted answer

if you order does not refer back to a customer, the trick is to first create a dataset which keeps the customers and orders linked together:

   .selectmany(c => => new {
       cust = c,
       ord = o

then on this customerorder (co) you can apply your join:

        co => co.ord.productid, 
        prod => prod.productid, 
        (co,prod) => new {
          orderamount = ord.quantity * prod.price});

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