Unable to cast object of type 'System.Data.DataRow' to type

I can't see any obvious reason why you'd need to use DataRowView at all. You could just use DataRow throughout:

private void addToOrder(DataRow drv)

{ 

DataRow dr = orderDataTable.NewRow(); 

dr["ProductID"] = drv["ProductID"]; 

dr["ProductBarcode"] = drv["ProductBarcode"]; 

dr["ProductName"] = drv["ProductName"];

dr["SalePrice"] = drv["SalePrice"]; 

dr["SoldQuantity"] = 1; 

dr["NotEditable"] = false; 

try

{

orderDataTable.Rows.Add(dr);

} 

catch (ConstraintException)

{

DataRow dr1 = orderDataTable.Rows.Find(drv["ProductID"]);
dr1["SoldQuantity"] = Convert.ToInt32(dr1["SoldQuantity"]) + 1;

} 

} 

private void btnAddToOrder_Click(object sender, RoutedEventArgs e)

{

// add to order

if(txtBarcodeAuto.Text.Length >0) 

{

DataTable dscrntProd = prods.SelCrntProd4Sale(crntProdID);//get data from sql server database for product details(productID, ProductName,ProductPrice) 

foreach (DataRow dr in dscrntProd.Rows) 

{ 

addToOrder(dr); 

}

}

}