System.IndexOutOfRangeException: while inserting data to mdf database file - vb.net

Question

Im getting this error while inserting data to my mdf database file
System.IndexOutOfRangeException: 'An SqlParameter with ParameterName '#qtysold' is not contained by this SqlParameterCollection.'
This is my code for order function
Function order(ByVal cart As List(Of Integer), ByVal product_qty As List(Of Integer))
If con.State = ConnectionState.Open Then
con.Close()
End If
con.Open()
'INSERT INTO PRODUCT TABLE
cmd.CommandType = CommandType.Text
cmd.CommandText = "UPDATE product SET product_qty= product_qty - #qty WHERE product_id= #id"
cmd.Parameters.Add("#qty", SqlDbType.Int)
cmd.Parameters.Add("#id", SqlDbType.Int)
For i As Integer = 0 To cart.Count - 1
cmd.Parameters("#id").Value = cart(i)
cmd.Parameters("#qty").Value = product_qty(i)
cmd.ExecuteNonQuery()
Next
'INSERT INTO SALES TABLE
cmd.CommandType = CommandType.Text
cmd.CommandText = "INSERT INTO sales VALUES (#id, #productname, #qtysold, #date)"
cmd.Parameters.Add("#id", SqlDbType.Int)
cmd.Parameters.Add("#productname", SqlDbType.VarChar)
cmd.Parameters.Add("#qtysold", SqlDbType.Int)
cmd.Parameters.Add("#date", SqlDbType.Date)
For i As Integer = 0 To cart.Count - 1
cmd.Parameters("#id").Value = cart(i)
cmd.Parameters("#productname").Value = getName(cart(i))
cmd.Parameters("#qtysold").Value = product_qty(i)
cmd.Parameters("#date").Value = datefield.Text
cmd.ExecuteNonQuery()
Next
cart.Clear()
product_qty.Clear()
refresh()
End Function
THE ERROR HAPPENS HERE
cmd.Parameters("#qtysold").Value = product_qty(i)

Answer 1

That seems an odd exception in that situation as the code looks OK to me but it's wrong even if it did work. Don't add the parameters and then get them back by name over and over. The Add method you're calling returns the SqlParameter object added so assign it to a variable and then use that:
Dim idParameter = cmd.Parameters.Add("#id", SqlDbType.Int)
Dim productionNameParameter = cmd.Parameters.Add("#productname", SqlDbType.VarChar)
Dim qtySoldParameter = cmd.Parameters.Add("#qtysold", SqlDbType.Int)
Dim dateParameter = cmd.Parameters.Add("#date", SqlDbType.Date)
For i As Integer = 0 To cart.Count - 1
idParameter.Value = cart(i)
productionNameParameter.Value = getName(cart(i))
qtySoldParameter.Value = product_qty(i)
dateParameter.Value = datefield.Text
cmd.ExecuteNonQuery()
Next
NEVER use the same complex expression over and over. ALWAYS use it once and assign the result to a variable, then use that variable over and over. In this case, you don't even have to use the complex expression once, because you already had the object but just ignored it.