Why I'm getting System.InvalidCastException when inserting in mdf database file - vb.net

Question

I'm just trying to insert some data, I've doubled check my table definition and my variables data type
This is my table definition in mdf database file
product_id - int
product_name - varchar(50)
qty_sold - int
date - date
and this is my list
Dim cart As New List(Of Integer)
Dim product_qty As New List(Of Integer)
I'm trying to insert using this code
For i As Integer = 0 To cart.Count - 1
cmd.CommandText = "insert into sales values('" + cart(i) + "', '" + getName(cart(i)) + "', '" + product_qty(i) + "', '" + datefield.Text + "')"
cmd.ExecuteNonQuery()
i = i + 1
Next
getName returns a String so it fits with the product_name column while datefield.Text returns a date format
and I'm getting this error, some sort of data type error with Double thing I think.
System.InvalidCastException: 'Conversion from string "insert into sales values('" to type 'Double' is not valid.'

Answer 1

Change the name of the date column to SaleDate. You will avoid extra work trying to except the name date as a reserved word or type in .net. Always list the fields you are inserting. Otherwise your inserts are positional. DBAs have been known to add fields and you may have missed the memo.
I created 2 classes to hold your data. Fill you class' instances with your data from the user interface or wherever. Add the instances to the lists.
Then pass the lists to the InsertSales method.
The the Connection, Command, and ParametersCollection are created only once outside the loops. The connection is opened outside the loop. Only the values of the values of the parameters change inside the loops. The command is executed on each iteration of the inner loop.
Public Class Cart
Public CartDate As Date
Public CartContents As New List(Of ProductSale)
End Class
Public Class ProductSale
Public ProcuctID As Integer
Public ProductName As String
Public QuantitySold As Integer
End Class
Private Carts As New List(Of Cart)
Private Products As New List(Of ProductSale)
Private Sub InsertSales(CartList As List(Of Cart), ProductList As List(Of ProductSale))
Dim sql = "insert into sales (product_id, product_name, qty_sold, saledate) values (#ProductId, #ProductName, #QuantitySold, #DateSold)"
Using con As New SqlConnection(ConStr),
cmd As New SqlCommand(sql, con)
cmd.Parameters.Add("#ProcuctID", SqlDbType.Int)
cmd.Parameters.Add("#ProductName", SqlDbType.VarChar, 50)
cmd.Parameters.Add("#QuanitySold", SqlDbType.Int)
cmd.Parameters.Add("#DateSold", SqlDbType.Date)
con.Open()
For Each c As Cart In CartList
cmd.Parameters("#DateSold").Value = c.CartDate
For Each p As ProductSale In c.CartContents
cmd.Parameters("#ProductID").Value = p.ProcuctID
cmd.Parameters("#ProductName").Value = p.ProductName
cmd.Parameters("#QuantitySold").Value = p.QuantitySold
Next
cmd.ExecuteNonQuery()
Next
End Using
End Sub
BTW, it is redundant to have columns for both product id and product name. I presume there is a Products table with product Id as the primary key and a column for product name. You are storing the same data twice. Suppose they change the product name. You would have to do a huge update.