Visual Basic - Actualizar base de datos

Dim cantidad As String

Dim id As String

id = Textcodigopro3.Text

cantidad = Textcantidad3.Text

cmd.Connection = conexionstock

cmd.CommandType = CommandType.Text

sql = " UPDATE STOCK SET CANTIDAD = CANTIDAD - " & cantidad & " WHERE ID = " & id & ""

cmd.CommandText = sql

Try

    cmd.ExecuteNonQuery()

Catch ex As Exception

    MsgBox(ex.ToString)

End Try

cmd.Connection = conexionstock ' le digo a la variable cmd que se conecte a la base

cmd.CommandType = CommandType.Text

For Each row As DataGridViewRow In DataGridView1.Rows 'con este codigo le digo al programa las filas que quiero del datagridview

    cmd.Parameters.AddWithValue("@cantidad", row.Cells("cantidad").Value)

    cmd.Parameters.AddWithValue("@codigo", row.Cells("codigo").Value)

Next

'For Each R As DataGridViewColumn In DataGridView1.Columns

sql = " UPDATE STOCK SET CANTIDAD = CANTIDAD - @cantidad WHERE ID = @codigo "

'Next

cmd.CommandText = sql

Try

    cmd.ExecuteNonQuery() ' le digo a la variable cmd que guarde el registro

Catch ex As Exception

    MsgBox(ex.ToString)

End Try