Question

I am trying to make a glossary. I have a form with a listbox, 2 textboxes, and a save button.

The listbox is now populated with words from the database, and when a word is selected, its definition will display in textbox2.

The user can add a record by filling the textbox1 with a new word and textbox2 with its definition,and clicking the save button. If the new word already existed it will not allow to save a new record, also if there’s a null value between the 2 textboxes. If it doesn’t exist it will be inserted on the table and the new word will be added to the listbox.

The user can also update the record by selecting first a word on the list then edit the word and/or definition and clicking the save button.

I already got the updating part to work but I have problem in inserting a new record. I can’t do it properly. The glossary table has only 2 fields: word, definition. Here’s my code:

Dim myCmd As New MySqlCommand
Dim myReader As MySqlDataReader
Dim myAdptr As New MySqlDataAdapter
Dim myDataTable As New DataTable
Private Sub btnSave_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnSave.Click
    Call Connect()
    With Me
        If Blank() = False Then
            If Duplicate() = False Then
                STRSQL = "insert into glossary values (@word, @def)"
                myCmd.Connection = myConn
                myCmd.CommandText = STRSQL
                myCmd.Parameters.AddWithValue("word", txtNew.Text)
                myCmd.Parameters.AddWithValue("def", txtdefine.Text)
                myCmd.ExecuteNonQuery()
                myCmd.Dispose()
                MsgBox("Record Added")
                Dim word As String
                word = txtNew.Text
                lstword.Items.Add(word)
                'myConn.Close()
                'Me.FillListbox()
            Else
                myConn.Open()
                STRSQL = "Update glossary set word = @term, definition = @mean where word = @term"
                myCmd.Connection = myConn
                myCmd.CommandText = STRSQL
                myCmd.Parameters.AddWithValue("term", txtNew.Text)
                myCmd.Parameters.AddWithValue("mean", txtdefine.Text)
                myCmd.ExecuteNonQuery()
                myCmd.Dispose()
                MsgBox("Record Updated", MsgBoxStyle.Information, "New word added")
            End If
        End If
    End With


End Sub

Public Function Blank() As Boolean
    Call Connect()
    With Me
        If .txtNew.Text = "" Or .txtdefine.Text = "" Then
            Blank = True
            MsgBox("Cannot save! Term and definition should not contain null value", MsgBoxStyle.Critical, "Unable to save")
        Else
            Blank = False
        End If
    End With
End Function

Public Function Duplicate() As Boolean
    Call Connect()
    With Me
        STRSQL = "Select * from glossary where word = '" & txtNew.Text & "'"
        myCmd.Connection = myConn
        myCmd.CommandText = STRSQL
        If myDataTable.Rows.Count <> 0 Then
            Duplicate = True
            'MsgBox("Word already exist. Please check the word.", MsgBoxStyle.Critical, "Duplicate.")

        Else
            Duplicate = False
        End If
        myConn.Close()
    End With
End Function

this is my connection module:

Public myConnectionString As String
Public STRSQL As String
Public myConn As New MySqlConnection
Public Sub Connect()
    With myConn
        Try
            If .State = ConnectionState.Open Then
                .Close()
            End If
            myConnectionString = "Database=firstaidcqs;Server=localhost;Uid=root;Password="
            .ConnectionString = myConnectionString
            .Open()
            'MsgBox("Successful Connection")
        Catch ex As Exception
            MsgBox(ex.Message, MsgBoxStyle.Critical, "Connection Error")
            .Close()
        End Try
    End With
End Sub
Public Sub Disconnect()
    With myConn
        .Close()
        .Dispose()
    End With
End Sub

How can I make this work properly?

Answer 1

You are using global variables in all the code above.

myConn and myCmd 

in particular, you call Dispose on myCmd but I can’t see anywhere the reinitialization of the object with New. Also, forgetting for a moment the myCmd.Dispose problem, you don’t reset the myCmd parameters collection. In this way you end up with a wrong Parameter collection for the command executed, Also don’t forget to open the connection for the insert part. (and close it for both parts)

You could easily avoid the use of unnecessary global variables

....
If Duplicate() = False Then
    STRSQL = "insert into glossary values (@word, @def)"
    Using myCmd = new MySqlCommand(STRSQL, myConn)
        myConn.Open()
        myCmd.Parameters.AddWithValue("word", txtNew.Text)
        myCmd.Parameters.AddWithValue("def", txtdefine.Text)
        myCmd.ExecuteNonQuery()
    End Using
    myConn.Close()
.....
Else
    STRSQL = "Update glossary set word = @term, definition = @mean where word = @term"
    Using myCmd = new MySqlCommand(STRSQL, myConn)
        myConn.Open()
        myCmd.Parameters.AddWithValue("term", txtNew.Text)
        myCmd.Parameters.AddWithValue("mean", txtdefine.Text)
        myCmd.ExecuteNonQuery()
    End Using
    myConn.Close()
.....

A better solution will be changing the Connect() method to return the initialized connection instead of using a global variable. In that way you could enclose also the creation and destruction of the connection in a Using statement

 Using myConn as MySqlConnection = Connect()
 .......
 End Using

For this to work you need to change the code of Connect in this way

Public Function Connect() as MySqlConnection
    Dim myConn As MySqlConnection
    myConnectionString = "Database=firstaidcqs;Server=localhost;Uid=root;Password="
    myConn = New MySqlConnection(myConnectionString)
    myConn.Open()
    return myConn
End Sub

No need of a Disconnect function because you will always use the Using statement that will close the connection for you, no need to have a global variable to keep the connection because you will reopen the connection every time you need it and close afterward. Don’t think that this is not performant because ADO.NET implements connection pooling (It is an MSDN article for SqlServer, but the concept applies also to MySql)