hello guys below is my code. when i select / fetch data from mysql database at the first time, it can run, the second time i run without changing anything, i get this error, "Object reference not set to an instance of an object." at line that i marked below (combo box). The combo box load list of tables from a database. Please help me. Thanks
Imports MySql.Data.MySqlClient
Public Class Form1
Dim conn As MySqlConnection
Dim reader As MySqlDataReader
Dim command As MySqlCommand
Dim adapter As MySqlDataAdapter
Dim dt As DataTable
Dim var As Integer
Dim p_error As Integer
Dim cmbselected As String
Private Sub MainFunction(ByVal lol)
If (var = 0) Then
MessageBox.Show("please Connect to a database.")
Exit Sub
End If
Select Case lol
Case "Select"
Try
cmbselected = cmbTable.Text
command.CommandText = "select * from " & cmbselected & "" <-----HERE'S THE PROBLEM OCCURED
command.CommandType = CommandType.Text
command.Connection = conn
cmbselected = ""
adapter = New MySqlDataAdapter(command)
dt = New DataTable
adapter.Fill(dt)
Dim cb As New MySqlCommandBuilder(adapter)
With DataGridView1
.AutoGenerateColumns = True
.DataSource = dt
End With
command.Dispose()
command = Nothing
conn.Close()
conn.Dispose()
Catch ex As MySqlException
MessageBox.Show("Error1: " & ex.Message)
End Try
Case "Update"
Try
DataGridView1.EndEdit()
adapter.Update(dt)
MessageBox.Show("Table Updated!")
Catch ex As MySqlException
MessageBox.Show("Error1: " & ex.Message)
End Try
End Select
End Sub
Private Sub setConnection()
Try
conn = New MySqlConnection
'locahost,test,root,quest
conn.ConnectionString = "Server='" & txtServer.Text & "';Database='" & txtDB.Text & "';UID='" & txtUID.Text & "';Password='" & txtPwd.Text & "'"
conn.Open()
p_error = 0
Catch ex As MySqlException
MessageBox.Show("Error1: " & ex.Message)
p_error = 1
End Try
End Sub
Private Sub BtnExit_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles BtnExit.Click
Application.Exit()
End Sub
Private Sub BtnFetch_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles BtnFetch.Click
MainFunction("Select")
End Sub
Private Sub btnConnect_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnConnect.Click
If (txtDB.Text = "" Or txtPwd.Text = "" Or txtServer.Text = "" Or txtUID.Text = "") Then
MessageBox.Show("Please complete above data! Thank you")
Exit Sub
End If
If (var = 1) Then
MessageBox.Show("A database already connected!")
Exit Sub
End If
setConnection()
If (p_error = 0) Then
var = ConnectionState.Open
MessageBox.Show("Connection Successfull!")
p_error = 1
FillCmbBox()
End If
End Sub
Private Sub btnUpdate_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnUpdate.Click
MainFunction("Update")
'DataGridView1.EndEdit()
'adapter.Update(dt)
End Sub
Private Sub FillCmbBox()
command = New MySqlCommand
dt = New DataTable
adapter = New MySqlDataAdapter
command.Connection = conn
command.CommandText = "SELECT TABLE_NAME FROM information_schema.TABLES WHERE TABLE_SCHEMA = 'test';"
command.CommandType = CommandType.Text
command.Connection = conn
adapter = New MySqlDataAdapter(command)
dt = New DataTable
adapter.Fill(dt)
Dim cb As New MySqlCommandBuilder(adapter)
With cmbTable
.DataSource = dt
.DisplayMember = "Table_Name"
End With
End Sub
End Class
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