I got this error for auto _Suggest and error is "Object reference not set an instance of an object "

Question

Public Sub auto_Suggest(ByVal member As String, ByVal table As String, ByVal txt As Object)
    Try
        dta = New DataTable
        'open the connection
        conn.Open()
        'holds the data in the database
        With cmda
            .Connection = conn
            .CommandText = "select " & member & " from " & table
        End With
        '''''''''''''''fill data in the table
        daa.SelectCommand = cmda
        daa.Fill(dta)

        ''function of autocomplete
        Dim r As DataRow
        txt.AutoCompleteCustomSource.Clear()


        For Each r In dta.Rows
            txt.AutoCompleteCustomSource.Add(r.Item(0).ToString)

        Next
        ''''''''''''''''''''''''
    Catch ex As Exception
        MsgBox(ex.Message)
    End Try
    ''''close the connection
    conn.Close()
    daa.Dispose()

End Sub




Private Sub Stock_Load(sender As Object, e As EventArgs) Handles MyBase.Load
    Try
        'call a public sub
        'the job_id is the field of a table , the employees is the name of a table and a textbox is an object
        auto_Suggest("ItemName", "stock", TxtItemName)

        Catch ex As Exception
        MessageBox.Show(ex.Message)

    End Try
End Sub

Answer 1

First, it's good to employ a "use it and lose it" approach and take advantage of Using blocks to automatically dispose of connections, commands, and readers. See the example below.

Second, please set a breakpoint on the first line after the Try statement and step through your source in the debugger. If the exception is reached, inspect the ex variable and potentially the ex.InnerException (if any).

Last, the example here uses SQLConnection and SQLCommand (SQL Server). Just swap out whatever library you are using for MySQL and you should be good to go.

Public Sub auto_Suggest(connectionString As String, member As String, table As String, txt As TextBox)
    Try
        txt.AutoCompleteCustomSource.Clear()

        Using cn = New SqlConnection(connectionString)
            cn.Open()

            Using cmd = New SqlCommand("SELECT " & member & " FROM " & table, cn)
                Using dr = cmd.ExecuteReader()
                    While dr.Read
                        txt.AutoCompleteCustomSource.Add(dr(member).ToString)
                    End While
                End Using
            End Using
        End Using
    Catch ex As Exception
        MsgBox(ex.Message)
    End Try
End Sub