错误 已经有一个打开的 DataReader 与此 Connection 关联，必须先关闭

Question

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Private Sub BtnSimpan_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles BtnSimpan.Click
        If BtnSimpan.Text = "&Simpan" Then
            If txtKode.Text = "" Then
                MsgBox("Kode Harus Di isi", MsgBoxStyle.Exclamation, "Peringatan")
                txtKode.Focus()
                Exit Sub
            End If
            Call bukaDB()
            CMD = New MySqlCommand("SELECT KodeBarang from tabelbarang WHERE KodeBarang = '" & txtKode.Text & "'", Conn)
            RD = CMD.ExecuteReader()
            RD.Read()
            If RD.HasRows Then
                MsgBox("Maaf, Data dengan Kode tersebut telah ada", MsgBoxStyle.Exclamation, "Peringatan")
            Else
                simpan = "INSERT INTO tabelbarang (KodeBarang,NamaBarang,HargaBeli,HargaJual,Stok) VALUES ('" & txtKode.Text & "','" & txtNamaBarang.Text & "','" & txtHargaBeli.Text & "','" & txtHargaJual.Text & "','" & txtStok.Text & "')"
                CMD = New MySqlCommand(simpan, Conn)
                CMD.ExecuteNonQuery() 
                Call isiGrid()
                BtnSimpan.Text = "&Tambah"
                Call Bersih()
            End If
        Else
            BtnSimpan.Text = "&Simpan"
            'Call Bersih()
            txtKode.Enabled = True
            txtNamaBarang.Enabled = True
            txtHargaBeli.Enabled = True
            txtHargaJual.Enabled = True
            txtStok.Enabled = True
            txtKode.Focus()
        End If
End Sub

Answer 1

问题的代码中有几个过时的做法。下面的代码针对现代编码风格进行了更新，仅仅使用现代风格的行为也将解决问题中的问题......也就是说，如果你从一开始就遵循良好的编码实践，这整个问题就解决了你永远不会遇到这个问题。

这还解决了原始版本中的巨大的安全问题，如果只是为了跟上现代编码标准，就可以完全避免这种问题。

Private Sub BtnSimpan_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles BtnSimpan.Click

    If BtnSimpan.Text = "&Simpan" AndAlso String.IsNullOrWhitespace(txtKode.Text)  Then
        MsgBox("Kode Harus Di isi", MsgBoxStyle.Exclamation, "Peringatan")
        txtKode.Focus()
        Exit Sub
    End If

    If BtnSimpan.Text <> "&Simpan" Then
        BtnSimpan.Text = "&Simpan"
        'Bersih()
        txtKode.Enabled = True
        txtNamaBarang.Enabled = True
        txtHargaBeli.Enabled = True
        txtHargaJual.Enabled = True
        txtStok.Enabled = True
        txtKode.Focus()
        Exit Sub
    End If

    'Do NOT try to re-use the same connection throughout your application!
    ' It really is more efficient to create a brand new object for most queries,
    '  and only share the connection string.

    'Also, JUST DO THE INSERT.
    'Make sure there is a unique constraint on the KodeBarang column,
    ' and handle the exception if it fails.

    ' Correct for *either one* of the above issues, and the
    ' problem in the question never would have happened.
 
    Try
        Using CN As New MySqlConnection("Connection string here"), _
              CMD As New MySqlCommand("INSERT INTO tabelbarang (KodeBarang,NamaBarang,HargaBeli,HargaJual,Stok) VALUES (@KodeBarang, @NamaBarang, @HargaBeli, @HargaJual, @Stok)", CN)

            CMD.Parameters.AddWithValue("@KodeBarang", txtKode.Text)
            CMD.Parameters.AddWithValue("@NamaBarang", txtNamaBarang.Text)
            CMD.Parameters.AddWithValue("@HargaBeli", txtHargaBeli.Text)
            CMD.Parameters.AddWithValue("@HargaJual", txtHargaJual.Text)
            CMD.Parameters.AddWithValue("@Stok", txtStok.Text)   

            CN.Open()
            CMD.ExecuteNonQuery() 
        End Using

        isiGrid()
        BtnSimpan.Text = "&Tambah"
    Catch ex As MySqlException When ex.Code = 1062 '1062 is Duplicate Key Violation
        MsgBox("Maaf, Data dengan Kode tersebut telah ada", MsgBoxStyle.Exclamation, "Peringatan")
    End Try
End Sub