vb.net ถ้าจะทำ log file error และ log file login จะต้องทำยังไง

Imports System.IO
Public Class Form1

    Private Sub btnLogin_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnLogin.Click
        Dim sw As StreamWriter = New StreamWriter("d://log.txt", True)
        'IF(login สำเร็จ) Then
        sw.WriteLine(Me.txtUsername.Text & ";" & Date.Now.ToString("dd/MM/yyyy"))
        ' ทำอะไรอื่นๆ 
        'End If
        sw.Close()
    End Sub
End Class

using System.Net;

string userIP = Request.ServerVariables["REMOTE_ADDR"].ToString();

IPAddress ip = IPAddress.Parse(userIP);
IPHostEntry GetIPHost = Dns.GetHostEntry(ip);
string hostName = GetIPHost.HostName;