how to validate a textbox in windows

i want this code to convert n vb.net
how to write null in vb.net


private void txtPassword_Validating(object sender, CancelEventArgs e)
{
if (string.IsNullOrEmpty(txtPassword.Text))
{
errorProvider1.SetError(txtPassword, "Password required!");
}
else if (!Regex.IsMatch(txtPassword.Text, @"[A-Za-z][A-Za-z0-9]{2,7}"))
{
errorProvider1.SetError(txtPassword, "Password invalid!");
}
else
{
errorProvider1.SetError(txtPassword, null);
}
}