private void okButton_Click(object sender, EventArgs e)
{
    var login = loginTextBox.Text.ToLowerInvariant();
    if (string.IsNullOrWhiteSpace(login))
    {
        MessageBox.Show(
            caption: "Error",
            text: "Login cannot be empty.",
            icon: MessageBoxIcon.Error,
            buttons: MessageBoxButtons.OK);
        DialogResult = DialogResult.None;
        return;
    }
    string pass = changePassword ? AuthorizeForm.MD5f(passTextBox.Text) : null;
    if (pass != null)
    {
        if (passConfirmTextBox.Enabled && pass != AuthorizeForm.MD5f(passConfirmTextBox.Text))
        {
            MessageBox.Show(
                caption: "Error",
                text: "Passwords do not match.",
                icon: MessageBoxIcon.Error,
                buttons: MessageBoxButtons.OK);
            DialogResult = DialogResult.None;
            return;
        }
    }

    var add = User == null;
    if (User == null)
        User = new User();

    var db = Context.Instance;
    if (db.Users.Any(u => u.Login == login && (add || u.Id != User.Id)))
    {
        MessageBox.Show(
            caption: "Error",
            text: "User with this login already exists.",
            icon: MessageBoxIcon.Error,
            buttons: MessageBoxButtons.OK);
        DialogResult = DialogResult.None;
        return;
    }

    User.Login = login;
    User.Pass = changePassword ? pass : User.Pass;
    User.IsAdmin = roleCombo.Text == "Teacher";

    if (add)
        db.Users.Add(User);
    db.SaveChanges();
}