LoginPage.cs

public void Login(string email, string password)
{
    if (string.IsNullOrEmpty(email))
    {
        throw new ArgumentException(nameof(email) + " cannot be null or empty.");
    }
    if (string.IsNullOrEmpty(password))
    {
        throw new ArgumentException(nameof(password) + " cannot be null or empty.");
    }
    // login the user
}