public JsonResult ChangePassword
    (string username, string currentPassword, string newPassword)
{
    switch (this.membershipService.ValidateLogin(username, currentPassword))
    {
        case UserValidationResult.BasUsername:
        case UserValidationResult.BadPassword:
            // abort: return JsonResult with localized error message
            // for invalid username/pass combo.
        case UserValidationResult.TrialExpired
            // abort: return JsonResult with localized error message
            // that user cannot login because their trial period has expired
        case UserValidationResult.Success:
            break;
    }

    // NOW change password now that user is validated
}

enum UserValidationResult
{
    BadUsername,
    BadPassword,
    TrialExpired,
    Success
}

public JsonResult ChangePassword
    (string username, string currentPassword, string newPassword)
{
    switch (this.membershipService.ValidateLogin(username, currentPassword))
    {
        case UserValidationResult.BasUsername:
        case UserValidationResult.BadPassword:
            // abort: return JsonResult with localized error message
            // for invalid username/pass combo.
        case UserValidationResult.TrialExpired
            // abort: return JsonResult with localized error message
            // that user cannot login because their trial period has expired
        case UserValidationResult.Success:
            break;
        default:
            throw new NotImplementedException
                ("Unrecognized UserValidationResult value.");
            // or NotSupportedException()
            break;
    }

    // Change password now that user is validated
}

enum UserValidationResult
{
    BadUsername,
    BadPassword,
    TrialExpired,
    ContactUs,
    Success
}

if( i == 0 ) {


    } else { // i > 0


    }

if( i == 0 ) {


    } else if ( i > 0 ) {


    } else {
        // i < 0 is now an enforced error
        throw new Illegal(...)
    }

if( i == 0 ) {


    } else { // i != 0
      // we can handle both positve or negative, just not zero
    }