Question

Is this action too redundant – is there a better way to simplify it?

[Authorize, AcceptVerbs(HttpVerbs.Post)]
public ActionResult ChangePassword(string oldPassword, string newPassword, string confirmPassword)
{
    var oldPasswordValidationResults = _validatorProvider.Validate<IStringLengthValidator>(oldPassword);
    oldPasswordValidationResults.Where(r => !r.Passed)
                                .Each(r => ModelState.AddModelError("OldPassword", "Please enter your old password."));


    var newPasswordValidationResults = _validatorProvider.Validate<IStringLengthValidator>(newPassword);
    newPasswordValidationResults.Where(r => !r.Passed)
                                .Each(r => ModelState.AddModelError("NewPassword", "Please enter a new password."));

    if (!ModelState.IsValid)
        return View();

    if (newPassword != confirmPassword)
        ModelState.AddModelError("ConfirmPassword", "The passwords do not match.");

    if (!ModelState.IsValid)
        return View();

    if (!_userMembershipService.ChangePassword(oldPassword, newPassword))
        ModelState.AddModelError("_FORM", "Unable to change your password.");

    if (!ModelState.IsValid)
        return View();

    return View("ChangePasswordSuccessful");
}

All of these seem to me to have a code smell…

if (!ModelState.IsValid)
    return View();

Answer 1

This change seems to preserve your original intentions a little better:

if (newPassword != confirmPassword)
{
    ModelState.AddModelError("ConfirmPassword", "The passwords do not match.");
    return View();
}

if (!_userMembershipService.ChangePassword(oldPassword, newPassword))
{
    ModelState.AddModelError("_FORM", "Unable to change your password.");
    return View();
}

return View("ChangePasswordSuccessful");