最终，决定取决于是使用异常处理更有助于处理此类应用程序级错误，还是通过您自己的机制(如返回状态代码)更有帮助。我不认为哪个更好有一个严格的规则，但我会考虑:

Who's calling your code? Is this a public API of some sort or an internal library? What language are you using? If it's Java, for example, then throwing a (checked) exception puts an explicit burden on your caller to handle this error condition in some way, as opposed to a return status which could be ignored. That could be good or bad. How are other error conditions in the same application handled? Callers won't want to deal with a module that handles errors in an idiosyncratic way unlike anything else in the system. How many things can go wrong with the routine in question, and how would they be handled differently? Consider the difference between a series of catch blocks that handle different errors and a switch on an error code. Do you have structured information about the error you need to return? Throwing an exception gives you a better place to put this information than just returning a status.

for me Exception should be thrown when a required technical or business rule fails. for instance if a car entity is associated with array of 4 tires ... if one tire or more are null ... an exception should be Fired "NotEnoughTiresException" , cuz it can be caught at different level of the system and have a significant meaning through logging. besides if we just try to flow control the null and prevent the instanciation of the car . we might never never find the source of the problem , cuz the tire isn't supposed to be null in the first place .

The simple answer is, whenever an operation is impossible (because of either application OR because it would violate business logic). If a method is invoked and it impossible to do what the method was written to do, throw an Exception. A good example is that constructors always throw ArgumentExceptions if an instance cannot be created using the supplied parameters. Another example is InvalidOperationException, which is thrown when an operation cannot be performed because of the state of another member or members of the class.

在您的情况下，如果调用Login(用户名，密码)这样的方法，如果用户名无效，抛出UserNameNotValidException或PasswordNotCorrectException(密码不正确)确实是正确的。用户不能使用提供的参数登录(即，这是不可能的，因为它将违反身份验证)，因此抛出异常。尽管我可能从ArgumentException继承了两个异常。

话虽如此，如果因为登录失败可能很常见而不希望抛出异常，一种策略是创建一个方法，该方法返回表示不同失败的类型。这里有一个例子:

{ // class
    ...

    public LoginResult Login(string user, string password)
    {
        if (IsInvalidUser(user))
        {
            return new UserInvalidLoginResult(user);
        }
        else if (IsInvalidPassword(user, password))
        {
            return new PasswordInvalidLoginResult(user, password);
        }
        else
        {
            return new SuccessfulLoginResult();
        }
    }

    ...
}

public abstract class LoginResult
{
    public readonly string Message;

    protected LoginResult(string message)
    {
        this.Message = message;
    }
}

public class SuccessfulLoginResult : LoginResult
{
    public SucccessfulLogin(string user)
        : base(string.Format("Login for user '{0}' was successful.", user))
    { }
}

public class UserInvalidLoginResult : LoginResult
{
    public UserInvalidLoginResult(string user)
        : base(string.Format("The username '{0}' is invalid.", user))
    { }
}

public class PasswordInvalidLoginResult : LoginResult
{
    public PasswordInvalidLoginResult(string password, string user)
        : base(string.Format("The password '{0}' for username '{0}' is invalid.", password, user))
    { }
}

Most developers are taught to avoid Exceptions because of the overhead caused by throwing them. It's great to be resource-conscious, but usually not at the expense of your application design. That is probably the reason you were told not to throw your two Exceptions. Whether to use Exceptions or not usually boils down to how frequently the Exception will occur. If it's a fairly common or an fairly expectable result, this is when most developers will avoid Exceptions and instead create another method to indicate failure, because of the supposed consumption of resources.

下面是一个使用Try()模式避免在类似刚刚描述的场景中使用exception的例子:

public class ValidatedLogin
{
    public readonly string User;
    public readonly string Password;

    public ValidatedLogin(string user, string password)
    {
        if (IsInvalidUser(user))
        {
            throw new UserInvalidException(user);
        }
        else if (IsInvalidPassword(user, password))
        {
            throw new PasswordInvalidException(password);
        }

        this.User = user;
        this.Password = password;
    }

    public static bool TryCreate(string user, string password, out ValidatedLogin validatedLogin)
    {
        if (IsInvalidUser(user) || 
            IsInvalidPassword(user, password))
        {
            return false;
        }

        validatedLogin = new ValidatedLogin(user, password);

        return true;
    }
}

一般来说，你想要为应用程序中可能发生的任何异常抛出一个"异常"

在您的示例中，这两个异常看起来都是通过密码/用户名验证调用的。在这种情况下，有人会输入错误的用户名/密码并不是什么例外。

它们是UML主要流程的“例外”，但在处理过程中是更多的“分支”。

如果您试图访问您的passwd文件或数据库，但无法访问，这将是一个异常情况，并需要抛出异常。

抛出异常会导致堆栈unwind，这对性能有一定影响(承认，现代托管环境在这方面有所改进)。仍然在嵌套的情况下反复抛出和捕获异常是一个坏主意。

可能比这更重要的是，例外是针对特殊情况的。它们不应该用于普通的控制流，因为这会损害代码的可读性。

最终，决定取决于是使用异常处理更有助于处理此类应用程序级错误，还是通过您自己的机制(如返回状态代码)更有帮助。我不认为哪个更好有一个严格的规则，但我会考虑:

Who's calling your code? Is this a public API of some sort or an internal library? What language are you using? If it's Java, for example, then throwing a (checked) exception puts an explicit burden on your caller to handle this error condition in some way, as opposed to a return status which could be ignored. That could be good or bad. How are other error conditions in the same application handled? Callers won't want to deal with a module that handles errors in an idiosyncratic way unlike anything else in the system. How many things can go wrong with the routine in question, and how would they be handled differently? Consider the difference between a series of catch blocks that handle different errors and a switch on an error code. Do you have structured information about the error you need to return? Throwing an exception gives you a better place to put this information than just returning a status.