避免抛出异常的主要原因是抛出异常涉及大量开销。

下面这篇文章指出的一件事是，例外是针对异常条件和错误的。

错误的用户名不一定是程序错误，而是用户错误……

下面是关于。net中的异常的一个不错的起点: http://msdn.microsoft.com/en-us/library/ms229030 (VS.80) . aspx

在决定异常是否合适时，需要考虑一些有用的事情:

what level of code you want to have run after the exception candidate occurs - that is, how many layers of the call stack should unwind. You generally want to handle an exception as close as possible to where it occurs. For username/password validation, you would normally handle failures in the same block of code, rather than letting an exception bubble up. So an exception is probably not appropriate. (OTOH, after three failed login attempts, control flow may shift elsewhere, and an exception may be appropriate here.) Is this event something you would want to see in an error log? Not every exception is written to an error log, but it's useful to ask whether this entry in an error log would be useful - i.e., you would try to do something about it, or would be the garbage you ignore.

一个经验法则是在您通常无法预测的情况下使用异常。例如数据库连接、磁盘上丢失的文件等。对于您可以预测的场景，例如用户试图使用错误的密码登录，您应该使用返回布尔值的函数，并知道如何优雅地处理这种情况。您不希望仅仅因为有人输入了密码错误而抛出异常，从而突然结束执行。

抛出异常的经验法则非常简单。当你的代码进入UNRECOVERABLE INVALID状态时，你可以这样做。如果数据被泄露，或者您无法回溯到目前为止发生的处理，那么您必须终止它。你还能做什么呢?您的处理逻辑最终将在其他地方失败。如果你能以某种方式恢复，那么就这样做，不要抛出异常。

在你的特定情况下，如果你被迫做一些愚蠢的事情，比如接受提款，然后才检查用户/密码，你应该通过抛出一个异常来终止这个过程，通知发生了一些不好的事情，并防止进一步的损害。

我认为只有在无法摆脱当前状态时才应该抛出异常。例如，如果您正在分配内存，但没有任何内存可以分配。在您提到的情况下，您可以清楚地从这些状态中恢复，并相应地将错误代码返回给调用者。

You will see plenty of advice, including in answers to this question, that you should throw exceptions only in "exceptional" circumstances. That seems superficially reasonable, but is flawed advice, because it replaces one question ("when should I throw an exception") with another subjective question ("what is exceptional"). Instead, follow the advice of Herb Sutter (for C++, available in the Dr Dobbs article When and How to Use Exceptions, and also in his book with Andrei Alexandrescu, C++ Coding Standards): throw an exception if, and only if

没有满足先决条件(通常会出现以下情况之一 不可能的)或 替代方案将无法满足后置条件或 替代方案将无法保持不变式。

为什么这样更好呢?它不是用几个关于前置条件，后置条件和不变量的问题代替了这个问题吗?这是更好的几个相关的原因。

Preconditions, postconditions and invariants are design characteristics of our program (its internal API), whereas the decision to throw is an implementation detail. It forces us to bear in mind that we must consider the design and its implementation separately, and our job while implementing a method is to produce something that satisfies the design constraints. It forces us to think in terms of preconditions, postconditions and invariants, which are the only assumptions that callers of our method should make, and are expressed precisely, enabling loose coupling between the components of our program. That loose coupling then allows us to refactor the implementation, if necessary. The post-conditions and invariants are testable; it results in code that can be easily unit tested, because the post-conditions are predicates our unit-test code can check (assert). Thinking in terms of post-conditions naturally produces a design that has success as a post-condition, which is the natural style for using exceptions. The normal ("happy") execution path of your program is laid out linearly, with all the error handling code moved to the catch clauses.

异常用于异常行为、错误、失败等事件。功能行为、用户错误等应该由程序逻辑来处理。由于错误的帐户或密码是登录例程中逻辑流的一部分，因此它应该能够毫无例外地处理这些情况。