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如何处理三层框架的异常

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Scenario	Description	Expected Result
1	Application called a stored procedure.	Happy path; everything should work fine.
2	Exception Raised by Database Management System such as Primary Key violated or Object does not exist, etc.	System should log the thrown exception in a sink (Text File). Replace the Exception with a User friendly Message and propagate it to UI passing through the Business Logic Layer.
3	Based upon some certain validation check / business rule a Custom Error is raised from Stored Procedure.	Thrown Custom Error message should be propagated to UI without Logging assuming these error messages will be some kind of alternative flows instead of real exception.
4	While processing the request, an error occurred in Business Logic such as Divide-by-zero, Object not set, Null reference exception, etc.	System should log the thrown exception in a sink (Text File). Replace the Exception with a User friendly Message and should propagate that to UI.
5	Based upon some certain business validation check, a custom exception was thrown from Business Logic Layer.	Thrown Custom Error message should be propagated to UI without Logging assuming these error messages will be some kind of alternative flows instead of real exception.
6	While processing the request an error occurred in UI. For example, `Null`reference exception, etc.	System should log the thrown exception in a sink (Text File). Replace the Exception with a User friendly Message and should propagate that to UI.

参考地址：https://www.codeproject.com/articles/85569/exception-handling-in-3-tier-architecture

标题：Exception Handling in 3-Tier Architecture

思想是：使用自定义异常替代原始的异常信息，如数据库错误信息，系统错误，组件错误等。

替换原始异常的信息，然后将异常传递到UI层，将更友善的信息展示给用户；

该记录的异常要记录到日志系统；

不需要记录的异常信息要直接抛给用户，让用户知道错在哪里；

需要仔细区分，哪些异常应该抛给用户，哪些异常应该进行替换展示友好信息，并记录异常信息，通知开发人员处理；

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参考：How to rethrow exceptions in a program with multiple layers?

地址：https://stackoverflow.com/questions/14060042/how-to-rethrow-exceptions-in-a-program-with-multiple-layers

(我认为)建议您抛出一个新的更简单的异常，就是将异常从较低层转换为新的、更高级别的异常，以便在外层中使用。较低级别的异常不适合在程序的上层使用。

例如，在LINQto实体中，当序列没有元素时，方法Single()将抛出InvalidOperationException。但是，这种异常类型非常常见，因此很难在用户界面级别捕获它：如何区分引发此异常的不同可能性(例如，修改只读集合)？解决方案是将异常转换为应用程序可以轻松处理的另一种(新的、用户定义的)类型。

Here is a simple example of the idea:

public class MyUserService {
    public User GetById(int id) {
        try {
            using(var ctx = new ModelContainer()) {
                return ctx.Where(u => u.Id == id).Single();
            }
        }
        catch(InvalidOperationException) {
            // OOPs, there is no user with the given id!
            throw new UserNotFoundException(id);
        }
    }
}

然后，Program层可以捕获UserNotFoundException并立即知道发生了什么，从而找到向用户解释错误的最佳方法。细节将取决于程序的确切结构，但类似的内容将在ASP.NETMVC应用程序中工作：

public class MyUserController : Controller {
    private MyUserService Service = new MyUserService();
    public ActionResult Details(int id) {
        User user;
        try {
            user = Service.GetById(id);
        }
        catch(UserNotFoundException) {
            // Oops, there is no such user. Return a 404 error
            // Note that we do not care about the InvalidOperationException
            // that was thrown inside GetById
            return HttpNotFound("The user does not exist!");
        }
        // If we reach here we have a valid user
        return View(user);
     }
}

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有更多的方法可以使用不同类型的异常处理。在功能上，您应该定义哪些层在异常情况下必须做什么。

就像数据层=>一样，除了DataException或SQLException之外，不要抛出其他任何东西。记录它们并将一个通用数据库异常抛回UI。

业务层=>日志并重新抛出简单的业务异常UI层=>只捕获业务异常并在业务异常内的消息中警告它。

一旦定义了所有这些，您就可以使用您所学到的和问题中的总结来构建它。

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20200602