问题
现在有一个FileStorageService类，继承自IStorageService，具体实现如下
public interface IStorageService{ void WriteAllBytes(string path, byte[] buffer); byte[] ReadAllBytes(string path);}public class FileStorageService : IStorageService{ public void WriteAllBytes(string path, byte[] buffer) { File.WriteAllBytes(path, buffer); } public byte[] ReadAllBytes(string path) { return File.ReadAllBytes(path); }}
假设调用其中任一一个方法出现异常，例如读写文件时候经常碰见的异常：IOException, DirectoryNotFoundException, FileNotFoundException,UnauthorizedAccessException… 甚至是 OutOfMemoryException
方案1-不是我的问题
IStorageService 不关心抛出的异常，那是使用者的职责。如此便将问题抛给了使用IStorageService接口的用户，它们必须要捕获有可能抛出的异常，往往一种偷懒的做法就是使用try catch语句将其包裹起来，如：
IStorageService myStorageService = Resolver.Resolve<IStorageService>();try{ myStorageService.ReadAllBytes("C:\stuff.data");}catch (Exception exception){ // please don't write generic error messages like this, be specific Logger.Log("Oops something went wrong: " + exception.Message);}
catch exception并不是什么好主意，而且每次调用都需要使用try catch很不方便
方案2-创建新的异常类
一种改进的方法就是创建我们自己的异常类如StorageReadException，不管以后具体的实现如何变化，我们仅捕获特定的异常来进行异常处理
public class StorageReadException : Exception{ public StorageReadException(Exception innerException) : base(innerException.Message, innerException) { }}
之前的FileStorageService实现更改为：
public byte[] ReadAllBytes(string path){ try { return File.ReadAllBytes(path); } catch (FileNotFoundException fileNotFoundException) { throw new StorageReadException(fileNotFoundException); }}
调用代码：
IStorageService myStorageService = Resolver.Resolve<IStorageService>();try{ myStorageService.ReadAllBytes(path);}catch (StorageReadException sre){ Logger.Log(String.Format("Failed to read file from path, {0}: {1}", path, sre.Message));}
同样存在try catch 包裹问题，而且用户必须依赖一个新的异常类型
方案3-Try 模式并返回一个Complex Result
我们可以使用Try模式来避免用户使用try catch,Try模式类似C#里int方法
bool TryParse(string s, out int result)，我们对接口进行更改
byte[] ReadAllBytes(string path)
变为
bool TryReadAllBytes(string path, out byte[] result)
但是这样不能提供用户更多的错误信息。如果我们想要显示更多的有帮助的异常信息给用户，可以返回一个通用的结果类OperationResult<TResult>
public class OperationResult<TResult>{ private OperationResult () { } public bool Success { get; private set; } public TResult Result { get; private set; } public string NonSuccessMessage { get; private set; } public Exception Exception { get; private set; } public static OperationResult<TResult> CreateSuccessResult(TResult result) { return new OperationResult<TResult> { Success = true, Result = result}; } public static OperationResult<TResult> CreateFailure(string nonSuccessMessage) { return new OperationResult<TResult> { Success = false, NonSuccessMessage = nonSuccessMessage}; } public static OperationResult<TResult> CreateFailure(Exception ex) { return new OperationResult<TResult> { Success = false, NonSuccessMessage = String.Format("{0}{1}{1}{2}", ex.Message, Environment.NewLine, ex.StackTrace), Exception = ex }; }}
FileStorageService的ReadAllBytes 方法变为
public OperationResult<byte[]> TryReadAllBytes(string path){ try { var bytes = File.ReadAllBytes(path); return OperationResult<byte[]>.CreateSuccessResult(bytes); } catch (FileNotFoundException fileNotFoundException) { return OperationResult<byte[]>.CreateFailure(fileNotFoundException); }}
调用代码：
var result = myStorageService.TryReadAllBytes(path);if(result.Success){ // do something}else{ Logger.Log(String.Format("Failed to read file from path, {0}: {1}", path, result.NonSuccessMessage));}
原文：Error Handling in SOLID C# .NET – The Operation Result Approach