错误处理(Operation Result)方法

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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