Analyze .NET exception handling

This article mainly introduces a summary of thoughts on .NET exception handling, which has certain reference value. Interested friends can refer to it.

The new year is approaching. For most programmers, it will be time to take some time off. However, in this free time, the only way to kill time is to argue about which language is better. It is estimated that this will be the case recently. There are many blog posts about java and .net. I said that as a melon-eater, I would quietly watch the big guys express their feelings.

The above nonsense is enough, so I won’t stop talking nonsense here, let’s get to the point.

In project development, there are corresponding requirements for the stability and fault tolerance of the system and code. The difference between the code in actual development projects and the sample code is more about the stability, fault tolerance, and scalability of the code's operation. Because for implementing a function, the core code to implement the function is the same, and it may just be optimized in terms of writing, but in terms of the classes used to implement a certain operation, this is the same most of the time. From this point of view, in the actual development process, we need to consider many issues, which are not limited to the implementation of a specific function, but more about the stability and scalability of the code.

The above are the problems that need to be faced in actual development. In the recent blog post, the author is also considering how to write this exception and how to understand the exception. I hope it will be helpful to everyone. , you are also welcome to put forward your own ideas and opinions, and share your knowledge and insights.

1. Overview of DotNET exceptions:

When it comes to exceptions, we need to know what exceptions are, and everything if we want to learn , we should know what we want to learn, so that we can have a general understanding in our hearts. An exception occurs when a member fails to perform the action that its name states it can perform. In .NET, there is no way to return error codes from constructors, getting and setting properties, adding and deleting events, calling operator overloads, calling conversion operators, etc. However, if errors need to be reported in these constructs, exceptions must be provided. processing mechanism.

In exception handling, the three blocks we often use are: try block; catch block; finally block. These three blocks can be used together or without a catch block. Exception handling blocks can be used nested. The specific method will be introduced below.

In the exception handling mechanism, there are generally three options: rethrow the same exception and notify the code higher in the call stack of the occurrence of the exception; throw a different exception and notify the code higher in the call stack One layer of code provides richer exception information; lets the thread exit from the bottom of the catch block.

There are some guiding suggestions on how to handle exceptions.

1. Use the finally block appropriately:

The finally block can ensure that no matter what type of exception the thread throws, it can be executed. The final block is generally used for cleanup. Those operations that have been successfully initiated will then return to the caller or the code after the finally block.

2. Exception catching needs to be appropriate:

Why should we catch exceptions appropriately? The following code, because we cannot catch all exceptions, after catching exceptions, we need to handle these exceptions. If we catch all exceptions, but do not foresee the exceptions that will occur, we will have no way to handle these exceptions. .

If the application code throws an exception, the other end of the application may expect to catch the exception, so it cannot be written as a "size-fits-all" exception block and the exception should be allowed to move up the call stack. Move and let the application code specifically handle this exception.

In the catch block, you can use System.Exception to catch exceptions, but it is best to rethrow the exception at the end of the catch block. The reason will be explained later.

   try
   {
    var hkml = GetRegistryKey(rootKey);
    var subkey = hkml.CreateSubKey(subKey);
    if (subkey != null && keyName != string.Empty)
     subkey.SetValue(keyName, keyValue, RegistryValueKind.String);
   }
   catch (Exception ex)
   {
    Log4Helper.Error("创建注册表错误" + ex);
    throw new Exception(ex.Message,ex);
   }

3. Recovery from exceptions:

After catching the exception, we can write some targeted exception recovery methods code that allows the program to continue running. When catching exceptions, you need to catch specific exceptions, fully understand under what circumstances an exception will be thrown, and know which types are derived from the caught exception type. Do not handle or catch System.Exception exceptions unless rethrown at the end of the catch block.

4. Maintain state:

Generally, when we complete an operation or a method, we need to call several method combinations to complete it. During the execution process, It appears that the first few methods are completed, but an exception occurs in the following methods. When an unrecoverable exception occurs, the partially completed operation is rolled back, because we need to restore the information, so when we catch the exception, we need to capture all the exception information.

5. Hide implementation details to maintain the contract:

Sometimes you may need to catch an exception and rethrow a different exception, so that the contract of the method can be maintained. The type of cardiac abnormality should be a specific abnormality. Look at the following code:

FileStream fs = null;
   try
   {
    fs = FileStream();
    
   }
   catch (FileNotFoundException e)
   {
　　　　　　　　　　//抛出一个不同的异常，将异常信息包含在其中，并将原来的异常设置为内部异常
    throw new NameNotFoundException();
   }
   catch (IOException e)
   {
 
    //抛出一个不同的异常，将异常信息包含在其中，并将原来的异常设置为内部异常
    throw new NameNotFoundException(); 
   } 
   finally 
   {
    if (fs != null) 
    { 
    fs.close(); 
   } 
   }

以上的代码只是在说明一种处理方式。应该让抛出的所有异常都沿着方法的调用栈向上传递，而不是把他们”吞噬“了之后抛出一个新的异常。如果一个类型构造器抛出一个异常，而且该异常未在类型构造器方法中捕获，CLR就会在内部捕获该异常，并改为抛出一个新的TypeInitialztionException。

二.DotNET异常的常用处理机制：

在代码发生异常后，我们需要去处理这个异常，如果一个异常没有得到及时的处理，CLR会终止进程。在异常的处理中，我们可以在一个线程捕获异常，在另一个线程中重新抛出异常。异常抛出时，CLR会在调用栈中向上查找与抛出的异常类型匹配的catch块。如果没有任何catch块匹配抛出的异常类型，就发生一个未处理异常。CLR检测到进程中的任何线程有一个位处理异常，都会终止进程。

1.异常处理块：

(1).try块：包含代码通常需要执行一些通用的资源清理操作，或者需要从异常中恢复，或者两者都需要。try块还可以包含也许会抛出异常的代码。一个try块至少有一个关联的catch块或finall块。      

(2).catch块：包含的是响应一个异常需要执行的代码。catch关键字后的圆括号中的表达式是捕获类型。捕获类型从System.Exception或者其派生类指定。CLR自上而下搜素一个匹配的catch块，所以应该教具体的异常放在顶部。一旦CLR找到一个具有匹配捕获类型的catch块，就会执行内层所有finally块中的代码，”内层finally“是指抛出异常的tey块开始，到匹配异常的catch块之间的所有finally块。

使用System.Exception捕捉异常后，可以采用在catch块的末尾重新抛出异常，因为如果我们在捕获Exception异常后，没有及时的处理或者终止程序，这一异常可能对程序造成很大的安全隐患，Exception类是所有异常的基类，可以捕获程序中所有的异常，如果出现较大的异常，我们没有及时的处理，造成的问题是巨大的。

(3).finally块：包含的代码是保证会执行的代码。finally块的所有代码执行完毕后，线程退出finally块，执行紧跟在finally块之后的语句。如果不存在finally块，线程将从最后一个catch块之后的语句开始执行。

备注：异常块可以组合和嵌套，对于三个异常块的样例，在这里就不做介绍，异常的嵌套可以防止在处理异常的时候再次出现未处理的异常，以上这些就不再赘述。

2.异常处理实例：

(1).异常处理扩展方法：

  /// <summary>
  /// 格式化异常消息
  /// </summary>
  /// <param name="e">异常对象</param>
  /// <param name="isHideStackTrace">是否隐藏异常规模信息</param>
  /// <returns>格式化后的异常信息字符串</returns>
  public static string FormatMessage(this Exception e, bool isHideStackTrace = false)
  {
   var sb = new StringBuilder();
   var count = 0;
   var appString = string.Empty;
   while (e != null)
   {
    if (count > 0)
    {
     appString += " ";
    }
    sb.AppendLine(string.Format("{0}异常消息：{1}", appString, e.Message));
    sb.AppendLine(string.Format("{0}异常类型：{1}", appString, e.GetType().FullName));
    sb.AppendLine(string.Format("{0}异常方法：{1}", appString, (e.TargetSite == null ? null : e.TargetSite.Name)));
    sb.AppendLine(string.Format("{0}异常源：{1}", appString, e.Source));
    if (!isHideStackTrace && e.StackTrace != null)
    {
     sb.AppendLine(string.Format("{0}异常堆栈：{1}", appString, e.StackTrace));
    }
    if (e.InnerException != null)
    {
     sb.AppendLine(string.Format("{0}内部异常：", appString));
     count++;
    }
    e = e.InnerException;
   }
   return sb.ToString();
  }

 (2).验证异常：

  /// <summary>
  /// 检查字符串是空的或空的，并抛出一个异常
  /// </summary>
  /// <param name="val">值测试</param>
  /// <param name="paramName">参数检查名称</param>
  public static void CheckNullOrEmpty(string val, string paramName)
  {
   if (string.IsNullOrEmpty(val))
    throw new ArgumentNullException(paramName, "Value can&#39;t be null or empty");
  }

  /// <summary>
  /// 请检查参数不是空的或空的，并抛出异常
  /// </summary>
  /// <param name="param">检查值</param>
  /// <param name="paramName">参数名称</param>
  public static void CheckNullParam(string param, string paramName)
  {
   if (string.IsNullOrEmpty(param))
    throw new ArgumentNullException(paramName, paramName + " can&#39;t be neither null nor empty");
  }

  /// <summary>
  /// 检查参数不是无效，并抛出一个异常
  /// </summary>
  /// <param name="param">检查值</param>
  /// <param name="paramName">参数名称</param>
  public static void CheckNullParam(object param, string paramName)
  {
   if (param == null)
    throw new ArgumentNullException(paramName, paramName + " can&#39;t be null");
  }

  /// <summary>
  /// 请检查参数1不同于参数2
  /// </summary>
  /// <param name="param1">值1测试</param>
  /// <param name="param1Name">name of value 1</param>
  /// <param name="param2">value 2 to test</param>
  /// <param name="param2Name">name of vlaue 2</param>
  public static void CheckDifferentsParams(object param1, string param1Name, object param2, string param2Name)
  {
   if (param1 == param2) {
    throw new ArgumentException(param1Name + " can&#39;t be the same as " + param2Name,
     param1Name + " and " + param2Name);
   }
  }

  /// <summary>
  /// 检查一个整数值是正的（0或更大）
  /// </summary>
  /// <param name="val">整数测试</param>
  public static void PositiveValue(int val)
  {
   if (val < 0)
    throw new ArgumentException("The value must be greater than or equal to 0.");
  }

(3).Try-Catch扩展操作:

  /// <summary>
  ///  对某对象执行指定功能与后续功能，并处理异常情况
  /// </summary>
  /// <typeparam name="T">对象类型</typeparam>
  /// <param name="source">值</param>
  /// <param name="action">要对值执行的主功能代码</param>
  /// <param name="failureAction">catch中的功能代码</param>
  /// <param name="successAction">主功能代码成功后执行的功能代码</param>
  /// <returns>主功能代码是否顺利执行</returns>
  public static bool TryCatch<T>(this T source, Action<T> action, Action<Exception> failureAction,
   Action<T> successAction) where T : class
  {
   bool result;
   try
   {
    action(source);
    successAction(source);
    result = true;
   }
   catch (Exception obj)
   {
    failureAction(obj);
    result = false;
   }
   return result;
  }

  /// <summary>
  ///  对某对象执行指定功能，并处理异常情况
  /// </summary>
  /// <typeparam name="T">对象类型</typeparam>
  /// <param name="source">值</param>
  /// <param name="action">要对值执行的主功能代码</param>
  /// <param name="failureAction">catch中的功能代码</param>
  /// <returns>主功能代码是否顺利执行</returns>
  public static bool TryCatch<T>(this T source, Action<T> action, Action<Exception> failureAction) where T : class
  {
   return source.TryCatch(action,
    failureAction,
    obj => { });
  }

  /// <summary>
  ///  对某对象执行指定功能，并处理异常情况与返回值
  /// </summary>
  /// <typeparam name="T">对象类型</typeparam>
  /// <typeparam name="TResult">返回值类型</typeparam>
  /// <param name="source">值</param>
  /// <param name="func">要对值执行的主功能代码</param>
  /// <param name="failureAction">catch中的功能代码</param>
  /// <param name="successAction">主功能代码成功后执行的功能代码</param>
  /// <returns>功能代码的返回值，如果出现异常，则返回对象类型的默认值</returns>
  public static TResult TryCatch<T, TResult>(this T source, Func<T, TResult> func, Action<Exception> failureAction,
   Action<T> successAction)
   where T : class
  {
   TResult result;
   try
   {
    var u = func(source);
    successAction(source);
    result = u;
   }
   catch (Exception obj)
   {
    failureAction(obj);
    result = default(TResult);
   }
   return result;
  }

  /// <summary>
  ///  对某对象执行指定功能，并处理异常情况与返回值
  /// </summary>
  /// <typeparam name="T">对象类型</typeparam>
  /// <typeparam name="TResult">返回值类型</typeparam>
  /// <param name="source">值</param>
  /// <param name="func">要对值执行的主功能代码</param>
  /// <param name="failureAction">catch中的功能代码</param>
  /// <returns>功能代码的返回值，如果出现异常，则返回对象类型的默认值</returns>
  public static TResult TryCatch<T, TResult>(this T source, Func<T, TResult> func, Action<Exception> failureAction)
   where T : class
  {
   return source.TryCatch(func,
    failureAction,
    obj => { });
  }

本文没有具体介绍try，catch，finally的使用，而是给出一些比较通用的方法，主要是一般的开发者对于三个块的使用都有一个认识，就不再做重复的介绍。

三.DotNET的Exception类分析：

CLR允许异常抛出任何类型的实例，这里我们介绍一个System.Exception类：

1.Message属性：指出抛出异常的原因。

[__DynamicallyInvokable]
public virtual string Message
{
 [__DynamicallyInvokable]
 get
 {
  if (this._message != null)
  {
   return this._message;
  }
  if (this._className == null)
  {
   this._className = this.GetClassName();
  }
  return Environment.GetRuntimeResourceString("Exception_WasThrown", new object[] { this._className });
 }
}

由以上的代码可以看出，Message只具有get属性，所以message是只读属性。GetClassName()获取异常的类。GetRuntimeResourceString()获取运行时资源字符串。

2.StackTrace属性：包含抛出异常之前调用过的所有方法的名称和签名。

public static string StackTrace
{
 [SecuritySafeCritical]
 get
 {
  new EnvironmentPermission(PermissionState.Unrestricted).Demand();
  return GetStackTrace(null, true);
 }
}

EnvironmentPermission()用于环境限制，PermissionState.Unrestricted设置权限状态，GetStackTrace()获取堆栈跟踪，具体看一下GetStackTrace()的代码。

internal static string GetStackTrace(Exception e, bool needFileInfo)
{
 StackTrace trace;
 if (e == null)
 {
  trace = new StackTrace(needFileInfo);
 }
 else
 {
  trace = new StackTrace(e, needFileInfo);
 }
 return trace.ToString(StackTrace.TraceFormat.Normal);
}

public StackTrace(Exception e, bool fNeedFileInfo)
{
 if (e == null)
 {
  throw new ArgumentNullException("e");
 }
 this.m_iNumOfFrames = 0;
 this.m_iMethodsToSkip = 0;
 this.CaptureStackTrace(0, fNeedFileInfo, null, e);
}

 以上是获取堆栈跟踪方法的具体实现，此方法主要用户调试的时候。

3.GetBaseException()获取基础异常信息方法。

[__DynamicallyInvokable]
public virtual Exception GetBaseException()
{
 Exception innerException = this.InnerException;
 Exception exception2 = this;
 while (innerException != null)
 {
  exception2 = innerException;
  innerException = innerException.InnerException;
 }
 return exception2;
}

InnerException属性是内在异常，这是一个虚方法，在这里被重写。具体看一下InnerException属性。

[__DynamicallyInvokable]
public Exception InnerException
{
 [__DynamicallyInvokable, TargetedPatchingOptOut("Performance critical to inline this type of method across NGen image boundaries")]
 get
 {
  return this._innerException;
 }
}

 4.ToString()将异常信息格式化。

private string ToString(bool needFileLineInfo, bool needMessage)
{
 string className;
 string str = needMessage ? this.Message : null;
 if ((str == null) || (str.Length <= 0))
 {
  className = this.GetClassName();
 }
 else
 {
  className = this.GetClassName() + ": " + str;
 }
 if (this._innerException != null)
 {
  className = className + " ---> " + this._innerException.ToString(needFileLineInfo, needMessage) + Environment.NewLine + " " + Environment.GetRuntimeResourceString("Exception_EndOfInnerExceptionStack");
 }
 string stackTrace = this.GetStackTrace(needFileLineInfo);
 if (stackTrace != null)
 {
  className = className + Environment.NewLine + stackTrace;
 }
 return className;
}

在此方法中，将获取的异常信息进行格式化为字符串，this.GetClassName() 获取异常类的相关信息。

以上我们注意到[__DynamicallyInvokable]定制属性，我们看一下具体的实现代码：

[TargetedPatchingOptOut("Performance critical to inline this type of method across NGen image boundaries")]
public __DynamicallyInvokableAttribute()
{
}

以上我们主要注释部分，”图像边界“这个属性的相关信息，请参见《Via CLR c#》,这里就不做具体的介绍。

四.总结：

以上在对异常的介绍中，主要介绍了CLR的异常处理机制，一些较为通用的异常代码，以及对Exception类的介绍。在实际的项目中，我们一般不要将异常直接抛出给客户，我们在编写程序时，已经考虑程序的容错性，在程序捕获到异常后，尽量去恢复程序，或者将异常信息写入日志，让程序进入错误页。如果出现比较严重的异常，最后将异常抛出，终止程序。

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