zoukankan      html  css  js  c++  java
  • Talk about how to write good C# code from a bug

    I have noticed one block code yesterday and found a bug in it, note down here to share with you.

    Background

    =============================================================================

    There is a asynchronous operation need to call BeginXXX and EndXXX, a COM object will be returned after BeginXXX called, and most important the COM object must be released manually otherwise it will cause leaking issue.

    =============================================================================

    Content

    A team guy wrote some sample to us, we can refer his code to use. I don’t remember his exact code and use similar code instead.

    namespace ConsoleApplication2
    {
        class Program
        {
            static void Main(string[] args)
            {
                try
                {
                    FakeReader reader = new FakeReader();
                    FakeLock fakeLock = reader.BeginRead();

                    try
                    {
                        reader.EndRead(fakeLock);
                    }
                    finally
                    {
                        fakeLock.Free();
                    }
                }
                catch (Exception ex)
                { /*handle ex*/}
                finally
                { }
            }

        }

        class FakeReader
        {
            public FakeLock BeginRead() { return new FakeLock(); }
            public void EndRead(FakeLock fakeLock) { }
        }

        class FakeLock
        {
            public static bool freed;
            ~FakeLock()
            {
                if (!freed)
                    throw new ApplicationException("FakeLock shoule be free.");
            }
            public void Free()
            {
                freed = true;
            }
        }
    }

    According the code, you can see he use nested try-finally to make sure fakeLock is freed. Yes, it can work on most parts of time, but what is the truth?

    I noticed if there are some code existed between the reader.BeginRead() and try block and unluckily there is happened throw out an exception, the code will  stopped immediately and go to outside try-catch-finally. The nested try-finally is skipped and leak issue is occurred.

    I sent an email to warn him and his reply is:

    • he will never add any codes between reader.BeginRead() and try block ;
    • he need the programmer who will refer his code to think clearly the importance before add codes in the special position

    It’s obvious he leave the risk to the programmer who will refer his code. This is not reasonable.

    BTW even there is no code between reader.BeginRead() and try block, the csc compiler will insert a NOP at the begin of each try. This increase the possibility of code exception. Of course the possibility is very minor.

    =============================================================================

    Tragedy

    I’m a unfortunate person who always watch tragedy. Sadly I saw another related code yesterday afternoon.

            static void Main(string[] args)
            {
                try
                {
                    FakeReader reader = new FakeReader();
                    FakeLock fakeLock = reader.BeginRead();

                    /*
                     there is a assertion here
                     */

                    if (fakeLock != null)
                    {
                        fakeLock.Free();
                    }
                }
                catch (Exception ex)
                { /*handle ex*/}
                finally
                { }
            }

    Correct, first it check fakeLock value with null then free it. The logic is perfect right to avoid NullReferenceException. But if the previous assertion is failed, only thespian result is performed.

    =============================================================================

    Solution

    The solution is very simple. Move the fakeLock’s assign into nested try-finaly or upgrade it outside of outside try-catch-finally to global variable. Free it in the last finally block.

            static void Main(string[] args)
            {
                try
                {
                    FakeReader reader = new FakeReader();
                    FakeLock fakeLock = null;

                    try
                    {

                        fakeLock = reader.BeginRead();
                        reader.EndRead(fakeLock);
                    }
                    finally
                    {
                        fakeLock.Free();
                    }
                }
                catch (Exception ex)
                { /*handle ex*/}
                finally
                { }
            }

            static void Main(string[] args)
            {

                FakeLock fakeLock = null;
                try
                {
                    FakeReader reader = new FakeReader();      

                    fakeLock = reader.BeginRead();
                    reader.EndRead(fakeLock);
                }
                catch (Exception ex)
                { /*handle ex*/}
                finally
                {

                    if (fakeLock != null)
                    {
                        fakeLock.Free();
                    }

                 }
            }

    =============================================================================

    END.

    We should pay more attention to each line of our code and know exactly the running condition of our program.

  • 相关阅读:
    framework 3 ,4
    C# 线程
    ReportingService错误:配置参数 SharePointIntegrated 被设置为 True,但无法加载 Share Point 对象模型
    C# 打印 word pdf PrintOut方法
    C# 多线程控制控件实例(例程简单,注释详细)
    Windchill MethodServer启动后自动关闭
    Press C#使用指定打印机打印Word,Excel等Office文件和打印PDF文件的代码 (转)
    ExtJs入门
    The transaction log for database 'wcadmin' is full. To find out why space in the log cannot be reused, see the log_reuse_wait_desc column in sys.datab
    JS+调用word打印功能实现在Webfrom客户端
  • 原文地址:https://www.cnblogs.com/diggingdeeply/p/Talk_about_how_to_write_good_CSharp_code_from_a_bug.html
Copyright © 2011-2022 走看看