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Defining Your Own Exception Class
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Unfortunately, designing your own exception is tedious and error prone . The main reason for this is because all Exception-derived types should be serializable so that they can cross an AppDomain boundary or be written to a log or database . There are many issues related to serialization and they are discussed in 24, Runtime Serialization . So, in an effort to simplify things, I made my own generic Exception<TExceptionArgs> class, which is defined as follows:
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[Serializable] public sealed class Exception<TExceptionArgs> : Exception, ISerializable where TExceptionArgs : ExceptionArgs { private const String c_args = "Args"; // For (de)serialization private readonly TExceptionArgs m_args; public TExceptionArgs Args { get { return m_args; } }
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Part IV
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Core Facilities
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public Exception(String message = null, Exception innerException = null) : this(null, message, innerException) { } public Exception(TExceptionArgs args, String message = null, Exception innerException = null): base(message, innerException) { m_args = args; } // The constructor is for deserialization; since the class is sealed, the constructor is // private. If this class were not sealed, this constructor should be protected [SecurityPermission(SecurityAction.LinkDemand, Flags=SecurityPermissionFlag.SerializationFormatter)] private Exception(SerializationInfo info, StreamingContext context) : base(info, context) { m_args = (TExceptionArgs)info.GetValue(c_args, typeof(TExceptionArgs)); } // The method for serialization; it s public because of the ISerializable interface [SecurityPermission(SecurityAction.LinkDemand, Flags=SecurityPermissionFlag.SerializationFormatter)] public override void GetObjectData(SerializationInfo info, StreamingContext context) { info.AddValue(c_args, m_args); base.GetObjectData(info, context); } public override String Message { get { String baseMsg = base.Message; return (m_args == null) baseMsg : baseMsg + " (" + m_args.Message + ")"; } } public override Boolean Equals(Object obj) { Exception<TExceptionArgs> other = obj as Exception<TExceptionArgs>; if (obj == null) return false; return Object.Equals(m_args, other.m_args) && base.Equals(obj); } public override int GetHashCode() { return base.GetHashCode(); } }
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And the ExceptionArgs base class that TExceptionArgs is constrained to is very simple and looks like this:
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[Serializable] public abstract class ExceptionArgs { public virtual String Message { get { return String.Empty; } } }
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Now, with these two classes defined, I can trivially define more exception classes when I need to . To define an exception type indicating the disk is full, I simply do this:
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[Serializable] public sealed class DiskFullExceptionArgs : ExceptionArgs { private readonly String m_diskpath; // private field set at construction time public DiskFullExceptionArgs(String diskpath) { m_diskpath = diskpath; }
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20 Exceptions and State Management
// Public read-only property that returns the field public String DiskPath { get { return m_diskpath; } } // Override the Message property to include our field (if set) public override String Message { get { return (m_diskpath == null) base.Message : "DiskPath=" + m_diskpath; } } }
And, if I have no additional data that I want to put inside the class, it gets as simple as this:
[Serializable] public sealed class DiskFullExceptionArgs : ExceptionArgs { }
And now I can write code like this, which throws and catches one of these:
public static void TextException() { try { throw new Exception<DiskFullExceptionArgs>( new DiskFullExceptionArgs(@"C:\"), "The disk is full"); } catch (Exception<DiskFullExceptionArgs> e) { Console.WriteLine(e.Message); } }
Note There are two issues to note about my Exception<TExceptionArgs> class . The first issue is that any exception type you define with it is always derived from System.Exception . In most scenarios, this is not a problem at all and, in fact, having a shallow and wide exception type hierarchy is preferred . The second issue is that Visual Studio s unhandled exception dialog box doesn t display Exception<T> type s generic type parameter, as you can see here:
Part IV
Core Facilities
Trading Reliability for Productivity
I started writing software in 1975 . I did a fair amount of BASIC programming, and as I got more interested in hardware, I switched to assembly language . Over time, I switched to the C programming language because it allowed me access to hardware with a much higher level of abstraction, making my programming easier . My background is in writing operating systems code and platform/library code, so I always work hard to make my code as small and as fast as possible since applications can only be as good as the OS and libraries they consume . In addition to creating small and fast code, I always focused on error recovery . When allocating memory (by using C++ s new operator or by calling malloc, HeapAlloc, VirtualAlloc, etc .), I would always check the return value to ensure that the memory I requested was actually given to me . And, if the memory request failed, I always had an alternate code path ensuring that the rest of the program s state was unaffected and would let any of my callers know that I failed so that the calling code can take corrective measures too . For some reason that I can t quite explain, this attention to detail is not done when writing code for the .NET Framework . Getting an out-of-memory situation is always possible and yet I almost never see any code containing a catch block to recover from an OutOfMemoryException . In fact, I ve even had some developers tell me that the CLR doesn t let a program catch an OutOfMemoryException . For the record, this is absolutely not true; you can catch this exception . In fact, there are many errors that are possible when executing managed code and I hardly ever see developers write code that attempts to recover from these potential failures . In this section, I d like to point out some of the potential failures and why it has become culturally acceptable to ignore them . I d also like to point out some of the significant problems that can occur when ignoring these failures and suggest some ways to help mitigate these problems . Object-oriented programming allows developers to be very productive . A big part of this is composability which makes it easy to write, read and maintain code . Take this line of code, for example:
Boolean f = "Jeff".Substring(1, 1).ToUpper().EndsWith("E");
There is a big assumption being made with the code above: no errors occur . But, of course, errors are always possible, and so we need a way to handle those errors . This is what the exception handling constructs and mechanisms are all about and why we need them as opposed to having methods that return true/false to indicate success/failure the way that Win32 and COM functions do . In addition to code composability, we are productive due to all kinds of great features provided by our compilers . For example, the compiler implicitly:
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