how to fix code 39 error network adapter Defining Your Own Exception Class in Visual Studio .NET

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Defining Your Own Exception Class
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When implementing your methods, you might come across scenarios in which you want to throw an exception. For example, I recommend that your nonprivate methods always validate their arguments, and if any argument doesn t live up to your method s implicit assumptions, an exception should be thrown. In this case, I recommend that you throw one of the exception classes already defined in the FCL: ArgumentNullException, ArgumentOutOfRangeException, or DuplicateWaitObjectException. I strongly suggest that you throw a specific exception class, a class that has no other classes derived from it. For example, don t throw an ArgumentException because it s too vague, it could mean any of its three derived types, and it doesn t provide as much information as possible to its catchers. You should never throw Exception, ApplicationException, or SystemException. Note Throwing an instance of an exception class that you ve defined provides your catch code with the capability to know exactly what happened and to recover in any way it sees fit. Now let s say that you re defining a method that s passed a reference to an object whose type must implement the ICloneable and IComparable interfaces. You might implement the method like this:
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class SomeType public void if (!((o throw { SomeMethod(Object o) { is ICloneable) && (o is IComparable))) new MissingInterfaceException(...);
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// Code that operates on o goes here. } }
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Because the FCL doesn t define an appropriate exception type, you must define the MissingInterfaceException type yourself. Note that by convention the name of an exception type should end with "Exception". When defining this type, you must decide what its base type will be. Should you choose Exception, ArgumentException, or a different type entirely I ve spent months thinking about this question, but unfortunately, I can t come up with a good rule of thumb to offer you, and here s why. If you derive MissingInterfaceException from ArgumentException, any existing code that s already catching ArgumentException will catch your new exception, too. In some ways this is a feature, and in some ways this is a bug. It s a feature because any code that wants to catch any kind of argument exception (via ArgumentException) now catches this new kind of argument exception (MissingInterfaceException) automatically. It s a bug because a MissingInterfaceException identifies a new event that wasn t anticipated when code was written to catch an ArgumentException. When you define the MissingInterfaceException type, you might think it s so similar to an ArgumentException that it should be handled the same way. However, this unanticipated relationship might cause unpredictable behavior. On the other hand, if you derive MissingInterfaceException directly from Exception, the code throws a new type that the application couldn t have known about. Most likely, this will become an unhandled exception that causes the application to terminate. I could easily consider this desired behavior because an implicit assumption was violated and the application never considered a remedy for it. Catching this new exception, swallowing it, and continuing execution might cause the application to run with unpredictable results. Answering questions like these is one of the reasons that application design is more of an art form than a science. When defining a new exception type, carefully consider how application code will catch your type (or base types of your type), and then choose a base type that has the least negative effect on your callers. When defining your own exception types, feel free to define your own subhierarchies, if applicable to what you re doing. You can define them directly from Exception or from some other base type. Again, just make sure that where you re putting the subhierarchy makes some sense for your callers. As a general rule, hierarchies should be broad and shallow: exception types should be derived from a type close to Exception and should typically be no more than two or three levels deep. If you define an exception type that s not going to be the base of other exception types, then mark the type as sealed. The Exception base type defines three public constructors: A parameterless (default) constructor that creates an instance of the type and sets all fields and properties to default values. A constructor taking a String that creates an instance of the type and sets a specific message. A constructor taking a String and an instance of an Exception derived type that creates an instance of the type and sets a specific message and an inner exception. Unfortunately, there are a lot of Exception derived types in the FCL that don t have this constructor. Microsoft will rectify this oversight in a future version of the .NET Framework. When defining your own exception type, your type should implement a set of three matching constructors and call the base type s corresponding constructor. Of course, your exception type will inherit all the fields and properties defined by Exception. In 320
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addition, you might add fields and properties of your own. For example, the System.ArgumentException exception adds a String property called ParamName. The ArgumentException type also defines new constructors (in addition to the three public constructors) that take an extra String as a parameter so that the ParamName property can be initialized to identify the name of the parameter that violated the method s implicit assumptions. When an ArgumentException is caught, the ParamName property can be read to determine exactly which parameter caused the problem. Let me tell you, this is incredibly handy when you re trying to debug an application! If you do add fields to your exception type, make sure you define some constructors that allow the fields to be initialized. Exception types should always be serializable so that the exception object can be marshaled across an AppDomain or a machine boundary and rethrown in the client s code. Making an exception type serializable also allows it to persist in a log or a database. To make your exception type serializable, you must apply the [Serializable] custom attribute to the type, and if the type defines any fields of its own, you must also implement the ISerializable interface along with its GetObjectData method and special protected constructor, both of which take SerializationInfo and StreamingContext parameters. The following code shows how to properly define your own exception type:
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using using using using System; System.IO; System.Runtime.Serialization; System.Runtime.Serialization.Formatters.Soap;
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// Allow instances of DiskFullException to be serialized. [Serializable] sealed class DiskFullException : Exception, ISerializable { // The three public constructors public DiskFullException() : base() { // Call base constructor. } public DiskFullException(String message) : base(message) { // Call base constructor. }
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public DiskFullException(String message, Exception innerException) : base(message, innerException) { // Call base constructor. } // Define a private field. private String diskpath; // Define a read only property that returns the field. public String DiskPath { get { return diskpath; } } // Override the public Message property so that the // field is included in the message. public override String Message { get { String msg = base.Message; if (diskpath != null) msg += Environment.NewLine + "Disk Path: " + diskpath; return msg; } } // Because at least one field is defined, define the
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// special deserialization constructor. Because this // class is sealed, this constructor is private. If this // class is not sealed, this constructor should be protected. private DiskFullException(SerializationInfo info, StreamingContext context) : base(info, context) { // Let the base deserialize its fields. // Deserialize each field. diskpath = info.GetString("DiskPath"); } // Because at least one field is defined, // define the serialization method. void ISerializable.GetObjectData(SerializationInfo info, StreamingContext context) { // Serialize each field. info.AddValue("DiskPath", diskpath); // Let the base type serialize its fields. base.GetObjectData(info, context); } // Define additional constructors that set the field. public DiskFullException(String message, String diskpath) : this(message) { // Call another constructor. this.diskpath = diskpath; } public DiskFullException(String message, String diskpath, Exception innerException) : this(message, innerException) { // Call another constructor. this.diskpath = diskpath; } }
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// The following code tests the serialization of the exception. class App { static void Main() { // Construct a DiskFullException object, and serialize it. DiskFullException e = new DiskFullException("The disk volume is full", @"C:\"); FileStream fs = new FileStream(@"Test", FileMode.Create); IFormatter f = new SoapFormatter(); f.Serialize(fs, e); fs.Close(); // Deserialize the DiskFullException object, and check its fields. fs = new FileStream(@"Test", FileMode.Open); e = (DiskFullException) f.Deserialize(fs); fs.Close(); Console.WriteLine("Type: {1}{0}DiskPath: {2}{0}Message: {3}", Environment.NewLine, e.GetType(), e.DiskPath, e.Message); } }
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