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How to Manage Objects
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The reason why this extra thought has to go into exceptions and error handling is because code can become complicated. Consider the following source code, which makes poor use of error handling and results in a more complicated implementation: NameValueCollection pluginConfig = (NameValueCollection) ConfigurationSettings.GetConfig("settings/tracer"); if( pluginConfig != null) { if( pluginConfig["isactive"] != null && pluginConfig[ "isactive"] == "true") { Console.WriteLine( "Woohoo is active"); } } You use the preceding example source code to retrieve a value from an application configuration file. Notice how many tests there are before a valid value can be tested. First, you test to check if the variable pluginConfig is not null to indicate that a configuration file has been loaded. Second, you test to check if the isactive variable exists. The problem with the two tests is that they re of no interest to the coder who wants to retrieve the value of a variable, and two tests aren t necessary.
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CHAPTER 2 LOGGING, ERRORS, AND TEST-DRIVEN DEVELOPMENT
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From a coding perspective, either an exception is generated, or the error is dealt with appropriately. The caller of the method shouldn t have to figure out if something strange happened. In the case of the preceding example, that means either pluginConfig references a valid object instance, or an exception is generated. Having pluginConfig reference a null object means something went wrong, but it s unknown what went wrong. For example, maybe the application execution directory is incorrect, or the application configuration file has XML problems, or doesn t exist. A better strategy is never to return a null object, and either return an empty object, return an object with some default data, or generate an exception that says something meaningful. The following rewritten example returns an empty object: class BetterExampleErrorHandling { private static readonly string EmptyValue = ""; public static int[] GetArrayValues() { return new int[ 0]; } public static string GetStringValue() { return EmptyValue; } } class ProcessErrorHandlingExamples { public static void BetterExamples() { Tracer.start( 10, "ProcessErrorHandlingExamples.BetterExamples"); int[] arr = BetterExampleErrorHandling.GetArrayValues(); for( int c1 = 0; c1 < arr.Length; c1 ++) { Tracer.output( 10, "Value is " + arr[ c1]); } if( BetterExampleErrorHandling.GetStringValue().Length > 0) { Tracer.output( 10, "Not an empty String"); } Tracer.end( 10); } } In the rewritten example, the methods GetArrayValues and GetStringValue return not a null value, but default object instances. In the examples, the valid object instances are nulllength arrays and empty string buffers. Looking at the implementation of the BetterExamples, the code has been immensely simplified because there s no need to test the validity of the objects. It s assumed that the individual methods always return a valid object. In the case of the for loop, the empty value is an array of length zero, which means no iterations are performed. In the end, simpler methods make it simpler to find errors and then fix them. If an empty object or an object with a set of data cannot be returned, then the application should throw an exception. If it s necessary to indicate an inconsistent state and you don t wish to throw an exception, then you can define a property. You d use the property to test the validity of the state, and the program uses it for further actions. The following example modifies the BetterExampleError Handling class to include a test property:
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CHAPTER 2 LOGGING, ERRORS, AND TEST-DRIVEN DEVELOPMENT
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class BetterExampleErrorHandling { private static readonly string EmptyValue = ""; public static bool IsArrayValid { get { return false; } } public static int[] GetArrayValues() { return new int[ 0]; } public static string GetStringValue() { return EmptyValue; } } The method IsArrayValid as it s implemented always returns false. Calling the property and returning false means that the method GetArrayValues will always return an object of inconsistent state.
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