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Initializing the Struct
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As with any struct, SmartDate can be created with or without calling a constructor. This means a business object could declare SmartDate fields using any of the following: private private private private private private SmartDate SmartDate SmartDate SmartDate SmartDate SmartDate _date1; _date2 = _date3 = _date4 = _date5 = _date6 = new new new new new SmartDate(false); SmartDate(DateTime.Today); SmartDate(DateTime.Today, true); SmartDate("1/1/2005", true); SmartDate("", true);
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In the first two cases, the SmartDate will start out being empty, with empty meaning that it has a value smaller than any other date. The _date3 value will start out containing the current date. It if is set to an empty value later, that empty value will correspond to a value smaller than any other date. The next two values are initialized either to the current date, or a fixed date based on a string value. In both cases, if the SmartDate is set to an empty value later, that empty value will correspond to a value larger than any other date. Finally, _date6 is initialized to an empty date value, where that value is larger than any other date. Handling this initialization is a bit tricky, since the C# compiler requires that all instance fields in a struct be assigned a value in any constructor before any properties or methods can be called. Yet the _date5 and _date6 fields in particular require that a method be called to parse the string value into a date value. Due to this compiler limitation, each constructor sets all instance fields to values (sometimes dummy values) and then calls properties or methods as needed. An additional complication is that a struct can t have a default constructor. Yet even in the previous case of _date1, some initialization is required. This is the purpose of the _initialized instance field. It, of course, defaults to a value of false, and so can be used in the properties of the struct to determine whether the struct has been initialized. As you ll see, this allows SmartDate to initialize itself the first time a property is called; assuming it hasn t been initialized previously.
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CHAPTER 5 s COMPLETING THE FRAMEWORK
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All the constructors follow the same basic flow. Here s one of them: public SmartDate(string value, bool emptyIsMin) { _emptyIsMax = !emptyIsMin; _format = null; _initialized = true; _date = DateTime.MinValue; this.Text = value; } Notice that all the instance fields are assigned values. Even the _date field is assigned a value, though as you ll see, the Text property immediately changes that value based on the value parameter passed into the constructor. This includes translation of an empty string value into the appropriate empty date value. Also look at the emptyIsMin parameter. Remember that SmartDate actually maintains an _emptyIsMax field the exact opposite of the parameter s meaning. This is why the parameter value is negated as _emptyIsMax is assigned. This is a bit awkward, but is necessary to preserve backward compatibility with the SmartDate type from the previous edition of this book and thus previous versions of CSLA .NET.
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s Note This highlights a key design consideration for frameworks in general. Backward compatibility is a key feature of frameworks, since breaking compatibility means going through every bit of code based on the framework to adjust to the change. While sometimes awkward, it is often worth adding extra code to a framework in order to preserve backward compatibility.
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The reason the field is the reverse of the property is that the default value for a SmartDate is that EmptyIsMin is true. Given that you can t initialize fields in a struct, it is simpler to accept the default value for a Boolean, which is false. Hence the use of _emptyIsMax as a field, since if it is false (the default), then EmptyIsMin is true by default.
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