9: Well-Formed Types in C#.NET

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9: Well-Formed Types
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overloaded, and the conditional operators comprise the logical operators, effectively it is possible to overload conditional operators x && y is processed as x & y, where y must evaluate to true Similarly, x || y is processed as x | y only if x is false To enable support for evaluating a type to true or false in an if statement, for example it is necessary to override the true/false unary operators
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Unary Operators (+, , !, ~, ++, , true, false) Overloading unary operators is very similar to overloading binary operators, except that they take only one parameter, also of the containing type Listing 99 overloads the + and operators for Longitude and Latitude and then uses these operators when overloading the same operators in Arc
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Listing 99: Overloading the and + Unary Operators
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public struct Latitude { // public static Latitude operator -(Latitude latitude) { return new Latitude(-latitudeDecimalDegrees); } public static Latitude operator +(Latitude latitude) { return latitude; } }
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public struct Longitude { // public static Longitude operator -(Longitude longitude) { return new Longitude(-longitudeDecimalDegrees); } public static Longitude operator +(Longitude longitude) { return longitude; } }
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public struct Arc { // public static Arc operator -(Arc arc)
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{ // Uses unary operator defined on // Longitude and Latitude return new Arc(-arcLongitudeDifference, -arcLatitudeDifference); } public static Arc operator +(Arc arc) { return arc; } }
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Just as with numeric types, the + operator in this listing doesn t have any effect and is provided for symmetry Overloading true and false has the additional requirement that they both be overloaded The signatures are the same as other operator overloads; however, the return must be a bool, as demonstrated in Listing 910
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Listing 910: Overloading the true and false Operators
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public { // } public { // } static bool operator false(IsValid item) static bool operator true(IsValid item)
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You can use types with overloaded true and false operators in if, do, while, and for controlling expressions
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Conversion Operators Currently, there is no support in Longitude, Latitude, and Coordinate for casting to an alternate type For example, there is no way to cast a double into a Longitude or Latitude instance Similarly, there is no support for assigning a Coordinate using a string Fortunately, C# provides for the definition of methods specifically to handle the casting of one type to another Furthermore, the method declaration allows for specifying whether the cast is implicit or explicit
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9: Well-Formed Types
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ADVANCED TOPIC Cast Operator (()) Implementing the explicit and implicit conversion operators is not technically overloading the cast operator (()) However, this is effectively what takes place, so defining a cast operator is common terminology for implementing explicit or implicit conversion
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Defining a conversion operator is similar in style to defining any other operator, except that the operator is the data type of the conversion Additionally, the operator keyword follows a keyword that indicates whether the conversion is implicit or explicit (see Listing 911)
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Listing 911: Providing an Implicit Conversion between Latitude and double
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public struct Latitude { // public double DecimalDegrees { get { return _DecimalDegrees; } set { _DecimalDegrees = Normalize(value); } } // public static implicit operator double(Latitude latitude) { return latitudeDecimalDegrees; } public static implicit operator Latitude(double degrees) { return new Latitude(degrees); } }
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With these conversion operators, you now can cast doubles implicitly to and from Latitude objects Assuming similar conversions exist for Longitude, you can simplify the creation of a Coordinate object by specifying
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Referencing Other Assemblies
the decimal degrees portion of each coordinate portion (for example, coordinate = new Coordinate(43, 172);)
NOTE
Either the return or the parameter must be of the enclosing type in support of encapsulation C# does not allow you to specify conversions outside the scope of the converted type
Guidelines for Conversion Operators The difference between defining an implicit and an explicit conversion operator centers on preventing an unintentional implicit cast that results in undesirable behavior You should be aware of two possible consequences of using the explicit conversion operator First, conversion operators that throw exceptions should always be explicit For example, it is highly likely that a string will not conform to the appropriate format that a conversion from string to Coordinate requires Given the chance of a failed conversion, you should define the particular conversion operator as explicit, thereby requiring that you be intentional about the conversion and ensure the format is correct, or that you provide code to handle the possible exception Frequently, the pattern for conversion is that one direction (string to Coordinate) is explicit and the reverse (Coordinate to string) is implicit A second consideration is the fact that some conversions will be lossy Converting from a float (42) to an int is entirely valid, assuming an awareness of the fact that the decimal portion of the float will be lost Any conversions that will lose data and not successfully convert back to the original type should be defined as explicit