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Some Useful Attributes
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The .NET Framework contains many attributes. Let s look at a few of them.
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The Obsolete Attribute
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The Obsolete attribute is one of the simplest of attributes. Try compiling the code in Listing 10-9. Listing 10-9. Using the Obsolete Attribute // obsolete.cpp using namespace System; ref class C { public: void Method2() {} [Obsolete("This method is obsolete; use Method2 instead.")] void Method1() {} };
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CHAPTER 10 EXCEPTIONS, ATTRIBUTES, AND REFLECTION
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int main() { C^ c = gcnew C(); c->Method1(); c->Method2(); } If you compile this, you should get output similar to the following: obsolete.cpp(16) : warning C4947: 'C::Method1' : marked as obsolete Message: 'This method is obsolete; use Method2 instead.' As you can see, attributes can be used to give a message to anyone who uses a class or method.
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The Out Attribute
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The Out attribute is useful when interoperating with other .NET languages, especially C#. It allows you to specify that a parameter is an out-only parameter, which to C# users means that its input value is not used (so it can be uninitialized). You must use a tracking reference as a parameter when using the OutAttribute (see Listing 10-10). Compiling with /clr:pure or /clr:safe makes interoperating with other .NET languages much easier, as you ll see in 12. Listing 10-10. Using the Out Attribute // outattribute.cpp // compile with: /clr:safe or /clr:pure using namespace System; using namespace System::Runtime::InteropServices; namespace OutAttrClass { public ref class C1 { public: void Func([Out] String^% text) { text = "testing"; } }; }
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CHAPTER 10 EXCEPTIONS, ATTRIBUTES, AND REFLECTION
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// consume_outattr.cs // compile with: csc /r:outattribute.dll consume_outattr.cs using System; using OutAttrClass; public class C { public static void Main() { C1 c1 = new C1(); String str = "old value"; c1.Func(out str); Console.WriteLine(str); } }; The output of Listing 10-10 is as follows:
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testing
Serialization Attributes
Some .NET Framework features rely heavily on attributes. For example, serialization of types as XML is primarily supported through applying certain attributes to classes and fields. The Serializable attribute, applied to a type, enables that type to be serialized as an XML stream. It can then be stored and re-created later by reading the XML stream and reconstructing the class. The NonSerialized attribute is used within a serializable class to identify a field that doesn t participate in serialization. Both attributes are used in Listing 10-11. Listing 10-11. Using Serialization Attributes // serialization.cpp #using "System.Xml.dll" using namespace System; using namespace System::IO; using namespace System::Xml::Serialization; // To participate in serialization, types must be public. [Serializable] public enum class SunEnum { FullSun, PartShade, Shade };
CHAPTER 10 EXCEPTIONS, ATTRIBUTES, AND REFLECTION
[Serializable] public enum class WaterEnum { Moist, Medium, Dry }; [ Serializable] public ref class Plant { // an internal counter to determine instance IDs static int counter;
// The instance ID keeps track of the plant objects. It will be // a different ID when the object is deserialized, so this does not need // to be serialized. We use the NonSerialized attribute to indicate that. [NonSerialized] int InstanceID; public: property property property property property property property
String^ Genus; String^ Species; String^ Cultivar; String^ CommonName; SunEnum Sun; WaterEnum Water; int Zone;
Plant() {} Plant(String^ genus, String^ species, String^ commonName, String^ cultivar, SunEnum sun, WaterEnum water, int zone) { Genus = genus; Species = species; Cultivar = cultivar; Sun = sun; Water = water; Zone = zone; InstanceID = counter++; } static Plant() { counter = 0; } }; void CreateAndSerialize(String^ genus, String^ species, String^ commonName, String^ cultivar, SunEnum sun, WaterEnum water, int zone) { Plant^ p = gcnew Plant(genus, species, commonName, cultivar, sun, water, zone);
CHAPTER 10 EXCEPTIONS, ATTRIBUTES, AND REFLECTION
// The XmlSerializer takes the Type object as a parameter. XmlSerializer^ serializer = gcnew XmlSerializer(Plant::typeid); // Create a StreamWriter object to write to a file. StreamWriter^ sw = gcnew StreamWriter("plants.xml"); // Serialize causes the XML to be generated. serializer->Serialize(sw, p); sw->Close(); } Plant^ Deserialize() { Plant^ p; XmlSerializer^ serializer = gcnew XmlSerializer(Plant::typeid); // To read the file, use a FileStream object. FileStream^ fs = gcnew FileStream("plants.xml", FileMode::Open); // Deserialize and cast to object type. p = safe_cast<Plant^>( serializer->Deserialize(fs) ); return p; } int main() { CreateAndSerialize("Ampelopsis", "brevipedunculata", "Porcelain Berry", nullptr, SunEnum::PartShade, WaterEnum::Medium, 4); Deserialize(); } Here is the plants.xml file Listing 10-11 produces: < xml version="1.0" encoding="utf-8" > <Plant xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://w ww.w3.org/2001/XMLSchema"> <Zone>4</Zone> <Water>Medium</Water> <Sun>PartShade</Sun> <Species>brevipedunculata</Species> <Genus>Ampelopsis</Genus> </Plant> This example demonstrates serialization and deserialization of a simple class. The attributes are very simple, and because they take no arguments, the parentheses may be omitted. Only the presence or absence of the attribute makes a difference; there is no internal structure to these, the simplest of attributes.
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