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16: Custom Attributes
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In this chapter, I ll discuss one of the most innovative features that the Microsoft .NET Framework has to offer: custom attributes. Custom attributes allow anyone (not just Microsoft) to define information that can be applied to almost any metadata table entry. This extensible metadata information can be queried at run time to dynamically alter the way code executes. As you use the various .NET Framework technologies (Windows Forms, Web Forms, XML Web services, and so on), you ll see that they all take advantage of custom attributes, allowing developers to express their intentions in code very easily. A solid understanding of custom attributes is necessary for any .NET Framework developer.
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Attributes, such as public, private, static, and so on, can be applied to types and members. I think we d all agree on the usefulness of applying attributes, but wouldn t it be even more useful if we could define our own attributes For example, what if I could define a type and somehow indicate that the type can be remoted via serialization Or maybe I could apply an attribute to a method to indicate that certain security permissions must be granted before the method can execute. Of course, creating and applying user defined attributes to types and methods would be great and convenient, but it would require the compiler to be aware of these attributes so that it would emit the attribute information into the resulting metadata. Because compiler vendors usually prefer not to release the source code for their compiler, Microsoft came up with another way to allow user defined attributes. This mechanism, called custom attributes, is an incredibly powerful mechanism that s useful at both application design time and run time. Anyone can define and use custom attributes, and all compilers that target the common language runtime (CLR) must be designed to recognize custom attributes and emit them into the resulting metadata. The first thing you should realize about custom attributes is that they re just a way to associate additional information with a target. The compiler emits this additional information into the managed module s metadata. Most attributes have no meaning for the compiler; the compiler simply detects the attributes in the source code and emits the corresponding metadata. The .NET Framework Class Library (FCL) ships with many predefined attributes. For example, the System.FlagsAttribute attribute causes an enumerated type to act like a set of bit flags, the System.SerializableAttribute attribute allows a type s fields to be serialized and deserialized (typically used for remoting of a method s arguments and return value), several security related attributes enable a method to ensure that it has a required privilege granted before attempting some particular kind of access, lots of interoperability related attributes serve to allow managed code to call unmanaged code, and so on. Following is some C# code with many attributes applied to it. It s not important to understand what this code does. I just want you to see what attributes look like.
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[StructLayout(LayoutKind.Sequential, CharSet=CharSet.Auto)] class OSVERSIONINFO { public OSVERSIONINFO() { OSVersionInfoSize = (UInt32) Marshal.SizeOf(this); } public UInt32 OSVersionInfoSize = 0; public UInt32 MajorVersion = 0; public UInt32 MinorVersion = 0;
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public UInt32 BuildNumber public Uint32 PlatformId
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= 0; = 0;
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[MarshalAs(UnmanagedType.ByValTStr, SizeConst=128)] public String CSDVersion = null; } class MyClass { [DllImport("Kernel32", CharSet=CharSet.Auto, SetLastError=true)] public static extern Boolean GetVersionEx( [In, Out] OSVERSIONINFO ver); }
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In C#, you apply a custom attribute to a target by placing the attribute in square brackets immediately before the target. So, in this case, the StructLayout attribute is applied to the OSVERSIONINFO class, the MarshalAs attribute is applied to the CSDVersion field, the DllImport attribute is applied to the GetVersionEx method, and the In and Out attributes are applied to GetVersionEx s ver parameter. Every programming language defines the syntax that a developer must use in order to apply a custom attribute to a target. Microsoft Visual Basic, for example, requires angle brackets (<, >) instead of square brackets. The CLR allows attributes to be applied to just about anything that can be represented in a file s metadata. Most commonly, attributes are applied to entries in the following definition tables: TypeDef (class, struct, enum, interfaces, and delegates), MethodDef (including constructors), ParamDef, FieldDef, PropertyDef, EventDef, AssemblyDef, and ModuleDef. Although it s rare, attributes can also be applied to entries in reference tables, such as AssemblyRef, ModuleRef, TypeRef, and MemberRef. Finally, custom attributes can be applied to other pieces of metadata such as security permissions, exported types, and resources. Although the CLR allows custom attributes to be applied to any of these entities, most programming languages allow attributes to be applied only to entries in the various definition metadata tables. This is certainly true of Microsoft s C# compiler. Specifically, C# allows you to apply an attribute only to source code that defines any of the following targets: assembly, module, type, field, method, method parameter, method return value, property, and event. When you re applying an attribute, C# allows you to specify a prefix specifically indicating what target the attribute applies to. The following code shows all the possible prefixes. In many cases, if you leave out the prefix, the compiler can still determine what target an attribute applies to, as shown in the previous example. However, explicitly specifying a prefix removes all ambiguity.
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using System; [assembly: MyAttribute(1)] [module: MyAttribute(2)] [type: MyAttribute(3)] class SomeType { // Applied to the assembly // Applied to the module // Applied to the type
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[property: MyAttribute(4)] // Applied to the property public String SomeProp { get { return null; } } [event: MyAttribute(5)] // Applied to the event public event EventHandler SomeEvent; [field: MyAttribute(6)] // Applied to the field public Int32 SomeField = 0; [return: MyAttribute(7)] // Applied to the return value
[method: MyAttribute(8)] // Applied to the method public Int32 SomeMethod( [param: MyAttribute(9)] // Applied to the parameter Int32 SomeParam) { return SomeParam; } }
Now that you know how to apply a custom attribute, let s find out what an attribute really is. A custom attribute is simply an instance of a type. For Common Language Specification (CLS) compliance, custom attribute types must be derived, directly or indirectly, from System.Attribute. C# allows only CLS compliant attributes. By examining the .NET Framework SDK documentation, you ll see that the following types (from the earlier example) are defined: StructLayoutAttribute, MarshalAsAttribute, DllImportAttribute, InAttribute, and OutAttribute. All these types happen to be defined in the System.Runtime.InteropService namespace, but attribute types can be defined in any namespace. Upon further examination, you ll notice that all these types are derived from System.Attribute, as all CLS compliant attribute types must be. As I mentioned, an attribute is an instance of a type. The type must have a public constructor to create an instance of it. So, when you apply an attribute to a target, the syntax is similar to that for calling one of the type s instance constructors. In addition, a language might permit some special syntax that allows you to set any public fields or properties associated with the attribute type. Let s look at an example. Recall the application of the DllImport attribute as it was applied to the GetVersionEx method earlier:
[DllImport("Kernel32", CharSet=CharSet.Auto, SetLastError=true)]
The syntax of this line should look pretty strange to you because you could never use syntax like this when calling a constructor. If you examine the DllImportAttribute type in the documentation, you ll see that its constructor requires a single String parameter. In this example, "Kernel32" is being passed for this parameter. A constructor s parameters are called positional parameters and are mandatory: the parameter must be specified when the attribute is applied. What are the other two parameters This special syntax allows you to set any public fields or properties of the DllImportAttribute object after the object is constructed. In this example, when the DllImportAttribute object is constructed and "Kernel32" is passed to the constructor, the object s public instance fields, CharSet and SetLastError, are set to CharSet.Auto and true, respectively. The "parameters" that set fields or properties are called named parameters and are optional: the parameters don t have to be specified when you re applying an instance of the attribute. A little later on, I ll explain what causes an instance of the DllImportAttribute type to actually be constructed. Also note that it s possible to apply multiple attributes to a single target. For example, the GetVersionEx method s ver parameter has both the In and Out attributes applied to it. When applying multiple attributes to a single target, be aware that the order of attributes has no significance. Also, in C# each attribute can be enclosed in square brackets or multiple attributes can be comma separated within a single set of square brackets. The Attribute suffix is optional, and if the attribute type s constructor takes no parameters, the parentheses are optional. The following lines behave identically and demonstrate all the possible ways of applying multiple attributes:
[Serializable][Flags] [Serializable, Flags] [FlagsAttribute, SerializableAttribute] [FlagsAttribute()][Serializable()]
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