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The C# Language
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The following program puts together all of the pieces and demonstrates the use of RemarkAttribute:
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// A simple attribute example using System; using SystemReflection; [AttributeUsage(AttributeTargetsAll)] public class RemarkAttribute : Attribute { string pri_remark; // underlies Remark property public RemarkAttribute(string comment) { pri_remark = comment; } public string Remark { get { return pri_remark; } } } [RemarkAttribute("This class uses an attribute")] class UseAttrib { // } class AttribDemo { static void Main() { Type t = typeof(UseAttrib); ConsoleWrite("Attributes in " + tName + ": "); object[] attribs = tGetCustomAttributes(false); foreach(object o in attribs) { ConsoleWriteLine(o); } ConsoleWrite("Remark: "); // Retrieve the RemarkAttribute Type tRemAtt = typeof(RemarkAttribute); RemarkAttribute ra = (RemarkAttribute) AttributeGetCustomAttribute(t, tRemAtt); ConsoleWriteLine(raRemark); } }
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The output from the program is shown here:
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Attributes in UseAttrib: RemarkAttribute Remark: This class uses an attribute
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R u n t i m e Ty p e I D , R e f l e c t i o n , a n d A t t r i b u t e s
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Positional vs Named Parameters
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In the preceding example, RemarkAttribute was initialized by passing the description string to the constructor, using the normal constructor syntax In this case, the comment parameter to RemarkAttribute( ) is called a positional parameter This term relates to the fact that the argument is linked to a parameter by its position in the argument list This is the way that all methods and constructors work in C# For example, given a method called test( ), declared as shown here:
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void test(int a, double b, string c)
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PART I PART I PART I
the following call to test( )
test(10, 11, "hello");
passes 10 to a, 11 to b, and hello to c because of the position (ie, order) of the arguments However, for an attribute, you can also create named parameters, which can be assigned initial values by using their name In this case, it is the name of the parameter, not its position, that is important A named parameter is supported by either a public field or property, which must be read-write and nonstatic Any such field or property is automatically able to be used as a named parameter A named parameter is given a value by an assignment statement that is located within the argument list when the attribute s constructor is invoked Here is the general form of an attribute specification that includes named parameters: [attrib(positional-param-list, named-param1 = value, named-param2 = value, )] The positional parameters (if they exist) come first Next, each named parameter is assigned a value The order of the named parameters is not important Named parameters do not need to be given a value In this case, their default value will be used To understand how to use a named parameter, it is best to work through an example Here is a version of RemarkAttribute that adds a field called Supplement, which can be used to hold a supplemental remark:
[AttributeUsage(AttributeTargetsAll)] public class RemarkAttribute : Attribute { string pri_remark; // underlies Remark property // This can be used as a named parameter: public string Supplement; public RemarkAttribute(string comment) { pri_remark = comment; Supplement = "None"; } public string Remark { get { return pri_remark; } } }
Part I:
The C# Language
As you can see, Supplement is initialized to the string None by the constructor There is no way of using the constructor to assign it a different initial value However, because Supplement is a public field of RemarkAttribute, it can be used as a named parameter, as shown here:
[RemarkAttribute("This class uses an attribute", Supplement = "This is additional info")] class UseAttrib { // }
Pay close attention to the way RemarkAttribute s constructor is called First, the positional argument is specified as it was before Next is a comma, followed by the named parameter, Supplement, which is assigned a value Finally, the closing ) ends the call to the constructor Thus, the named parameter is initialized within the call to the constructor This syntax can be generalized Position parameters must be specified in the order in which they appear Named parameters are specified by assigning values to their name Here is a program that demonstrates the Supplement field:
// Use a named attribute parameter using System; using SystemReflection; [AttributeUsage(AttributeTargetsAll)] public class RemarkAttribute : Attribute { string pri_remark; // underlies Remark property public string Supplement; // this is a named parameter public RemarkAttribute(string comment) { pri_remark = comment; Supplement = "None"; } public string Remark { get { return pri_remark; } } } [RemarkAttribute("This class uses an attribute", Supplement = "This is additional info")] class UseAttrib { // } class NamedParamDemo { static void Main() { Type t = typeof(UseAttrib); ConsoleWrite("Attributes in " + tName + ": ");
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