Chapter95:Attributes

Section95.1:Creatingacustomattribute

//1)AllattributesshouldbeinheritedfromSystem.Attribute

//2)Youcancustomizeyourattributeusage(e.g.placerestrictions)byusingSystem.AttributeUsage Attribute

//3)Youcanusethisattributeonlyviareflectioninthewayitissupposedtobeused

//4)MethodMetadataAttributeisjustaname.Youcanuseitwithout”Attribute”postfix-e.g. [MethodMetadata(“Thistextcouldberetrievedviareflection”)].

//5)Youcanoverloadanattributeconstructors [System.AttributeUsage(System.AttributeTargets.Method|System.AttributeTargets.Class)] publicclassMethodMetadataAttribute:System.Attribute

{

//thisiscustomfieldgivenjustforanexample

//youcancreateattributewithoutanyfields

//evenanemptyattributecanbeused-asmarker

publicstringText{get;set;}

//thisconstructorcouldbeusedas[MethodMetadata]

publicMethodMetadataAttribute()

{

}

//Thisconstructorcouldbeusedas[MethodMetadata(“String”)]

publicMethodMetadataAttribute(stringtext)

{

Text=text;

}

}

Section95.2:Readinganattribute

MethodGetCustomAttributesreturnsanarrayofcustomattributesappliedtothemember.Afterretrievingthis arrayyoucansearchforoneormorespecificattributes.

varattribute=typeof(MyClass).GetCustomAttributes().OfType<MyCustomAttribute>().Single();

Oriteratethroughthem

foreach(varattributeintypeof(MyClass).GetCustomAttributes()){ Console.WriteLine(attribute.GetType());

}

GetCustomAttributeextension method from System.Reflection.CustomAttributeExtensionsretrieves a custom attributeofaspecifiedtype,itcanbeappliedtoanyMemberInfo.

varattribute=(MyCustomAttribute)typeof(MyClass).GetCustomAttribute(typeof(MyCustomAttribute));

GetCustomAttributealsohasgenericsignaturetospecifytypeofattributetosearchfor.

varattribute=typeof(MyClass).GetCustomAttribute<MyCustomAttribute>();

Booleanargumentinheritcanbepassedtobothofthosemethods.Ifthisvaluesettotruetheancestorsof element would be also to inspected.

Section95.3:Usinganattribute

[StackDemo(Text=”Hello,World!”)]

publicclassMyClass

{

[StackDemo(“Hello,World!”)]

staticvoidMyMethod()

{

}

}

Section95.4:DebuggerDisplayAttribute

AddingtheDebuggerDisplayAttributewillchangethewaythedebuggerdisplaystheclasswhenitishoveredover.

Expressions that are wrapped in {}will be evaluated by the debugger. This can be a simple property like in the following sample or more complex logic.

[DebuggerDisplay(“{StringProperty}-{IntProperty}”)]

publicclassAnObject

{

publicintObjectId{get;set;}

publicstringStringProperty{get;set;}

publicintIntProperty{get;set;}

}

Adding,nqbeforetheclosingbracketremovesthequoteswhenoutputtingastring.

[DebuggerDisplay(“{StringProperty,nq}-{IntProperty}”)]

Eventhoughgeneralexpressionsareallowedinthe{}theyarenotrecommended.TheDebuggerDisplayattribute willbewrittenintotheassemblymetadataasastring.Expressionsin{}arenotcheckedforvalidity.Soa DebuggerDisplayattributecontainingmorecomplexlogicthani.e.somesimplearithmeticmightworkfineinC#, butthesameexpressionevaluatedinVB.NETwillprobablynotbesyntacticallyvalidandproduceanerrorwhile debugging.

AwaytomakeDebuggerDisplaymorelanguageagnosticistowritetheexpressioninamethodorpropertyandcall it instead.

[DebuggerDisplay(“{DebuggerDisplay(),nq}”)]

publicclassAnObject

{

publicintObjectId{get;set;}

publicstringStringProperty{get;set;}

publicintIntProperty{get;set;}

privatestringDebuggerDisplay()

{

return$”{StringProperty}-{IntProperty}””;

}

}

OnemightwantDebuggerDisplaytooutputallorjustsomeofthepropertiesandwhendebuggingandinspecting also the type of the object.

The example below also surrounds the helper method with #ifDEBUGas DebuggerDisplayis used in debugging environments.

[DebuggerDisplay(“{DebuggerDisplay(),nq}”)]

publicclassAnObject

{

publicintObjectId{get;set;}

publicstringStringProperty{get;set;}

publicintIntProperty{get;set;}

#ifDEBUG

privatestringDebuggerDisplay()

{

return

$”ObjectId:{this.ObjectId},StringProperty:{this.StringProperty}, Type:{this.GetType()}”;

}

#endif

}

Section95.5:Callerinfoattributes

Caller info attributes can be used to pass down information about the invoker to the invoked method. The declaration looks like this:

usingSystem.Runtime.CompilerServices;

publicvoidLogException(Exceptionex,

[CallerMemberName]stringcallerMemberName=””,

[CallerLineNumber]intcallerLineNumber=0,

[CallerFilePath]stringcallerFilePath=””)

{

//performlogging

}

Andtheinvocationlookslike this:

publicvoidSave(DBContextcontext)

{

try

{

context.SaveChanges();

}

catch(Exceptionex)

{

LogException(ex);

}

}

Notice that only the first parameter is passed explicitly to the LogExceptionmethod whereas the rest of them willbe provided at compile time with the relevant values.

ThecallerMemberNameparameterwillreceivethevalue”Save”-thenameofthecallingmethod.

ThecallerLineNumberparameterwillreceivethenumberofwhicheverlinetheLogExceptionmethodcallis written on.

Andthe’callerFilePath’parameterwillreceivethefullpathofthefileSavemethodisdeclaredin.

Section95.6:ObsoleteAttribute

System.Obsolete is an attribute that is used to mark a type or a member that has a better version, and thus should not be used.

[Obsolete(“Thisclassisobsolete.UseSomeOtherClassinstead.”)]classSomeClass

{

//

}

Incasetheclassaboveisused,thecompilerwillgivethewarning”Thisclassisobsolete.UseSomeOtherClass instead.”

Section95.7:Readinganattributefrominterface

Thereisnosimplewaytoobtainattributesfromaninterface,sinceclassesdoesnotinheritattributesfroman interface.Wheneverimplementinganinterfaceoroverridingmembersinaderivedclass,youneedtore-declare theattributes.SointheexamplebelowoutputwouldbeTrueinallthreecases.

usingSystem;

usingSystem.Linq;

usingSystem.Reflection;

namespaceInterfaceAttributesDemo{

[AttributeUsage(AttributeTargets.Interface,Inherited=true

)] classMyCustomAttribute:Attribute{

publicstringText{get;set;}

}

[MyCustomAttribute(Text=”Hellofrominterfaceattribute”)]

interfaceIMyClass{

voidMyMethod();

}

classMyClass:IMyClass{

publicvoidMyMethod() { }

}

publicclassProgram{

publicstaticvoidMain(string[]args){

GetInterfaceAttributeDemo();

}

privatestaticvoidGetInterfaceAttributeDemo(){

varattribute1=(MyCustomAttribute)

typeof(MyClass).GetCustomAttribute(typeof(MyCustomAttribute),true);

Console.WriteLine(attribute1==null);//True

varattribute2=

typeof(MyClass).GetCustomAttributes(true).OfType<MyCustomAttribute>().SingleOrDefault();

Console.WriteLine(attribute2==null);//True

varattribute3=typeof(MyClass).GetCustomAttribute<MyCustomAttribute>(true); Console.WriteLine(attribute3==null);//True

}

}

}

Onewaytoretrieveinterfaceattributesistosearchforthemthroughalltheinterfacesimplementedbyaclass.

varattribute=typeof(MyClass).GetInterfaces().SelectMany(x=>x.GetCustomAttributes(

).OfType<MyCustomAttribute>()).SingleOrDefault();

Console.WriteLine(attribute==null);//False

Console.WriteLine(attribute.Text);//Hellofrominterfaceattribute