C # Features

An attribute is a class that needs to inherit or indirectly inherit System.Attribute.

1. Common features

　　AttributeUsage: defines the attribute definition to the target element.

　　Flags: The enumeration value is used as a bit marker, not as a numeric value.

        [Flags]        publicenum  Animal        {            0x0001,             0x0002,            0x0004,            0x0008        }         = animal.dog|　　Animal.cat;    WriteLine (animal. ToString ()); // Animal Use the flags attribute, output dog,cat, and do not use the flags     attribute, output 3

　　DllImport: call unmanaged code.

     Public classDllimportdemo {[DllImport ("User32.dll")]         Public Static extern intMessageBox (intHparent,stringMsgstringCaptioninttype);  Public Static intMain () {returnMessageBox (0,"Using Attributes",". NET Attribute",0); }    }    //calledDllimportdemo.main ();

2. Try to write a feature yourself

[AttributeUsage (Attributetargets.property,//Apply to PropertiesAllowMultiple =false,//do not allow multiple applicationsinherited =false)]//do not inherit to a derived class     Public classTrimAttribute:System.Attribute { PublicType type {Get;Set; }  Publictrimattribute (Type type) {type=type; }    }    /// <summary>    ///Trimattribute: Implement extension method Trim ()--Must be static class, static method, first parameter with this decoration/// </summary>     Public Static classTrimattributeext { Public Static voidTrim ( This Objectobj) {Type Tobj=obj.            GetType (); foreach(varPropinchTobj. GetProperties ()) {//GetCustomAttributes (typeof (Trimattribute), false), returns the attribute of the Trimattribute identity                foreach(varattrinchProp. GetCustomAttributes (typeof(Trimattribute),false) {trimattribute tab=(Trimattribute) attr; if(Prop. GetValue (obj)! =NULL&& tab. Type = =typeof(string) {Prop. SetValue (obj, getpropvalue (obj, prop. Name). ToString (). Trim (),NULL); }                }            }        }        Private Static ObjectGetPropValue (ObjectObjstringpropname) {            //invokes the specified member by using the specified binding constraint and matching the specified argument list            returnObj. GetType (). InvokeMember (propname, BindingFlags.GetProperty,NULLObjNew Object[] { }); }    }

Attribute Class Code

     Public classUser { Public intId {Get;Set; } [Trim (typeof(string))]         Public stringUserName {Get;Set; } [Trim (typeof(string))]         Public stringPassword {Get;Set; } } using StaticSystem.Console;//static classes can use the using staticnamespaceconsoletest{classProgram {Static voidMain (string[] args) {            //Dllimportdemo.main ();User User=NewUser {Id =1, UserName ="Admin", Password ="1234" }; User. Trim ();//the line is commented out with a blank spaceWriteLine ("|"+ user. UserName +"|");        ReadKey (); }    }}

Using Code

C # Features