. Net: Creation and viewing of custom attributes (named Attributes)

Sometimes we see something like this on a class or method:

          [Obsolete ("Please Update Method")]      

At first , he also gave it a name-small label, for example, usually said: "Your data contract is not labeled Ah!" ~ hehe ~ Later with the code found a lot more do not know the custom label. So, how are these tags defined and used?

define attribute

as follows, a attribute is defined that records class changes, does not allow inheritance, and allows multiple uses.

[AttributeUsage (AttributeTargets.Class, inherited = False, AllowMultiple = true)]//Public   Class Recordattribute: Attribute    {       private string recordtype;//record type: Update/create       private string author;//author       private DateTime date ;//Create/Update date       private string memo;//note       //constructor, the parameters of the constructor are also referred to as "positional parameters" in the attribute. Public       Recordattribute (string recordtype, string author, string date) {           this.recordtype = RecordType;           This.author = author;           This.date = convert.todatetime (date);       }       For positional parameters, typically only get accessor public       string RecordType {get {return recordtype;}}       public string Author {get {return Author;}}       Public DateTime Date {get {return Date;}}       Constructs an attribute, also called the "named parameter" public       string Memo {           get {return Memo;}           in the attribute set {memo = value;}       }    }

When used on a class:

    [Record ("Create", "LHC", "April 25, 2015 15:38:24", memo= "Here is the note information")]    [Record ("Modify", "LHC", "April 25, 2015 15:38:24", Memo = "Here is the note information")]      Public sealed class Democlass:baseclass, Idemointerface, IdemoInterface2 {        private string name;        private string City;        Public  readonly string title;        Public Const string Text = "const field";        public event Demodelegate MyEvent;        public string Name {            private get {return Name;}            set {name = value;}                }        Public DemoClass () {            title = "Readonly field";                        }        public class NestedClass {} public        void Saygreeting (string name)        {            Console.WriteLine ("Morning:" +name) ;        }            }

whether it is a constructor parameter or a property, it is all written into the parentheses of the constructor, and for the parameters of the constructor, the order and type of the constructor arguments must be taken, so called positional arguments; for attributes, the attribute = values, they are separated by commas, called named parameters.

It feels like the initialization information for a class is condensed to a single line ... The compression rate is quite high ah ~

Viewing custom attributes with Reflection

After using attributes, we can read the information inside. The basic idea is to get the same kind of metadata, first use the type, and then, hey, you know ~ to have everything ....

#region View custom attributes using reflection                Type t = typeof (Test.democlass);                Console.WriteLine ("The Recordattributes attribute applied to {0} is listed below:" + t);                Get all Reocordattributes properties                object[] records = T.getcustomattributes (typeof (Test.recordattribute), false);// Gets all Recordattribute label                foreach (Test.recordattribute item in records)                {                    Console.WriteLine ("{0}", item. ToString ());                    Console.WriteLine ("Type: {0}", item.) RecordType);                    Console.WriteLine ("{0}", item. Author);                    Console.WriteLine ("Date: {0}", item.) Date.tostring ());                    if (!string. IsNullOrEmpty (item. Memo)                    {                        Console.WriteLine ("Note: {0}", item.) Memo);                    }                }                            #endregion

. Net: Creation and viewing of custom attributes (named Attributes)