CSharp Advanced L05-Attributes+Reflection

Mohammad Shaker

mohammadshaker.com

@ZGTRShaker

2011, 2012, 2013, 2014

C# AdvancedL05-Attributes, Conditional Methods and

Reflection

Attributes

AttributesMuch of the C# language enables the programmer to specify declarative information about the entities defined

in the program. For example, the accessibility of a method in a class is specified by decorating it with the method-modifiers public, protected, internal, and private. C# enables programmers to invent new kinds of declarative information, called attributes. Programmers can then attach attributes to various program

entities, and retrieve attribute information in a run-time environment. For instance, a framework might define a HelpAttribute attribute that can be placed on certain program elements (such as classes and methods) to

provide a mapping from those program elements to their documentation. (from MSDN)

"DLL Hell“ problem


• can be caused by one application overwriting a shared library of another

application, usually with a different version.

• Imagine you write an application with a shared library and then years later create

a new application with an updated library.

• In order not to break old programs, what do you do with the new library? You

give it a different name, store it in a different location, or overwrite the old ones?


• can be caused by one application overwriting a shared library of another

application, usually with a different version.

• Imagine you write an application with a shared library and then years later create

a new application with an updated library.

• In order not to break old programs, what do you do with the new library? You

give it a different name, store it in a different location, or overwrite the old ones?

Assemblies

Assemblies

• What’s an assembly?

– An assembly is a file that is automatically generated by the compiler upon successful

compilation of every.NET application. It can be either a Dynamic Link Library or an executable

file. It is generated only once for an application and upon each subsequent compilation the

assembly gets updated.

– contains the intermediate code, resources, and the metadata for itself.

– We can have a look inside the assembly using the ildasm (Intermediate Language

Disassembler) tool that comes as part of Visual Studio.

– To access it you need to open the Visual Studio Command Prompt and type ildasm.exe. This

will launch a Windows application that you can use to explore any.Net application.

Attributes and Reflection

• Attributes

– Attributes provide a powerful method of associating declarative information with C# code

(types, methods, properties, and so forth).

– Once associated with a program entity, the attribute can be queried at run time and used in

any number of ways.

Attributes and Reflection

• Example usage of attributes includes:

– Associating help documentation with program entities (through a Help attribute).

– Associating value editors to a specific type in a GUI framework (through

a ValueEditor attribute).

Example at the start!

Example at the start!If you understand this, it’s over

Example at the start

enum CarColour

{

[Description("Ice Cream White")]

White,

[Description("Carbon Black")]

Black,

[Description("Sterling Silver")]

Silver,

Red

}


enum CarColour

{


White,


Black,


Silver,

Red

}

Attributes


public static class EnumExtensions{

public static string Description(this Enum enumValue){

var enumType = enumValue.GetType();var field = enumType.GetField(enumValue.ToString());var attributes = field.GetCustomAttributes(typeof(DescriptionAttribute), false);return ((DescriptionAttribute)attributes[0]).Description;

}}

enum CarColour

{


White,


Black,


Silver,

Red

}





}}

enum CarColour

{


White,


Black,


Silver,

Red

}

Extension Method





}}

enum CarColour

{


White,


Black,


Silver,

Red

}

Reflection





}}

enum CarColour

{


White,


Black,


Silver,

Red

}





}}

enum CarColour

{


White,


Black,


Silver,

Red

}





}}

for (CarColour cc = CarColour.White; cc <= CarColour.Red; cc++){

Console.WriteLine(cc.Description());}

enum CarColour

{


White,


Black,


Silver,

Red

}





}}

for (CarColour cc = CarColour.White; cc <= CarColour.Red; cc++){

Console.WriteLine(cc.Description());}

enum CarColour

{


White,


Black,


Silver,

Red

}

Ice Cream WhiteCarbon BlackSterling SilverRed

Concept of Attributes

Attributes Basics

• Attributes are generally applied physically in front of type and type member

declarations. They're declared with square brackets, "[" and "]", surrounding the

attribute such as the followingObsoleteAttribute attribute:

• [ObsoleteAttribute]

AttributeUsage Describes how a custom attribute class can be used.

Conditional Marks a conditional method, a method whose execution depends on a specified preprocessing identifier.

Obsolete Marks a program entity that should not be used.

http://msdn.microsoft.com/en-us/library/tw5zxet9(v=vs.71).aspx

http://msdn.microsoft.com/en-us/library/4xssyw96(v=vs.71).aspx

http://msdn.microsoft.com/en-us/library/22kk2b44(v=vs.71).aspx

Attribute, Obsolete

http://msdn.microsoft.com/en-us/library/22kk2b44(v=vs.71).aspx

Pre-Defined Attributes using System;

class BasicAttributeDemo{

[Obsolete]public void MyFirstdeprecatedMethod(){

Console.WriteLine("Called MyFirstdeprecatedMethod().");}

[ObsoleteAttribute]public void MySecondDeprecatedMethod(){

Console.WriteLine("Called MySecondDeprecatedMethod().");}

[Obsolete("You shouldn't use this method anymore.")]public void MyThirdDeprecatedMethod(){

Console.WriteLine("Called MyThirdDeprecatedMethod().");}

























Pre-Defined Attributes // make the program thread safe for COM[STAThread]static void Main(string[] args){

BasicAttributeDemo attrDemo = new BasicAttributeDemo();

attrDemo.MyFirstdeprecatedMethod();attrDemo.MySecondDeprecatedMethod();attrDemo.MyThirdDeprecatedMethod();

}}

Pre-Defined Attributes // make the program thread safe for COM[STAThread]static void Main(string[] args){

BasicAttributeDemo attrDemo = new BasicAttributeDemo();

attrDemo.MyFirstdeprecatedMethod();attrDemo.MySecondDeprecatedMethod();attrDemo.MyThirdDeprecatedMethod();

}}

Attribute, AttributeUsage

http://msdn.microsoft.com/en-us/library/tw5zxet9(v=vs.71).aspx

AttributeUsageDescribes how a custom attribute class can be used.

[AttributeUsage(validon,AllowMultiple=allowmultiple,Inherited=inherited

)]

User-Defined Attributes

[Author("H. Ackerman", version = 1.1)] class SampleClass



Author anonymousAuthorObject = new Author("H. Ackerman"); anonymousAuthorObject.version = 1.1;



Author anonymousAuthorObject = new Author("H. Ackerman"); anonymousAuthorObject.version = 1.1;

conceptually equivalent

Attribute, Conditional

http://msdn.microsoft.com/en-us/library/4xssyw96(v=vs.71).aspx

Conditional Methodsallow developers to create methods whose calls can be placed in the code and then either

included or omitted during compilation based on a preprocessing symbol.

Conditional Methods

• The Conditional attribute takes one parameter — the preprocessing

identifier that controls whether the method call is included when clients are

compiled. If the preprocessing identifier is defined, the method is called;

otherwise, the call is never inserted in the client's code.

using System; using System.Diagnostics;namespace TraceFunctions{

public class Trace {

[Conditional("DEBUG")] public static void Message(string traceMessage) {

Console.WriteLine("[TRACE] - " + traceMessage); }

} }

[Serializable]

[Serializable]Says that the object is Serializable

[Serializable]Says that the object is Serializable

So you can Stream it and save the object into, like, a file!

Reflection

ReflectionAsk the object itself about itself

Types

Types and Reflection

• Reflection

– Provides objects (of type Type) that encapsulate assemblies, modules and types.

– You can use reflection to dynamically create an instance of a type, bind the type to an existing

object, or get the type from an existing object and invoke its methods or access its fields and

properties.

– If you are using attributes in your code, Reflection enables you to access them.

http://msdn.microsoft.com/en-us/library/system.type(v=vs.80).aspx


• Reflection is useful in the following situations:

– When you need to access attributes in your program's metadata.

• “Accessing Attributes With Reflection”

– For examining and instantiating types in an assembly.

– For building new types at runtime. Use classes in System.Reflection.Emit

– For performing late binding, accessing methods on types created at run time.


• Reflection is useful in the following situations:

– When you need to access attributes in your program's metadata.

• “Accessing Attributes With Reflection”

– For examining and instantiating types in an assembly.

– For building new types at runtime. Use classes in System.Reflection.Emit

– For performing late binding, accessing methods on types created at run time.

System.Type class

System.Type class

• The System.Type class is central to reflection.

• The common language runtime creates the Type for a loaded type when

reflection requests it. You can use a Type object's methods, fields, properties, and

nested classes to find out everything about that type.

http://msdn.microsoft.com/en-us/library/system.type(v=vs.80).aspx

System.Type class

• Simple example of Reflection using the static method GetType - inherited by all

types from the Object base class - to obtain the type of a variable:

// Using GetType to obtain type information:int i = 42;System.Type type = i.GetType();System.Console.WriteLine(type);

System.Type class

• Simple example of Reflection using the static method GetType - inherited by all

types from the Object base class - to obtain the type of a variable:

// Using GetType to obtain type information:int i = 42;System.Type type = i.GetType();System.Console.WriteLine(type);

System.Int32


• Identifying all Types that Implement an Interface (http://www.blackwasp.co.uk/)public interface IParent { }public class Parent { }public class Child1 : Parent { }public class Child2 : Parent { }public class Grandchild11 : Child1 { }public class Grandchild12 : Child1 { }public class Grandchild21 : Child2 { }public class Grandchild22 : Child2 { }public class NotAChild { }public class NotAGrandchild : NotAChild { }

Type parentType = typeof(IParent);Assembly assembly = Assembly.GetExecutingAssembly();Type[] types = assembly.GetTypes();


• Identifying all Types that Implement an Interfacepublic interface IParent { }public class Parent { }public class Child1 : Parent { }public class Child2 : Parent { }public class Grandchild11 : Child1 { }public class Grandchild12 : Child1 { }public class Grandchild21 : Child2 { }public class Grandchild22 : Child2 { }public class NotAChild { }public class NotAGrandchild : NotAChild { }

IEnumerable<Type> imp = types.Where(t => t.GetInterfaces().Contains(interfaceType));

foreach (Type type in imp){

Console.WriteLine(type.Name);}

/* OUTPUT ParentChild1Child2Grandchild11Grandchild12Grandchild21Grandchild22 */



• Identifying all Subclasses of a Class (http://www.blackwasp.co.uk/)public interface IParent { }public class Parent { }public class Child1 : Parent { }public class Child2 : Parent { }public class Grandchild11 : Child1 { }public class Grandchild12 : Child1 { }public class Grandchild21 : Child2 { }public class Grandchild22 : Child2 { }public class NotAChild { }public class NotAGrandchild : NotAChild { }



• Identifying all Subclasses of a Classpublic interface IParent { }public class Parent { }public class Child1 : Parent { }public class Child2 : Parent { }public class Grandchild11 : Child1 { }public class Grandchild12 : Child1 { }public class Grandchild21 : Child2 { }public class Grandchild22 : Child2 { }public class NotAChild { }public class NotAGrandchild : NotAChild { }

IEnumerable<Type> subclasses = types.Where(t => t.IsSubclassOf(parentType));

foreach (Type type in subclasses){

Console.WriteLine(type.Name);}

/* OUTPUT Child1Child2Grandchild11Grandchild12Grandchild21Grandchild22 */


Test CaseGetting Constructors of String Class

Types and Reflection// This program lists all the public constructors // of the System.String class.using System;using System.Reflection;class ListMembers{

public static void Main(String[] args){

Type t = typeof(System.String);Console.WriteLine("Listing all the public constructors of the {0} type", t);// Constructors.ConstructorInfo[] ci = t.GetConstructors(BindingFlags.Public | BindingFlags.Instance);Console.WriteLine("//Constructors");PrintMembers(ci);

}public static void PrintMembers(MemberInfo[] ms){

foreach (MemberInfo m in ms){

Console.WriteLine("{0}{1}", " ", m);}Console.WriteLine();

}}

Types and Reflection// This program lists all the public constructors // of the System.String class.using System;using System.Reflection;class ListMembers{

public static void Main(String[] args){

Type t = typeof(System.String);Console.WriteLine("Listing all the public constructors of the {0} type", t);// Constructors.ConstructorInfo[] ci = t.GetConstructors(BindingFlags.Public | BindingFlags.Instance);Console.WriteLine("//Constructors");PrintMembers(ci);

}public static void PrintMembers(MemberInfo[] ms){

foreach (MemberInfo m in ms){

Console.WriteLine("{0}{1}", " ", m);}Console.WriteLine();

}}

Listing all the public constructors of the System.String type//Constructors

Void.ctor(Char*)Void.ctor(Char*, Int32, Int32)Void.ctor(SByte*)Void.ctor(SByte*, Int32, Int32)Void.ctor(SByte*, Int32, Int32, System.Text.Encoding)Void.ctor(Char[], Int32, Int32)Void.ctor(Char[])Void.ctor(Char, Int32)

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Invoking Overloaded Methods Using Reflectionhttp://www.blackwasp.co.uk/

Invoking Overloaded Methods Using Reflectionpublic class Test{

public void ShowMessage(){

Console.WriteLine("Hello, world");}

public void ShowMessage(string msg, int timesToShow){

for (int i = 0; i < timesToShow; i++){

Console.WriteLine(msg);}

}}







}}

Test test = new Test();Type type = test.GetType();MethodInfo method = type.GetMethod("ShowMessage", new Type[] { typeof(string), typeof(int) });method.Invoke(test, new object[] { "Exterminate!", 3 });







}}








}}


Exterminate!Exterminate!Exterminate!

Accessing Attributes Through Reflection

Accessing Attributes Through ReflectionYou don’t have to read..


class MainClass{

public static void Main(){

System.Reflection.MemberInfo info = typeof(MyClass);object[] attributes = info.GetCustomAttributes(true);for (int i = 0; i < attributes.Length; i++){

System.Console.WriteLine(attributes[i]);}

}}


class MainClass{

public static void Main(){

System.Reflection.MemberInfo info = typeof(MyClass);object[] attributes = info.GetCustomAttributes(true);for (int i = 0; i < attributes.Length; i++){

System.Console.WriteLine(attributes[i]);}

}}


• Let’s have the following giant example!

public class IsTestedAttribute : Attribute{

public override string ToString(){

return "Is Tested";}

}

Concept of Attributes[AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct)]public class AuthorAttribute : Attribute{

public AuthorAttribute(string name){

this.name = name;this.version = 0;

}public string Name{ get

{return name;

}}public int Version{

get{

return version;}set{

version = value;}

}public override string ToString(){

string value = "Author : " + Name;if (version!= 0){

value += " Version : " + Version.ToString();}return value;

}private string name;private int version;

}





{return name;


get{


version = value;}





}





{return name;


get{


version = value;}





}





{return name;


get{


version = value;}





}


[Author("Joe Programmer")]class Account{

// Attach the IsTestedAttribute custom attribute to this method.[IsTested]public void AddOrder(Order orderToAdd){

orders.Add(orderToAdd);}

private ArrayList orders = new ArrayList();}

[Author("Jane Programmer", Version = 2), IsTested()]class Order{

// add stuff here...}















Accessing Attributes Through Reflectionclass MainClass{

private static bool IsMemberTested(MemberInfo member){

foreach (object attribute in member.GetCustomAttributes(true)){

if (attribute is IsTestedAttribute){

return true;}

}return false;

}

private static void DumpAttributes(MemberInfo member){

Console.WriteLine("Attributes for : " + member.Name);foreach (object attribute in member.GetCustomAttributes(true)){

Console.WriteLine(attribute);}

}

Concept of Attributespublic static void main(){ DumpAttributes(typeof(Account));

foreach (MethodInfo method in (typeof(Account)).GetMethods()){

if (IsMemberTested(method)){

Console.WriteLine("Member {0} is tested!", method.Name);}else{

Console.WriteLine("Member {0} is NOT tested!", method.Name);}

}Console.WriteLine();

// display attributes for Order classDumpAttributes(typeof(Order));

// display attributes for methods on the Order classforeach (MethodInfo method in (typeof(Order)).GetMethods()){




}Console.WriteLine(); } }


























Attributes for : AccountAuthor : Joe ProgrammerMember AddOrder is tested!Member ToString is NOT tested!Member Equals is NOT tested!Member GetHashCode is NOT tested!Member GetType is NOT tested!

Attributes for : OrderAuthor : Jane Programmer Version : 2Is TestedMember ToString is NOT tested!Member Equals is NOT tested!Member GetHashCode is NOT tested!Member GetType is NOT tested!

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https://www.goodreads.com/user/show/11193121-mohammad-shaker

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https://www.youtube.com/channel/UCvJUfadMoEaZNWdagdMyCRA

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CSharp Advanced L05-Attributes+Reflection

Software

new application

descriptioncarbon blackblack

windows application

new library

descr cream whitewhite

updated library

shared library

different location