Reflection in .Net

1
Reflection in .Net

• Siun-Wai Seow

2
Objective

• Explain .NET Reflection
• When to use it?
• How to use it?
• Topic is in the final exam

3
Overview

• Reflection
• .Net Execution Model
• Show how applications can use Reflection to
  explore type information and how they can build
  types at runtime
• Demo for each sub-topic

4
What is Reflection

• A way to acquire important information at run
  time about the assemblies it loads.

5
Uses of Reflection

• Viewing metadata (Attributes)
• Performing type discovery
• Late binding to methods and properties
• Dynamic Loading/Creating types at runtime
  (Reflection Emit)
• How is it done?

6
.Net Execution Model
VB
Cobol
C
C
.NET languages
Language compilers
CIL code ( metadata)
Loader/verifier
JIT compiler
Uncompiled method call
Managed Code
Execution
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Metadata

• Metadata
• Single location for type information and code
• Types' metadata can be explored with Reflection
• Code is literally contained within type
  information
• Every .NET object can be queried for its type

8
Uses of Metadata

• Dynamic Type System
• Allows on-the-fly creation of assemblies
• .NET Compilers use .NET to emit .NET code

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MetaData Type Info at Runtime
serializablepublic class Person
public event OnSaveChange onsv public Date
DOB public string FirstName public
string LastName public string Name
get return FirstName " "
LastName public
Person(string First,string Last)
FirstNameFirstLastNameLast public
bool Save() System.Type t
this.GetType() foreach( FieldInfo f in
t.GetFields() ) ...
System.Type
Attributes
Events
Fields
Properties
Constructors
Parameters
Methods
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Viewing metadata

• Add custom attributes to a compiled executables
  metadata
• Why/When to use this?

11
Scenario

• Suppose that your organization wants to keep
  track of bug fixes. You already keep a database
  of all your bugs, but you'd like to tie your bug
  reports to the fixes in the code.
• How do implement this?

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• BugFixAttribute(121, "Jesse Liberty",
  "01/03/05")
• BugFixAttribute(107, "Jesse Liberty",
  "01/04/05", Comment "Fixed off by one errors")
• public class MyMath
• AttributeUsage(AttributeTargets.Class
• AttributeTargets.Cons
  tructor
• AttributeTargets.Fiel
  d
• AttributeTargets.Meth
  od
• AttributeTargets.Prop
  erty,
• AllowMultiple
  true)
• public class BugFixAttribute
  System.Attribute

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• public static void Main()
• ..
• object attributes
• attributes inf.GetCustomAttributes(
  typeof(BugFixAttribute), false)
• // iterate through the attributes,
  retrieving the
• // properties
• foreach (Object attribute in
  attributes)
• BugFixAttribute bfa
  (BugFixAttribute)attribute
• Console.WriteLine("\nBugID 0",
  bfa.BugID)
• Console.WriteLine("Programmer
  0", bfa.Programmer)
• Console.WriteLine("Date 0",
  bfa.Date)
• Console.WriteLine("Comment 0",
  bfa.Comment)

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Demo1

• demo1.doc
• Project Programming_Csharp

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Performing type discovery

• This allows you to examine the types in an
  assembly.
• Project Programming_Csharp2

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Reflecting on an assembly

• using System
• using System.Reflection
• public class Tester
• //Reflecting on an assembly
• public static void Main()
• // what is in the assembly
• Assembly a Assembly.Load("Mscorlib.d
  ll")
• Type types a.GetTypes()
• foreach (Type t in types)
• Console.WriteLine("Type is 0",
  t)
• Console.WriteLine(
• "0 types found", types.Length)

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Reflecting on a Type

• public static void Main()
• // examine a single object
• Type theType
• Type.GetType(
• "System.Reflection.Assembly")
• Console.WriteLine(
• "\nSingle Type is 0\n",
  theType)

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Reflecting on the members of a type

• public static void Main()
• // examine a single object
• Type theType
• Type.GetType(
• "System.Reflection.Assembly")
• Console.WriteLine(
• "\nSingle Type is 0\n",
  theType)
• // get all the members
• MemberInfo mbrInfoArray
• theType.GetMembers()
• foreach (MemberInfo mbrInfo in
  mbrInfoArray)
• Console.WriteLine("0 is a 1",
• mbrInfo, mbrInfo.MemberType)

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Finding type methods

• public static void Main()
• // examine a single object
• Type theType
• Type.GetType(
• "System.Reflection.Assembly")
• Console.WriteLine(
• "\nSingle Type is 0\n",
  theType)
• MemberInfo mbrInfoArray
  theType.GetMethods()
• foreach (MemberInfo mbrInfo in
  mbrInfoArray)
• Console.WriteLine("0 is a 1",
• mbrInfo, mbrInfo.MemberType)

20
Late binding to methods and properties

• Performing late binding by dynamically
  instantiating and invoking methods on types.
• Done in homework 3 (application domain/stack
  walk/permission).

21
Creating types at runtime

• The ultimate use of reflection is to create new
  objects/types at runtime and then to use those
  objects to perform tasks. You might do this when
  a new class, created at runtime, will run
  significantly faster than more generic code
  created at compile time.

22
Static Method

• Wrote before runtime (before compilation)
• public int DoSumLooping(int initialVal)
• int result 0
• for(int i 1i ltinitialVali)
• result i
• return result
• Summation of N 123456..N

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Dynamic Method created at run time

• // Push zero onto the stack. For each
  'i'
• // less than 'theValue',
• // push 'i' onto the stack as a constant
• // add the two values at the top of the
  stack.
• // The sum is left on the stack.
• generator.Emit(OpCodes.Ldc_I4, 0)
• for (int i 1 i lt theValuei)
• generator.Emit(OpCodes.Ldc_I4, i)
• generator.Emit(OpCodes.Add)

24
Demo

• Programming_CSharp3

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Problems

• Reflection APIs are known to cause problem on
  obfuscated assemblies.

26
References

• http//www.oreilly.com/catalog/progcsharp/chapter/
  ch18.html
• http//en.csharp-online.net/Introduction_to_CLIE2
  8094Execution_and_Deployment_Models
• http//www.dcl.hpi.uni-potsdam.de/teaching/compone
  ntVl05/slides/Net_VL2_02_Reflection.pdf

27
Reference

• medialab.di.unipi.it/web/AP/Reflection.ppt
• Programming Microsoft .NET by Jeff Prosise

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• Questions

29

• Thank you