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Policy Injection in ASP.NET using Enterprise Library 3.0

Alex Homeralex@stonebroom.com

http://www.daveandal.net

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Agenda

Terminology and Buzzwords

The Policy Injection Application Block

• what it does, how it works

Configuring Policies

Using Attributes

Extending the Block

• custom Handlers and Matching Rules

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Terminology and Buzzwords

Aspect Oriented Programming (AOP) Policy Injection Remoting Proxies Intercepting Proxies Method Interception

The overall aim is better management of Crosscutting Concerns

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Core Concerns

What are Crosscutting Concerns?

Application

Business

Object

Business

Object

Data Access Object

Application

Business

Object

Data Access Object

Validation

Logging

Exception Handling

Caching

Authorization

Crosscutting Concerns

Performance

Counters

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What is Policy Injection? “... techniques for creating policies that

help to manage crosscutting concerns, and applying these policies across all relevant objects and applications ...”

Using the Policy Injection Block you can:• Create handlers for each crosscutting concern• Define the conditions where they will apply• Define the behavior in specific circumstances • Adjust runtime behavior through configuration• Manage runtime behavior using Group Policy

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The Handlers Pipeline

Objects must be “interceptable” Must use factory Create or Wrap method Each handler can execute code in the pre-

and/or post-processing stages Handlers can short-circuit (abort)

execution

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Interceptable Object Types (1)

C#:object = PolicyInjection.Create<type>(ctor_params)object = PolicyInjection.Create<type>(IConfigurationSource,

ctor_params)

object = PolicyInjection.Wrap<type>(existing_object)object = PolicyInjection.Wrap<type>(IConfigurationSource,

existing_object)

Types that inherit MarshalByRefObject:

VB.NET:object = PolicyInjection.Create(Of type)(ctor_params)object = PolicyInjection.Create(Of type)(IConfigurationSource,

params)

object = PolicyInjection.Wrap(Of type)(existing_object)object = PolicyInjection.Wrap(Of type)(IConfigurationSource,

object)

C#

Visual Basic .NET

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Interceptable Object Types (2)

interface = PolicyInjection.Create<type, interface>(ctor_params)interface = PolicyInjection.Create<type, interface> (IConfigurationSource, ctor_params)interface = PolicyInjection.Wrap<interface>(existing_object)interface = PolicyInjection.Wrap<interface> (IConfigurationSource, existing_object)

Types that implement a known interface:C#

interface = PolicyInjection.Create(Of type, interface)(ctor_params)interface = PolicyInjection.Create(Of type, interface) _

(IConfigurationSource, ctor_params)interface = PolicyInjection.Wrap(Of interface) (existing_object)interface = PolicyInjection.Wrap(Of interface) _

(IConfigurationSource, existing_object)

Visual Basic .NET

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Configuring Policies Enterprise Library Configuration Console Visual Studio 2005 Configuration Editor Manually (recommended for masochists

only)

Add the PIAB to your app configuration Create the required Policies

• Each policy has one or more matching rules Must all evaluate to TRUE to select a target

• Each policy has one or more handlers Processed in the order they occur in

configuration

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Configuring Policies Demo

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Demo Business Object Members

Member

Attributed Customer Model

Interface Customer Model

Notes

GetCustomerList() GetCustomerName(customerID)

GetCustomerNameWithWildcard

(customerID)

Uses GetCustomerName method with a wildcard.

GetCustomerDetails(customerID)

Not defined in interface.

GetCityList(minimumCount) CityCount Property. Uses

GetCityListPublic members exposed by the two business objects

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Demo PoliciesPolicy Name Matching Rules Call Handlers

InterfaceModelPolicy Member Name = GetCustomerNameWithWildcard or GetCustomerList

Type Name = InterfaceCustomerModel

Logging Handler with Categories = "AuditFile" and "EventLog"

Caching Handler with cache duration 20 seconds

Exception Handler with Exception Policy = "CustomerModelPolicy”

AttributeModelPolicy Method Signature, Name = * and parameter = System.String

Type Name = AttributedCustomerModel

Logging Handler with Categories = "AuditFile"

CacheByTagPolicy Tag Attribute = "ApplyTenSecondCaching"

Caching Handler with cache duration 10 seconds

The policies defined in the example application

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Using Attributes (1) Use an attribute to apply a handler, and

optionally set properties, at design-time:

[CachingCallHandler(0, 0, 30)]public DataTable GetCustomerList(){ …}

C#

<CachingCallHandler(0, 0, 30)> _Public Sub GetCustomerList() As DataTable

…End Sub

Visual Basic .NET

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Using Attributes (2) Use Validation attributes (from the Validation

Application Block) to validate parameters:

[ValidationCallHandler][ValidationCallHandler]public String GetCustomerName( [StringLengthValidator(3,

RangeBoundaryType.Inclusive, 5,

RangeBoundaryType.Inclusive)] String customerID)

C#

<ValidationCallHandler()> _Public Function GetCustomerName( _ <StringLengthValidator(3,

RangeBoundaryType.Inclusive, _ 5,

RangeBoundaryType.Inclusive)> _ ByVal customerID As String) As String

Visual Basic .NET

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Using Attributes (3) Prevent configuration of any policies:

[ApplyNoPolicies]public DataTable GetCustomerList()…

C#

< ApplyNoPolicies> _Public Sub GetCustomerList() As DataTable

…

Visual Basic .NET

Other handler attributes: [AuthorizationCallHandler],

[ExceptionHandlingCallHandler],

[LoggingCallHandler],

[PerformanceCounterCallHandler]

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Using Attributes Demo

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Factory

Proxy

Handlers

Matching Rules

Pipeline

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Extending the PIAB

Create a new Matching Rule• implement the IMatchingRule interface

Create a new Handler• implement the ICallHandler interface

Create a new Handler Attribute• derive from HandlerAttribute or

Attribute

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Inside a Matching Rulepublic interface IMatchingRule{ bool Matches(MethodBase member);}

// for example… the built-in Tag Attribute Matching Rule uses:public bool Matches(MethodBase member){ foreach (TagAttribute tagAttribute in

GetAllAttributes(member, true)) { if (string.Compare(tagAttribute.Tag, tagToMatch,

ignoreCase) == 0) return true; } return false;}

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Inside a Pipeline Handlerpublic interface ICallHandler{ IMethodReturn Invoke(IMethodInvocation input, GetNextHandlerDelegate getNext);}

public delegate IMethodReturn InvokeHandlerDelegate(IMethodInvocation input,

GetNextHandlerDelegate getNext);

public delegate InvokeHandlerDelegate GetNextHandlerDelegate();

Handlers must implement the Invoke method

IMethodInvocation contains method invocation or property access information (including parameters)

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Handler Process Overviewpublic class ExampleHandler : ICallHandler{ public IMethodReturn Invoke(IMethodInvocation input, GetNextHandlerDelegate getNext) { // Perform any pre-processing tasks required in the custom

handler. ... // Invoke next handler that the block should execute. This

code gets the current return message that you must pass back to the caller:

IMethodReturn msg = getNext()(input, getNext); ... // Perform any post-processing tasks required in the custom

handler. // You can modify the return message if required. // Return the message to the previous handler or the client. return msg; }

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Messages and Short-CircuitingEach handler invokes the next one, passing the invocation message along the pipeline and the return message back again

A handler can instead simply refuse to call

the next handler, and can add information

(e.g. an exception) to the return message.

Note that previous handlers still execute

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Short-Circuiting Executionpublic IMethodReturn Invoke(IMethodInvocation input, GetNextHandlerDelegate getNext){ GregorianCalendar cal = new GregorianCalendar(); DayOfWeek weekDay = cal.GetDayOfWeek(DateTime.Now); if (weekDay == DayOfWeek.Saturday || weekDay ==

DayOfWeek.Sunday) { // create an Exception to return and the return message. Exception ex = new Exception("Available on weekdays only"); IMethodReturn msg = input.CreateExceptionMethodReturn(ex) return msg; } else { return getNext()(input, getNext); // invoke the next handler }

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Custom Handler Attributes[AttributeUsage(AttributeTargets.Class | AttributeTargets.Property |

AttributeTargets.Method)]public class CachingCallHandlerAttribute : HandlerAttribute{ private TimeSpan expirationTime;

public CachingCallHandlerAttribute(int hours, int minutes, int seconds)

{ // constructor, save the parameter values expirationTime = new TimeSpan(hours, minutes, seconds); }

public override ICallHandler CreateHandler() { // create the appropriate handler instance and return it return new CachingCallHandler(expirationTime); }}

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References Book covering Enterprise Library:

• "Effective Use of Microsoft Enterprise Library: Building Blocks for Creating Enterprise Applications and Services" (Addison-Wesley ISBN 0-321-33421-3).

All about Enterprise Library:• http://www.codeplex.com/entlib/

Code and Slides for this session:• http://www.daveandal.net/download/

Contact: alex@stonebroom.com

http://www.codeplex.com