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Exception Handling in 3-Tier Architecture

By , , 15 Oct 2010
 

Introduction

We have been thinking of working on Exception handling in 3-Tier Architecture using Enterprise Library since ages but never got time or got into the situation where it should have become a requirement. Finally, we got a chance to work on it.

Our main objective was to handle exceptions seamlessly in three tiered architecture. We checked internet but couldn’t find appropriate code sample doing stuff that we wanted. Therefore, we decided to write a re-useable code to help ourselves and others. Below is what we think, exception handling should do:

Exception_Handling_Model.png

Scenario Description Expected Result
1 Application called a stored procedure. Happy path; everything should work fine.
2 Exception Raised by Database Management System such as Primary Key violated or Object does not exist, etc. System should log the thrown exception in a sink (Text File). Replace the Exception with a User friendly Message and propagate it to UI passing through the Business Logic Layer.
3 Based upon some certain validation check / business rule a Custom Error is raised from Stored Procedure. Thrown Custom Error message should be propagated to UI without Logging assuming these error messages will be some kind of alternative flows instead of real exception.
4 While processing the request, an error occurred in Business Logic such as Divide-by-zero, Object not set, Null reference exception, etc. System should log the thrown exception in a sink (Text File). Replace the Exception with a User friendly Message and should propagate that to UI.
5 Based upon some certain business validation check, a custom exception was thrown from Business Logic Layer. Thrown Custom Error message should be propagated to UI without Logging assuming these error messages will be some kind of alternative flows instead of real exception.
6 While processing the request an error occurred in UI. For example, Null reference exception, etc. System should log the thrown exception in a sink (Text File). Replace the Exception with a User friendly Message and should propagate that to UI.

Using the Code

What things do you need to use/run this code?

  • Microsoft.NET Framework 3.5
  • Microsoft Visual Studio 2008
  • Enterprise Library 5.0
  • SQL Server (Server, Desktop or SQL Express Edition - Adjust connection string accordingly.)

Let us explain how this application is broken down into pieces:

  • User Interface: Web Application using ASP.NET and C# as code behind
  • Business Logic: We have created separate folder inside the Web Application for Business Logic purpose. However, it can easily be moved into a separate Class Library project.
  • Helpers: Windows Class Library using C#. We created this project to combine below functionality:
    • Data Access Helper Class which is using Enterprise Library to interact with Database. This way we can enjoy Enterprise Library’s power to work with any type of database management system such as SQL and Oracle.
    • Exception handling which is also using Enterprise Library to log, replace and propagate exception using different policies.

To handle scenarios described above, following Exception Type classes are being created:

BaseException: It is a base class inherited from System.Exception class. All other classes in this project will be inherited from this class.

using System;
using System.Linq;
using System.Text;
using System.Collections.Generic;
using System.Runtime.Serialization;

namespace Common.Helpers.ExceptionHandling
{
   public class BaseException : System.Exception, ISerializable
   {
      public BaseException()
         : base()
      {
         // Add implementation (if required)
      }

      public BaseException(string message)
         : base(message)
      {
         // Add implementation (if required)
      }

      public BaseException(string message, System.Exception inner)
         : base(message, inner)
      { 
         // Add implementation (if required)
      }

      protected BaseException(SerializationInfo info, StreamingContext context)
         : base(info, context)
      { 
         // Add implementation (if required)
      }
   }
}

DataAccessException: This is used by the Exception handling Block to replace the original exception with a User Friendly message such as “An unknown error has occurred at database level while processing your request. Please contact Technical Help Desk with this identifier XXXXXXXXXXXXXX”. 

using System;
using System.Linq;
using System.Text;
using System.Collections.Generic;
using System.Runtime.Serialization;

namespace Common.Helpers.ExceptionHandling
{
   public class DataAccessException : BaseException, ISerializable
   {
      public DataAccessException()
         : base()
      { 
         // Add implementation (if required)
      }

      public DataAccessException(string message)
         : base(message)
      { 
         // Add implemenation (if required)
      }

      public DataAccessException(string message, System.Exception inner)
         : base(message, inner)
      { 
         // Add implementation
      }

      protected DataAccessException(SerializationInfo info, StreamingContext context)
         : base(info, context)
      { 
         //Add implemenation
      }

   }
}

DataAccessCustomException: This is used by the Exception Handling block to send raised Custom Error message towards UI.

PassThroughException: This is used to replace DataAccessException and DataAccessCustomException types with this type at Business Logic level. Otherwise, Business Logic will also handle the exception as Data access is being called from Business Logic.

BusinessLogicException: This is used by the Exception Handling block to replace the original exception with a user friendly message such as “An unknown error has occurred in Business Logic Layer while processing your request. Please contact Technical Help Desk with this identifier XXXXXXXXXXXXXX”.

BusinessLogicCustomException: This is used by the Exception Handling block to send raised Custom Error message towards UI.

UserInterfaceException: This is used by the Exception Handling block to replace the original exception with a user friendly message such as “An unknown error has occurred at User Interface while processing your request. Please contact Technical Help Desk with this identifier XXXXXXXXXXXXXX”

The following polices are created to support design:

DataAccessPolicy: This policy will log the original exception in defined sink will replace the original exception with DataAccessException type. 

 <add name="DataAccessPolicy">
    <exceptionTypes>
       <add name="All Exceptions" type="System.Exception, mscorlib, 
	Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089"
          postHandlingAction="ThrowNewException">
          <exceptionHandlers>
             <add name="DataAccessLoggingHandler" 
		type="Microsoft.Practices.EnterpriseLibrary.ExceptionHandling.
		Logging.LoggingExceptionHandler, 
		Microsoft.Practices.EnterpriseLibrary.ExceptionHandling.Logging, 
		Version=5.0.414.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"
                logCategory="General" eventId="100" severity="Error" 
		title="Enterprise Library Exception Handling"
                formatterType="Microsoft.Practices.EnterpriseLibrary.
		ExceptionHandling.TextExceptionFormatter, 
		Microsoft.Practices.EnterpriseLibrary.ExceptionHandling"
                priority="0" />
             <add name="DataAccessReplaceHandler" 
		type="Microsoft.Practices.EnterpriseLibrary.ExceptionHandling.
		ReplaceHandler, Microsoft.Practices.EnterpriseLibrary.ExceptionHandling,
		Version=5.0.414.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"
                exceptionMessage="An unknown error has occurred in Data Access Layer 
		while processing your request. Please contract Help Desk Support at 
			X-XXX-XXX-XXXX with Error Token ID {handlingInstanceID}."
                replaceExceptionType=
		"Common.Helpers.ExceptionHandling.DataAccessException, Helpers, 
		Version=1.0.0.0, Culture=neutral, PublicKeyToken=null" />
          </exceptionHandlers>
       </add>
    </exceptionTypes>
 </add>

DataAccessCustomPolicy: This policy will only replace the original exception with DataAccessCustomException type.

<add name="DataAccessCustomPolicy">
   <exceptionTypes>
      <add name="All Exceptions" type="System.Exception, mscorlib, 
	Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089"
         postHandlingAction="NotifyRethrow">
         <exceptionHandlers>
            <add name="Replace Handler" type="Microsoft.Practices.EnterpriseLibrary.
		ExceptionHandling.ReplaceHandler, 
		Microsoft.Practices.EnterpriseLibrary.ExceptionHandling, 
		Version=5.0.414.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"
               replaceExceptionType="Common.Helpers.ExceptionHandling.
		DataAccessCustomException, Helpers, Version=1.0.0.0, 
		Culture=neutral, PublicKeyToken=null" />
         </exceptionHandlers>
      </add>
   </exceptionTypes>
</add>

PassThroughPolicy: This policy will replace the original exceptions of type DataAccessException or DataAccessCustomException with PassThroughException type. 

<add name="PassThroughPolicy">
   <exceptionTypes>
      <add name="All Exceptions" type="System.Exception, mscorlib, 
	Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089"
         postHandlingAction="NotifyRethrow">
         <exceptionHandlers>
            <add name="PassThroughReplaceHandler" 
		type="Microsoft.Practices.EnterpriseLibrary.ExceptionHandling.
		ReplaceHandler, Microsoft.Practices.EnterpriseLibrary.ExceptionHandling,
		 Version=5.0.414.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"
               replaceExceptionType="Common.Helpers.ExceptionHandling.PassThroughException,
		 Helpers, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null" />
         </exceptionHandlers>
      </add>
   </exceptionTypes>
</add>

BusinessLogicPolicy: This policy will log the original exception in defined sink will replace the original exception with BusinessLogicException type.

<add name="BusinessLogicPolicy">
   <exceptionTypes>
      <add name="All Exceptions" type="System.Exception, 
	mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089"
         postHandlingAction="ThrowNewException">
         <exceptionHandlers>
            <add name="BusinessLogicLoggingHandler" 
		type="Microsoft.Practices.EnterpriseLibrary.ExceptionHandling.
		Logging.LoggingExceptionHandler, 
		Microsoft.Practices.EnterpriseLibrary.ExceptionHandling.Logging, 
		Version=5.0.414.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"
               logCategory="General" eventId="100" severity="Error" 
		title="Enterprise Library Exception Handling"
               formatterType="Microsoft.Practices.EnterpriseLibrary.
		ExceptionHandling.TextExceptionFormatter, 
		Microsoft.Practices.EnterpriseLibrary.ExceptionHandling"
               priority="0" />
            <add name="BusinessLogicReplaceHandler" 
		type="Microsoft.Practices.EnterpriseLibrary.ExceptionHandling.
		ReplaceHandler, Microsoft.Practices.EnterpriseLibrary.ExceptionHandling,
		 Version=5.0.414.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"
               exceptionMessage="An unknown error has occurred in Business Logic 
		Layer while processing your request. Please contract Help Desk Support 
		at X-XXX-XXX-XXXX with Error Token ID {handlingInstanceID}."
               replaceExceptionType="Common.Helpers.ExceptionHandling.
		BusinessLogicException, Helpers, Version=1.0.0.0, 
		Culture=neutral, PublicKeyToken=null" />
         </exceptionHandlers>
      </add>
   </exceptionTypes>
</add>

BusinessLogicCustomPolicy: This policy will only replace the original exception with BusinessLogicCustomException type.

<add name="BusinessLogicCustomPolicy">
   <exceptionTypes>
      <add name="All Exceptions" type="System.Exception, 
	mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089"
         postHandlingAction="NotifyRethrow">
         <exceptionHandlers>
            <add name="Replace Handler" 
		type="Microsoft.Practices.EnterpriseLibrary.ExceptionHandling.
		ReplaceHandler, Microsoft.Practices.EnterpriseLibrary.ExceptionHandling,
		 Version=5.0.414.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"
               replaceExceptionType="Common.Helpers.ExceptionHandling.
		BusinessLogicCustomException, Helpers, Version=1.0.0.0, 
		Culture=neutral, PublicKeyToken=null" />
         </exceptionHandlers>
      </add>
   </exceptionTypes>
</add>

UserInterfacePolicy: This policy will log the original exception in defined sink will replace the original exception with UserInterfaceException type.

<add name="UserInterfacePolicy">
   <exceptionTypes>
      <add name="All Exceptions" type="System.Exception, mscorlib, 
	Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089"
         postHandlingAction="ThrowNewException">
         <exceptionHandlers>
            <add name="UserInterfaceReplaceHandler" 
		type="Microsoft.Practices.EnterpriseLibrary.ExceptionHandling.
		Logging.LoggingExceptionHandler, 
		Microsoft.Practices.EnterpriseLibrary.ExceptionHandling.Logging, 
		Version=5.0.414.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"
               logCategory="General" eventId="100" severity="Error" 
		title="Enterprise Library Exception Handling"
               formatterType="Microsoft.Practices.EnterpriseLibrary.
		ExceptionHandling.TextExceptionFormatter, 
		Microsoft.Practices.EnterpriseLibrary.ExceptionHandling"
               priority="0" />
            <add name="Replace Handler" type="Microsoft.Practices.
		EnterpriseLibrary.ExceptionHandling.ReplaceHandler, 
		Microsoft.Practices.EnterpriseLibrary.ExceptionHandling, 
		Version=5.0.414.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"
               exceptionMessage="An error occord at front end. please check."
               replaceExceptionType="Common.Helpers.ExceptionHandling.
		UserInterfaceException, Helpers, Version=1.0.0.0, Culture=neutral, 
		PublicKeyToken=null" />
         </exceptionHandlers>
      </add>
   </exceptionTypes>
</add>

Following exception handlers are being created to manage exception logging, replacement and propagating:

DataAccessExceptionHandler: This handler will take care of "DataAccessException" and "DataAccessCustomException" exception types. Based upon the type of the exception, this handler will decide which policy should work. Below is the code calling different policy: 

using System;
using System.Linq;
using System.Text;
using System.Data.Common;
using System.Collections.Generic;
using System.Data.SqlClient;
using Microsoft.Practices.EnterpriseLibrary.ExceptionHandling;

namespace Common.Helpers.ExceptionHandling
{
   public static class DataAccessExceptionHandler
   {
      public static bool HandleException(ref System.Exception ex)
      {
         bool rethrow = false;
         if ((ex is SqlException))
         {
            SqlException dbExp = (SqlException)ex;
            if (dbExp.Number >= 50000)
            {
               rethrow = ExceptionPolicy.HandleException(ex, "DataAccessCustomPolicy");
               ex = new DataAccessCustomException(ex.Message);
            }
            else
            {
               rethrow = ExceptionPolicy.HandleException(ex, "DataAccessPolicy");
               if (rethrow)
               {
                  throw ex;
               }
            }
         }
         else if (ex is System.Exception)
         {
            rethrow = ExceptionPolicy.HandleException(ex, "DataAccessPolicy");
            if (rethrow)
            {
               throw ex;
            }
         }
         return rethrow;
      }
   }
}

BusinessLogicExceptionHandler: This handler will take care of “PassThroughException”, “BusinessLogicException” and “BusinessLogicCustomException” exception types. Based upon the type of the exception, this handler will decide which policy should work. Below is the code calling different policy:

using System;
using System.Linq;
using System.Text;
using System.Collections.Generic;
using Microsoft.Practices.EnterpriseLibrary.ExceptionHandling;
using Common.Helpers.ExceptionHandling;

namespace Common.Helpers.ExceptionHandling
{
   public static class BusinessLogicExceptionHandler
   {
      public static bool HandleExcetion(ref System.Exception ex)
      {
         bool rethrow = false;
         if ((ex is DataAccessException) || (ex is DataAccessCustomException))
         {
            rethrow = ExceptionPolicy.HandleException(ex, "PassThroughPolicy");
            ex = new PassThroughException(ex.Message);
         }
         else if (ex is BusinessLogicCustomException)
         {
            rethrow = ExceptionPolicy.HandleException(ex, "BusinessLogicCustomPolicy");
         }
         else
         {
            rethrow = ExceptionPolicy.HandleException(ex, "BusinessLogicPolicy");
         }
         if (rethrow)
         {
            throw ex;
         }
         return rethrow;
      }
   }
}

If you examine the above code, you will see that in case of DataAccessException or DataAccessCustomException, this handler is using PassThroughExceptionPolicy. Means, exception has already been logged it needs to be propagated towards caller. Afterwards, the system is checking whether exception is raised intentionally by the Business Logic Layer or it is a uncontrolled exception. Based upon type handler is calling appropriate Exception Handling Policy.

UserInterfaceExceptionHandler: This handler will take care of “UserInterfaceException” exception type. Based upon the type of the exception, this handler will decide which policy should work. Below is the code calling different policy:

using System;
using System.Linq;
using System.Text;
using System.Collections.Generic;
using Microsoft.Practices.EnterpriseLibrary.ExceptionHandling;

namespace Common.Helpers.ExceptionHandling
{
   public static class UserInterfaceExceptionHandler
   {

      public static bool HandleExcetion(ref System.Exception ex)
      {
         bool rethrow = false;
         try
         {
            if (ex is BaseException)
            {
               // do nothing as Data Access or Business Logic exception 
               // has already been logged / handled
            }
            else
            {
               rethrow = ExceptionPolicy.HandleException(ex, "UserInterfacePolicy");
            }
         }
         catch (Exception exp)
         {
            ex = exp;
         }
         return rethrow;
      }
   }
}

That's it. Download the code and enjoy and do leave your feedback.

Thanks!

Sohail Maroof Naushahi, Farrukh Hashmi

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

About the Authors

 Sohail Maroof
United States United States
Member
No Biography provided

Farrukh Hashmi
United States United States
Member
No Biography provided

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FirstPrevNext
GeneralMy vote of 5memberjim lahey18 Oct '11 - 2:35 
Nice article. I like the flow diagram, very concise overview.
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QuestionNeed Clarification on Code.memberkonda reddy 198423 Jul '11 - 8:46 
Hi Sohail/Hashmi,
 
It is very nice article.but I have a some doubts in MyClass.cs file at the catch block places.
The below code one of the method in MyClass.cs file.
here what is the use of "rethow" variable and if block of if(rethrow).
 
if postHandlingAction="ThrowNewException" then execution will jump to caller method.
if postHandlingAction="NotifyRethrow" then it will return true, in ExceptionHandler class only you are throwing exception. so this if block never execute, right?. if it is not needed Can I remove it?
 
Can you please suggest me on this.
 
 public static DataTable GetCustomersList()
      {
         try
         {
            DataTable dt = SqlHelper.GetDataTable("GetCustomerList");
            return dt;
         }
         catch (Exception ex)
         {
            bool rethrow = false;
            rethrow = BusinessLogicExceptionHandler.HandleExcetion(ref ex);
            if (rethrow)
            {
               throw ex;
            }
            return null;
 
         }
      }
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GeneralMy vote of 5memberMember 810288021 Jul '11 - 10:42 
Excellent article!
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QuestionActivation erroe occur while trying to handle error on UI levelmembervarshapdesale10 Jul '11 - 23:11 
Hi sohali,
 
I am trying to implementing this error handling but it give me activation error whilw error occur in UI.
 
Please help me for the same.
 
Thanks
Varsha
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GeneralGood article, but...memberChris G.14 Jun '11 - 1:49 
I think you're over using Try / Catch blocks. They are expensive in nature and you're applying them in almost every method. In a web application where performance and memory are crucial, I believe a global exception handler is appropriate. The problem with your design and using a global exception handler is that the global exception handler would not know from which layer the exception occurred, so it wouldn't know which policy to use. Maybe part of the solution is to incorporate your design for local level exception handling, when needed, and design a "Global Exception Policy" for everything else.
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QuestionProblem in UserInterfaceExceptionHandlermemberkonda reddy 198430 Apr '11 - 5:07 
HI Sohail,
First of all I'm appreciating you for providing gud article. this is very good suited to my app.
But i found a problem UserInterfaceExceptionHandler.cs file.
When i got exception in UI layer , passing to the UserInterfaceExceptionHandler file for process.
when i log exception message into the database, the stack trace is missing.
 
Can you please look into this and provide any work around.
Below Code is UI Layer
 

catch (Exception exp)
{
bool rethrow = false;
rethrow = UserInterfaceExceptionHandler.HandleExcetion(ref exp);
SessionManager.DisplayActionMessage(false, exp.Message);
}
 

-Konda
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AnswerRe: Problem in UserInterfaceExceptionHandlermemberSohail Maroof30 Apr '11 - 13:05 
Thanks for the comments and glad this article is suitable for your application.
In given code you're using exp.Message. This method will only give you message.
To get Stack Trace, you will have to use exp.stacktrace.
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QuestionRe: Problem in UserInterfaceExceptionHandlermemberkonda reddy 19841 May '11 - 7:00 
HI Sohail,
Thanks for quick reply.
this exp.message i am showing to user.it is a generic message.
 
rethrow = UserInterfaceExceptionHandler.HandleExcetion(ref exp); this line of code wil process the exception and insert the exception details into the the database.here exception stack trace missing. this is my problem.
 
Can you please look into this.
 
-Konda
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GeneralActivation Error in Business Logic Custom Policy.membermaheshgrapesoft21 Apr '11 - 0:04 
I am getting the following error:
"Activation error occurred while trying to get instance of type ExceptionPolicyImpl, key "BusinessLogicCustomPolicy" ...
 
when I am throwing new custom exception in Business Logic Layer. Can you please help me in this regard ?
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GeneralRe: Activation Error in Business Logic Custom Policy.memberSohail Maroof30 Apr '11 - 13:07 
Sure. We can try troubleshooting the problem. But you will have to send me code.
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About Article
Exception handling in 3-Tier Architecture using Enterprise Library
Type Article
Licence CPOL
First Posted 3 Jun 2010
Views 83,693
Downloads 4,551
Bookmarked 187 times

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