Error and Exception Handling Policy

DRAFT DOCUMENT

TODO: This document is a DRAFT and needs to be revised and extended, especially in the areas of Client and Server error handling.

Input should be given on this page.

Exception Handling

Overall

Exceptions should be thrown...

• whenever exceptional conditions occur in a program (e.g. a Method failure or unexpected states of Objects) and the program cannot continue because of this;
• when other error states (determined by program logic) are detected in code and the handling for those is outside the current program scope (e.g. Method, Property Getter/Setter, etc.).
• for signalling data verification errors. Use the EVerificationResultsException for this. To do this,
  • create (a) TVerificationResult instance(s) and an TVerificationResultCollection instance for this;
  • add the TVerificationResult instance(s) to the TVerificationResultCollection instance;
  • add the TVerificationResultCollection instance to the EVerificationResultsException by setting its 'VerificationResults' Property (that can be conveniently be done in a Constructor overload).
  • throw that EVerificationResultsException instance;
  • Important: Callers of the Method that can throw EVerificationResultsException(s) must be able to handle that Exception, otherwise an 'Unhandled Exception' would occur in the application!

Exceptions must not be thrown...

• simply to change the flow of a program Use other control logic (e.g. if or switch statements) for such purposes.

Raising of Exceptions is not considered costly nowadays. We are happy to throw Exceptions across .NET Remoting boundaries even if this costs some resources because the Exceptions need to be Serialized/Deserialized - Exceptions should be thrown in exceptional circumstances only and therefore this should not be a problem.

Practices That Need to be Adhered to

General

'Swallowing' Exceptions - *** DON'T DO IT! ***

Exceptions must never be 'swallowed', that is, get caught and not get re-raised again! Would this be done, one would let the program carry on as if everything is fine, while in reality an exceptional condition has occurred that would need attention. As a result, the program would likely become unstable and would be left in a non-deterministic state.

Catching Exceptions That Result From Programming Errors - *** DON'T DO IT! ***

Exceptions which are a result of a programming error shall never be caught, e.g. NullPointerException. This would mask the original error, therefore making it hard to find it.

Catching an Exception and Re-raising the Same Exception

An Exception shall not be caught and then the same Exception be thrown again.

There are three notable exceptions to this rule, though:

1. Logging of the Exception should take place in that very Exception Handler.
2. Resources need to be freed in case of an Exception, e.g. DB transaction rolled backed, handles freed, ...
3. User Interface update needs to happen that can not be achieved otherwise, e.g. wait cursor resetted, status bar text set, ...

Raising Another Exception Than The Original Exception (Translating Exceptions)

Catching of a specific Exception (not System.Exception!!!) and throwing of another Exception (possibly being a more precise one) instead in that Exception Handler is allowed. HOWEVER, throwing different Exceptions should generally be avoided, as it makes it harder to see what has originally happened. (An exception to that rule is the throwing of an OpenPetra-specific Exception [such as EOPAppException or one of the Exceptions that derive from it] --> see below!) Use of the 'InnerException' Property (see below) helps in mitigating this, but it can still be tricky to find the root of a problem if Exceptions are 'translated' --- if a different Exception is thrown, the reason for doing so should be carefully assessed and the pros and cons need to be considered, as it must not cause confusion in any situation in which it might be handled by an Exception Handler in any position in the Call Stack.

The new Exception may be a different Exception altogether if this adds meaning or allows adding more information (in the exception message, or in Fields of Custom Exceptions) but it needs careful assessment as to whether this is appropriate (see previous paragraph!).

Throwing of a new Exception which is a generalisation of the original Exception (e.g. catching a System.NullReferenceException and raising a System.Exception instead) is strongly discouraged, as valuable Type information is lost in doing so! (While this could be mitigated by putting the original Exception in the InnerException Property of the new Exception [see below], a generalisation is still making it harder to find the root of a problem as InnerExceptions are not usually evaluated programmatically, but only by a programmer when (s)he is debugging.)

Special Case: Throwing an OpenPetra-specific Exception: Expected (System-)Exceptions shall be caught and be put into an EOPAppException or into one of the Exceptions that derive from it. This helps to differentiate between program errors and expected Exception conditions. Always add the original Exception as the 'InnerException' of the OpenPetra-specific Exception (see below)!

• Put an Exception into an EOPDBException rather than into an EOPAppException if the source for the Exception is Database-related, Database Access-Layer related (TTypedDataAccess Class), or Database Abstraction-Layer related (Classes contained in Project Ict.Common.DB).

Use of InnerException Property: When another Exception is raised than the original Exception, the original Exception must be added as the 'InnerException' of the newly raised Exception. In doing so, the original Exception information isn't lost (incl. the code line where the original Exception occurred [if the Assembly is built in the Debug rather than the Release configuration]!).

• This is done by adding the instance of the original Exception as the second Argument when creating the instance of the new Exception.
  • Example: throw new EOPAppException("Error caused by trying ThrowInner.", Exp);, where Exp is the instance of the original Exception.
  • Full Example: see Exception.InnerException Property

Use of Custom Exceptions

.NET allows the creation of Custom Exceptions.

They are useful for several reasons:

• Their Name can tell much more about an Exception situation than a built-in Exception of the .NET Framework;
• They can contain arbitrary additional data;
• It is possible to create custom Exceptions that themselves derive from another Custom Exception. Through that a hierarchy of Custom Exceptions can be created.

Type Naming conventions: See Exception Naming Guidelines

In many cases the creation of a new Custom Exception that has no additional information is sufficient as the Type Name of the new Exception already conveys enough. For Exception handling situations where additional information should be passed to Exception Handlers higher up in the Call Stack, extra data should be added to the custom Exception that is thrown. This helps in handling the Exception because it is possible to process the additional information in Exception Handlers that know about this information.

Examples in \csharp\ICT\Petra\Shared\RemotedExceptions.cs:

• Simple Custom Exception: Class 'EPagedTableNoRecordsException'
• Custom Exception that holds additional data: Class 'ESecurityPartnerAccessDeniedException'

See also: A Use Case for a Custom Exception

Good article about Custom Exceptions: Creating Custom Exceptions in .NET

Custom Exceptions crossing .NET Remoting Boundaries

All our Custom Exceptions need to be able to cross .NET Remoting boundaries and keep its specific data in doing so. All the information that is encapsulated in an Exception is transported without loss of fidelity across .NET Remoting boundaries (this works fine from Linux/mono to Windows/MS .NET as well!).

To ensure that Exception-specific data is kept when crossing .NET Remoting boundaries, care needs to be taken that any data that is specific to a Custom Exception is not lost during Serialization/Deserialization! Crucial for that is the correct implementation of the Constructor that has the Arguments 'SerializationInfo' and 'StreamingContext' and of the Method 'GetObjectData'.

• Example: \csharp\ICT\Petra\Shared\RemotedExceptions.cs, Class 'ESecurityPartnerAccessDeniedException'

Custom Exceptions Must Be Specified In Shared Assemblies

All Custom Exceptions must be specified in DLL's that are shared between Client and Server so that they can be passed from Server to Client (their Type information needs to be available to the Client as .NET Remoting needs to be able to deserialize the exact Type whenever they are thrown on the server side). Therefore Custom Exceptions need to be specified in the Namespace Ict.Petra.Shared or one of its Subnamespaces.

File Operations

TODO

Server-specific Considerations

DB Access

At the moment our auto-generated '...SubmitChanges' Methods in the openPETRA Datastore return a TVerificationResultCollection, but that should be changed to Exceptions being thrown instead - see DB Discussions!

TODO

Client-specific Considerations

Aborting a Data Saving Operation

TODO

TODO

• All OpenPetra-specific Exceptions should derive from a common Base Class XXX.
  • The Base Class should add support for I18N of the Exception for showing an error message to the user in various languages.
    • It should be possible to show an I18N exception including the additional data.
  • Exceptions should always be logged in English to the log file (no I18N)!
  • Introduce specialized Base Classes for Exception for database, IO, server-communication, ... situations. This helps to catch specific areas of expected Exceptions, which could be handled specifically and separately in Exception Handlers.

More TODO?

Error Handling

Past Implementation and Anticipated Changes

• In Petra 2.x, a DB Table ('s_error_message') held error codes and the associated error messages centrally. A central function existed which displayed these errors when the software developer supplied the error code.
• In openPETRA we will probably do this differently - a global static Class which holds error codes is envisaged, and a message library which will display errors.

Error Codes

See Error Codes in Openpetra.

Central Place for Maintenance of Unique Error Codes

The goal is to have a numbering scheme for unique error codes that

• allows new error codes to be introduced easily;
• ensures that error codes are used and re-used appropriately by the software developers;
• distinguishes between data verification errors and exceptions.

See the generated list of already existing error codes in OpenPetra: https://ci.openpetra.org/job/OpenPetraCodeDoc/doclinks/1/