Posts in category .net - page 8

LINQ to SQL templates updated, now on CodePlex

My templates that allow you to customize the LINQ to SQL code-generation process (normally performed by SQLMetal/LINQ to SQL classes designer) have been updated once again.

Updates

  • Now licensed under the Microsoft Public License and hosted at CodePlex
  • User options specified with a var options block at the start of the template
  • Option for each class to be a separate file that is reflected in the VS project EntityPerFile=true
  • Detection and support of IsComposable functions
  • General code clean-up and better error handling such as missing DBML file

CodePlex

CodePlex makes it easier for people to be able to see and merge updates in with their own modified versions as well as report issues via the issue tracker etc. There is also an RSS feed that lets you keep track of releases, source updates or whatever else you are interested in.

For now it is a grab-the-source style release but I hope to publish downloadable tested releases wrapped up in a Visual Studio Installer (VSI) package to make getting started easier soon.  Feel free to grab the sources directly via TFS/Subversion to be able to diff them with your own modified versions.

Enjoy!

[)amien

Changing type, the state pattern and LINQ to SQL

A question I see from time-to-time on LINQ to SQL relates to changing an entity’s class.

C# and VB.NET don’t allow a class to change its type at run-time and LINQ to SQL specifically doesn’t provide a mechanism for changing the underlying discriminator for this reason.

Discarding the current object and creating a new one is fraught with issues. What do we do about existing references, unsaved data, established associations and caches?

Start with an example

Consider an abstract Account class with SavingsAccount and CurrentAccount sub-classes. Bank accounts don’t change type once created (in my experience) so that’s good so far.

When we get into processing and validation logic its tempting to create ClosedAccount and OpenAccount classes but what happens during execution when closing an account?

A further consideration is how exactly ClosedAccount and OpenAccount fit into the hierarchy given the single-inheritance limitation of C# and VB.NET.

Enter the State Pattern

The ever-resourceful Gang of Four describe the State Pattern as:

Allow an object to alter its behavior when its internal state changes. The object will appear to change its class.

Taking the bank accounts example and applying the state pattern gives:

State Pattern class diagram

Account no longer needs to change type at run-time yet is still able to have clients call Validate and process Methods that end up in discrete methods as if inheritance had been used.

To achieve this we have:

  1. Created a State hierarchy that contains the logic we need to change at run-time
  2. Introduced a private member in Account that points to the current state instance
  3. Ensured the Account methods call the current state class via the private member

Because the state member is private and only accessed by Account itself we can happily create and dispose it as the conditions that affect the state change as much as we like without worrying about references to it.

This is best illustrated with code. Here rather than just calling the state’s validation logic there is a combination of core Account validation (balance), state validation (closed) and CheckingAccount validation (transaction limits):

public abstract class Account {
  private AccountState state;

  public virtual Status Validate(ITransaction tx) {
    Status result = state.Validate(tx);
    if (tx.Amount > Balance)
      result.Add(TransactionFailures.InsufficientFunds);
    return result;
  }
}

public class SavingsAccount : Account {
  public override Status Validate(ITransaction tx) {
    Status result = base.Validate(tx);
    if (Transactions.Count > TransactionLimit)
      result.Add(TransactionFailures.TransactionLimitReached);
    return result;
  }
}

public class ClosedAccountState : AccountState {
  public override Status Validate(ITransaction tx) {
    return new Status(TransactionFailures.InvalidSourceAccount);
  }
}

This is less complex than selectively replacing objects within our application at run-time and can bring additional benefits:

Like all guidance, patterns and principles do not blindly follow these guidelines or patterns but consider how it affects and fits with your application. For this particular example it not only solves the problem but helps maintainability – at least at this simple stage. Once Validation becomes sufficiently complex it would likely move out entirely into a new set of orchestrated classes just for that.

With LINQ to SQL (and other mappers)

Moving this example into an object-relational mapper requires two – not unexpected – database-mapped properties.

  1. The inheritance discriminator (Type)
  2. A state indicator (Active)

Sample class diagrams for accounts using state pattern

The only thing we need to ensure is the Account’s state member always refers to either a ClosedAccountState or OpenedAccountState depending upon the Active flag.

Given that LINQ to SQL code-generates the property for Active we could:

  1. Make Active private, wrap it in another property and set the state member when it changes and at initialization
  2. Make the state member a read-only property instead of an instance variable

The second works well here and given that AccountState is itself stateless (perhaps not the best choice of name) we can use a singleton to avoid multiple instances. The state instance variable in the Account class is replaced with:

private AccountState State {
  get {
    if (Active)
      return OpenAccountState.Instance;
    else
      return ClosedAccountState.Instance;
  }
}

The code continues to work and now changing the Active flag results in different behavior.

Best of all we still have the code in separate classes, no switch/case/if statements relating to validation or account types, a clean inheritance hierarchy and no running around trying to invalidate existing references.

Hitting the discriminator directly

There may be times when claims are made that a type has to change – perhaps data was entered incorrectly.

Before delving into the database or providing a tool to flip the underlying discriminator value consider:

  1. Does the new class interpret the same data in a different manner? Has a $1,000 credit limit just become a 1,000 transactions per-month limit?
  2. Would the object be valid in the new class? Did a ProposedCustomer just become ApprovedCustomer without a policy-enforced credit check?
  3. Are associations still honored? Are 300 unshipped orders for a GameProduct still honored for a BookProduct?

If in doubt don’t do it.

An inconsistent database bleeding through your application isn’t good for anyone and will take a lot longer to sort out than setting up a new entity.

[)amien

ALT.NET Seattle

One of the cool things about living in Seattle is the sheer number of passionate developers around. Whether you’re dropping into offices, heading across campus for lunch, meeting downtown for music and beer or, in my case, last month taking a Saturday out to participate in ALT.NET Seattle, there are ideas, enthusiasm and discussions with great developers to be had everywhere.

The ALT.NET event was interesting. If you didn’t already know the name encapsulates:

  • the desire to improve the art, process, individual and product
  • the understanding that the right tool for the job doesn’t always ship with the .NET

The event follows the same minimal up-front planning modern development practices enjoy relying on talented people and simple structure to achieving something great in a short space of time.1

This open-spaces format involves a small deck of cards to write topics you’d like to talk about and a board with rooms and times to use. I was surprised at how well the event unfolded (bar one session that veered off-course), given our unplanned attempts at the Guernsey developer group always resulted in five people having dinner in a bar :)

I had an enjoyable day, and my thanks go to Brad Wilson (ASP.NET) for driving and listening to my nostalgic 8-bit discussion with fellow Brit Ade Miller (P&P) who previously worked at Future Publishing (Your Sinclair magazine). I’m now messing around with a .NET based Spectrum emulator I started years ago, and I’m going to blame him for that.

The best news is that there are more events planned for the coming months:

  • January 17th - location TBA
  • February – provisionally the 7th, location TBA

ALT.NET Seattle 2009 Conference

This is the big one immediately preceding the MVP Summit by running February 27th to March 1st at Digipen in Redmond. Glenn Block has details.

Registration will open on Tuesday, 6pm (GMT-8), attendance is free, and spaces are limited - so get in quick!

Consider joining the Seattle area ALT.NET group or ALT.NET Facebook group for more details or to get involved.

[)amien

1And as either Glen or Brad said with a nod to the end-of-credits bit in Ferris Bueller – “When it’s over it’s over. Go home already.”

LINQ to SQL template for Visual Studio 2008

A newer version of this LINQ to SQL template is available.

If you want to customize the LINQ to SQL code generation phase in your project without additional tool dependencies this could be what you’re looking for.

11 Dec 2008 Fixes to association code with one-to-one’s and with no serialization required. 7 Dec 2008 Added IsDelayLoaded, IsUnique. Fixed association code, stored proc update overrides. 17 Nov 2008 Added IsInheritanceDefault read/emit IsDefault on type 26 Oct 2008 Emit column Name attribute if it doesn’t match the member (thanks Steele) 20 Oct 2008 Handle class naming better, fix associations for renamed or out-of-sequence keys and their access modifiers 25 Sep 2008 Figure out missing association keys by using either sides primary key 25 Sep 2008 Empty DBML name-space is now no namespace instead of “MyApplication” 23 Sep 2008 fixed stored procedures with 0 parameters 22 Sep 2008 fixed VB.NET IsForeignKey attribute for associations 18 Sep 2008 now generates stored procedures including insert/update/delete with concurrency checking.

New since ‘reloaded’ version

  • Inheritance – generates sub-classes with all properties and code mappings.
  • VB.NETCSharpDataContext.tt is joined by a VB.NET emitting VBNetDataContext.tt.
  • DataContract SP1 – additional mode to emit SP1-compatible DataContract serialization via Roger Jennings.
  • Composite keys – both as the primary key and as a foreign key in an association.
  • Type attributes – the data context and entity types can now be sealed or abstract as well as public, private, protected, internal or protected internal.
  • Associations – prevents foreign key values changing once the object association is made and updates parent side of one-to-many associations.
  • Stored procedures – generates method wrappers and associated methods to facilitate insert/update/delete with concurrency support.

Functionality compared to designer

A primary goal in developing the template was to allow for easy switching between the template and the LINQ to SQL designer so things are very similar.

Missing

  • Comprehensive sanity checking on the DBML.
  • The Custom Tool Namespace and project namespaces are not pulled in when the DBML namespaces not specified.

Fixed

The designer has a few bugs which helpfully this template doesn’t suffer from.

  • Modifying a table via a stored procedure using original values for concurrency will throw ChangeConflictException and not silently fail.
  • Protected internal virtual property doesn’t forget to be virtual.
  • Checks all associations based on a foreign key are not loaded before allowing change and not just the first one.

Improved

  • Fully customizable with full source.
  • Serialization mode to support DataContract improvements in .NET 3.5 SP1 To use uncomment the line // data.Serialization = SerializationMode.DataContractSP1; in xDataClasses.tt
  • CanBeNull attribute generated for value types (useful when working with metadata).

Source compared to designer

The designer generated code can be difficult to read and isn’t well suited to template generation so the output from this template is different in a number of ways:

Sequence

Everything related to a column mapping – the storage variable, event signatures, attribute and the property itself – is batched together so it can be hit with a single loop making the template shorter and easier to work with.

#regions

Opinion may be divided on the usefulness of #regions in your own code but for code generation of large files I found it invaluable. There are regions for the logical parts of the data context and within each entity such as construction, column mapping, associations and serialization.

Style

The code generated should be a little easier to follow – if/else ordering, no redundant casts or extra brackets etc.

Namespace

I don’t believe adding “this” everywhere or fully qualifying attributes and exceptions makes things easy to read. I realize this might cause some name conflicts for some people but it is easy to change yourself and means the code is shorter and easier to work with for the majority.

Getting started

Although I work on the LINQ to SQL team this template should be treated as a third-party sample and is not supported by Microsoft.

Download via CodePlex

  1. Add the L2ST4.ttinclude and either CSharpDataContext.tt or VBNetDataContext.tt to your project depending on your language type
  2. Rename the xDataContext.tt to match your DBML file but with .tt extension instead of .dbml
  3. Set the existing DBML file’s .designer.cs/vb Build Action property to None to ignore the LINQ to SQL built-in code generation

Note that the template will only regenerate when it has been changed so use Run Custom Tool from the template’s right-mouse button menu in Solution Explorer when you’ve changed the DBML.

Should you wish to switch back to using the designer code then set the DBML file’s .designer.cs/vb Build Action to Compile and either remove the .tt and .ttinclude file for permanent removal or just set the .generated.cs/vb Build Action to None to keep it around. VB.NET users will need to use Show All Files to see the .designer.vb file.

Customization

The template run-time built into Visual Studio 2008 is called T4 and requires no additional tools however if you do a lot of editing you might want to install the Clarius T4 Editor for syntax highlighting and also check out the treasure trove of T4 material that is Oleg Sych’s blog.

The template is simple to follow, it loads the DBML file as an XML document then uses LINQ to XML to instantiate wrapper objects over the elements. This gives you a simple way to change default naming and behavior while making the template simpler to work with.

Let me know how you get on by leaving a comment here.

[)amien