How To Implement A Generic Entity List Repository And Business Logic For SharePoint 2010 Using The T4 Templating Engine

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Introduction

The SharePoint API offers developers great possibilities to access all kinds of information, especially data stored in SharePoint lists. But in real-world scenarios and complex SharePoint projects, you often need a more efficient way to access list data. Therefore I've developed a solution that is implementing a generic entity list repository which will be partially created using the T4 templating engine, also known as the Text Template Transformation Toolkit from Microsoft. The template engine is part of Visual Studio 2008 and 2010. For more information about T4 toolkit, check out this link.

The entity list repository provides a type-safe and extensible business logic layer (BAL) and data access layer (DAL) for SharePoint lists based on content types. Each list repository is associated with a specific SharePoint content type, but multiple list repositories can point to the same list. For the associated content type, the T4 template files will automatically generate an entity class that will be used for the list repository.

The SharePoint solution we will create with the Visual Studio 2010 Tools for SharePoint Development and C# contains three projects. The repository project containing the entities and repository code, a T4 templates helper project with code to support and simplify the template generation process as well as the SharePoint project itself, that will use the repository.

The SharePoint solution creates a new content type named Mail Template and a list called Mail Templates associated with the content type. The mail template itself contains the type name, subject, body and BCC information. The following code fragment shows an example how to use the repository:

MailTemplateRepository mr = new MailTemplateRepository();
MailTemplate mt = mr.GetByType("INVITATION");    

The GetByType method is a repository method that internally queries the SharePoint list by executing a CAML query. The query result is automatically converted into the MailTemplate entity and returned to the caller.

For each repository, the developer can define an execution scope. The scope allows to define if an entity update or deletion shall be executed with the SystemUpdate or Update method of the SPList class or if the task shall be executed with privileged rights. There are also other setting options available. The following sample defines a entity repository scope which uses the SystemUpdate method, does not trigger any list item events and executes the update with privileged rights (using RunWithElevatedPrivileges):

MailTemplateRepository mr = new MailTemplateRepository();
mr.Scope = new EntityRepositoryScope(true); 

MailTemplate mt = mr.GetByType("INVITATION"); 
mt.Subject = "...";
mt.Body = "...";  

mr.Update(mt); 

Scoping is very useful in many scenarios where developers have to update or delete entities without programming special logic themselves. It just helps in unifying this process.

The Repository Project

There are basically three main classes in the repository project: Entity, EntityRepository and EntityRepositoryScope. The classes Entity and EntityRepository are used to create specific entity and repository classes, like the MailTemplate or MailTemplateRepository class. The EntityRepositoryScope class is used to control the update and deletion process of the SharePoint list items (see above).

The repository project contains a couple of helper and automatically created classes. Those files are all organized in special folders within the project:

Folder Attributes

This folder contains a class called FieldIDAttribute which is internally used by the EntityRepository class to read the field ID of each public entity property. The repository class is using the field ID to access the field object. The ID is a GUID value defining the field within the SharePoint list item. The template MailTemplate.tt is adding the FieldID attribute automatically to the generated entity class code.

To use system entities like the content type Item, the entity class must be created manually and the FieldID attribute needs to be added to each public property (see the Item.cs file in code). Instead of using the GUID of build-in fields, it's possible to use a special notation: [FieldID("[Title]")] for the Title field.

Folder Constants

The Constants folder contains classes with list (Lists class) and enumeration (Enums class) constants. In addition, the template file ContentTypes.tt in the subfolder Generated generates a ContentTypes class that contains a partial subclass with ID and name properties for each content type defined in the template file. The fileNames variable in the template file itself defines the content types for which code will be produced. The FieldIDs.tt template generates similar to the ContentTypes.tt template file a class with all field IDs.

Folder Contracts

All the repository interfaces are defined in this folder. The generic interface IEntityRepository is the main interface and defines all the basic methods of a repository. Each custom repository contract must be inherited from this interface, e.g. the IMailTemplateRepository interface implemented by the MailTemplateRepository class.

Folder Entities

The folder Entities contains all automatically generated and all manually created entity classes. The templatebased entity classes are all defined as partial classes and stored in the subfolder Generated, so they can be extended with attributes, methods (overrides) and base classes. The MailTemplate class generated by the T4 template (in the subfolder Generated) will look like this:

[CompilerGenerated]
public partial class MailTemplate
{ 
  [FieldID("{B0D7F541-0F86-4794-88AE-9CEEA9BC6F02}")] public string Type { get; set; }
  [FieldID("{31C4A2D3-2928-47D3-A534-9C92993538D7}")] public string Body { get; set; }
  [FieldID("{D41A10DE-5A90-4904-8758-1C6EC9EB54EB}")] public string BCC { get; set; }

  protected override string AssociatedList
    { get { return Lists.MailTemplates; } }
  protected override SPContentTypeId AssociatedContentTypeID
    { get { return ContentTypes.MailTemplate.ID; } }
} 

This class will be extended with the partial code from the MailTemplate.cs file in the folder Entities:

[Serializable]
public partial class MailTemplate : Item
{ 
  public override bool Validate()
  {
     // TODO: Implement specific object validations!
     return base.Validate();
  }

  public void SetPlaceholders(Dictionary<string, string> placeholderValues)
  {
     // TODO: Replace placeholders in subject and body texts if available!
  }
} 

In order to support object serialization, the Serializable attribute will be added to the class. The MailTemplate class as well as the content type inherits from the Item entity class and content type in SharePoint. To reflect the inheritance situation, the base class will be the Item class. The definition for the Item class looks like this:

[Serializable]
public partial class Item : Entity
{ 
  [FieldID("[Title]")]
  public string Title { get; set; }
} 

Since the base class of the Item class is the Entity class, it is possible to override a couple of methods (overrides). Those overridable methods are Validate, PrepareBeforeUpdate and others. The Validate method is overridden by the MailTemplate class to allow object validation, e.g. to check if a given BCC email address is valid or does exist in the system. The PrepareBeforeUpdate method can be used to change values before the list item update is executed.

The SetPlaceholders method for instance can be used to replace placeholders in the subject and/or body text of a mail template record. The method is not implemented yet. It only shows the many options developers have when using such a repository system.

Folder Helpers

The Helpers folder contains utility classes used by the repository.

Folder Repositories

The Repositories folder contains all custom repository classes like the MailTemplateRepository. To create a new class, the EntityRepository class is used as base class. For MailTemplateRepository, the base class is EntityRepository<MailTemplate>. The EntityRepository class is implementing the IEntityRepository interface and the custom MailTemplateRepository class is implementing the IMailTemplateRepository interface.

The specific repository class should implement at least one constructor that is calling the base class constructor with the SharePoint list name and the associated content type ID for initialization. The implementation of the GetByType method of the MailTemplateRepository class shows an example of creating a CAML query and executing it by calling the base class method Get:

public IList<MailTemplate> GetByType(string type)
{ 
  if(string.IsNullOrEmpty(type))
    throw new ArgumentNullException("type");

   return Get(
     string.Format("<Eq><FieldRef ID='{0}'/><Value Type='String'>{1}</Value></Eq>",
     FieldIDs.MailTemplate.Type, type));
} 

The Get method is transforming the CAML statement into a final query, executes the query, materializes the list items received from SharePoint and returns a list of entities. The execution is running with the default or given execution scope. Another option is to use methods that are executing CAML queries and returning a list of entities within the same repository class like the GetByType method as input for LINQ to Objects in order to sort, change or transform the list.

This article can only give you an overview of the repository logic. The best way to find out more about how to use it the system efficiently and create new repository classes is to look at the source code and do tests by yourself. The SharePoint project includes an application page (Test.aspx) which will use the MailTemplateRepository repository class to display the list data:

protected void Page_Load(object sender, EventArgs e)
{ 
   _templateRepository = new MailTemplateRepository();

   if(IsPostBack)
     return;

   var types = _templateRepository.GetTypes();

   if(types == null || types.Count == 0)
     WriteMessage("No mail templates found.");
   else
   {
     MailTemplateTypeList.DataSource = types;
     MailTemplateTypeList.DataBind();
     SetTemplateData(types[0]);
   }
}

...

protected void SetTemplateData(string type)
{ 
   MailTemplate mt = _templateRepository.GetByType(type).FirstOrDefault();

   if(mt == null)
     WriteMessage(string.Format("No mail templates of type '{0}' found.", type));
   else
   {
     SubjectLabel.Text = mt.Title ?? string.Empty;
     BodyLabel.Text = HttpUtility.HtmlEncode(mt.Body ?? string.Empty);
     BccLabel.Text = mt.BCC ?? string.Empty;
   
     Views.SetActiveView(DefaultView);
   }
} 

The next section will shortly describe how the T4 templates helper project will work.

The T4 Templating Project

This project is only used to simplify the process of template generation. It is not part of the production output nor is it deployable on a SharePoint system. Creating a custom class library which can be called during template processing is currently the only way for developers to implement reusable template helper methods. Within the template files, the class library will be referenced (in Visual Studio 2010 only):

<#@ assembly name="$(SolutionDir)MailTemplate.T4\bin\MailTemplate.T4.dll" #> 

All the templates used in this article or project are first checking which Visual Studio project is containing the project item (the content type definition). The file names of the content type definitions are defined in the template files in a variable called fileNames. The helper methods will open the files and analyze the XML/CAML definition to find out about the defined field types and IDs. That information is returned to the template code to generate the final source code.

The classes generated by the template files are decorated with the CompilerGenerated attribute. Feel free to change the template and adapt the template code to your specific needs. It is also possible to debug template code, but this goes beyond the scope of this article. The T4 templating engine or Text Template Transformation Toolkit from Microsoft is a great option to get certain things done automated.

Summary

This article gives an overview about several interesting topics in the area of SharePoint development. The repository system has helped me to develop large SharePoint project more efficiently. The T4 templating engine helped to automate annoying tasks and reduced errors by forgetting to adjust entity classes. The power of the system will come up with the number of entities you have to handle.

History

• 8^th August, 2010: Initial post