Using LINQ to SharePoint

The LINQ to SharePoint provider is a new feature in SharePoint 2010 that allows you to use a strongly-typed entity model and the language integrated query (LINQ) query syntax to query list data. Essentially, LINQ to SharePoint hides the complexity of developing CAML queries from developers, which can reduce development time and make code more readable. The LINQ to SharePoint provider converts the LINQ expressions into CAML queries at run time.

Using LINQ to SharePoint in your own solutions consists of three main steps:

• Generate the entity classes. Before you can start writing LINQ queries against your SharePoint lists, you must create or generate the strongly-typed entity classes that represent your list data and lookup column relationships.
• Develop the solution. After you add the entity classes to your Visual Studio 2010 project, you can write LINQ queries against the strongly-typed entities that represent your data model.
• Run the solution. At run time, the LINQ to SharePoint provider dynamically converts your LINQ expressions into CAML queries, executes the CAML, and then maps the returned items to your strongly-typed data entities.

Although you can manually develop your entity classes, in most cases, you will want to use the SPMetal command line tool. This is included in SharePoint Foundation 2010 and can be found in the BIN folder in the SharePoint root. The SPMetal tool targets an individual SharePoint site and, by default, generates the following code resources:

• A data context class that derives from DataContext. This is the top-level entity class. It represents the content of your site and provides methods that allow you to retrieve list entities. The data context class uses the EntityList<TEntity> class to represent the lists in your site, where TEntity is a class that represents a content type.
• Classes that represent content types. These are marked with the ContentTypeAttribute. Content type classes are generated for implicit content types as well as content types that are explicitly defined on the site. For example, if a user adds a column to an existing list, the user is creating an implicit content type and a representative class will be generated.
• Classes and properties that represent relationships between lists. SPMetal can detect relationships based on lookup columns. Within the entity class that represents a content type, SPMetaluses the EntityRef<TEntity> class to represent the singleton side of a one-to-many relationship and the EntitySet<TEntity> class to represent the “many” side of one-to-many or many-to-many relationships (known as a reverse lookup). Properties that are mapped to a field in a related list are decorated with the AssociationAttribute.

Reference: 

http://msdn.microsoft.com/en-us/library/ff798478.aspx

http://blogs.msdn.com/b/ocarpen/archive/2011/01/18/manage-linq-to-sharepoint-entities-with-linq-to-sharepoint-dsl-extension.aspx