Programming Entity Framework: DbContext

Preface

Microsoft’s principal data access technology, ADO.NET Entity Framework, has had two major releases as part of the .NET Framework. .NET 3.5 brought us the first version of Entity Framework, which is covered in the first edition of Programming Entity Framework (O’Reilly). In 2010, Microsoft .NET 4 was released, containing the next version of Entity Framework, referred to as Entity Framework 4. The completely revised second edition of Programming Entity Framework (O’Reilly) was dedicated to teaching readers how to use this version of Entity Framework in Visual Studio 2010.

When .NET 4 was released, the Entity Framework team was already hard at work on a new addition, called Code First, to provide an alternative way of building the Entity Data Model that is core to Entity Framework. Rather than using a visual designer, Code First allows you to create the model from your existing classes. At the same time, the team devoted resources to making Entity Framework easier to use. They focused on the most commonly used features and tasks in Entity Framework and built a new API called the DbContext API.

This book is dedicated to teaching readers how to use the features of the DbContext API. In addition to the DbContext class, you’ll find the DbSet class for performing set operations, improved APIs for change tracking and handling concurrency conflicts, and a Validation API that integrates with validation features already present in .NET.

In this book, you will learn how to query and update data using the new API, whether you are working with individual objects or graphs of objects and their related data. You’ll learn how to take advantage of the change tracking features and Validation. You’ll find myriad samples and delve into taking advantage of advanced features presented by the API.

Audience

This book is designed for .NET developers who have experience with Visual Studio and database management basics. Prior experience with Entity Framework is beneficial but not required. The code samples in the book are written in C#, with some of these samples also expressed in Visual Basic. There are a number of online tools you can use to convert snippets of C# into Visual Basic.

Contents of This Book

This book contains nine chapters.

Chapter 1, Introducing the DbContext API

This chapter provides a high-level, end-to-end overview of the DbContext API. You’ll learn why the Entity Framework team decided to create the DbContext API and how it makes the Entity Framework easier to use. You’ll find example code, but there are no walkthroughs in this first chapter.

Chapter 2, Querying with DbContext

In this chapter you’ll learn about retrieving data from the database using Entity Framework’s query capabilities. You’ll learn how to find an entity based on its key and how to load all entities of a given type. You’ll learn how to use Language Integrated Query (LINQ) to sort and filter data. This chapter also explores the various strategies for loading related data.

Chapter 3, Adding, Changing, and Deleting Entities

Once you’ve learned how to query for data, this chapter will cover how to make changes to that data and save those changes to the database. You’ll see how to add new data as well as change and delete existing data. You’ll learn how Entity Framework keeps track of changes as you make them and how it saves them using the SaveChanges method.

Chapter 4, Working with Disconnected Entities Including N-Tier Applications

In this chapter, you’ll learn about using Entity Framework to persist changes that were made to entities while they were not being managed by a context. This challenge is most common in N-Tier applications where a server component is responsible for retrieving data and returning it to a client application. The client application then modifies this data and sends it back to the server to be saved. You’ll learn about various approaches to solving this challenge and how the Change Tracker API can be used to implement them.

Chapter 5, Change Tracker API

The Change Tracker API is first introduced in Chapter 4 and this chapter is dedicated to exploring the remaining functionality of the change tracker. You’ll learn how to access the information that Entity Framework keeps about the state of your entity instances. You’ll also learn about the operations that can be performed from the Change Tracker API, including refreshing an entity from the database. This chapter wraps up with some examples of how the Change Tracker API can be used to solve some common application requirements.

Chapter 6, Validating with the Validation API

Chapter 6 introduces the new Validation API that integrates with the DbContext and how it can be used to validate changes to your data before they are sent to the database. This chapter covers how the Validation API makes use of the existing validation functionality included in the .NET Framework. You’ll learn how validation is integrated into the SaveChanges pipeline and how you can also trigger validation manually. You’ll learn how to set up validation rules and how to inspect validation errors when your data violates these rules.

Chapter 7, Customizing Validations

This chapter explores some more advanced features of the Validation API, which was introduced in Chapter 6. You’ll learn how to customize the logic used to validate entities, including customizing the logic that determines which entities need to be validated. These advanced techniques will allow you to write validation that interacts with the context, which opens up more validation possibilities, such as validating the uniqueness of a column. This chapter will also provide guidance regarding the dangers of using the Validation API for tasks other than validation.

Chapter 8, Using DbContext in Advanced Scenarios

Chapter 8 is devoted to covering some advanced functionality that’s available in the DbContext API. You’ll learn about techniques for unit testing and how to write tests that don’t hit a database. You’ll also see how to bypass Entity Framework’s query pipeline and interact directly with the database when the need arises. Should your requirements exceed what is possible from the DbContext API, you’ll see how to drop down to the underlying ObjectContext API. The chapter wraps up with a look at creating smaller bounded contexts that allow you to interact with a subset of your complete model.

Chapter 9, What’s Coming Next for Entity Framework

This book was written based on the features of the DbContext API available in the Entity Framework 4.3 release. At the time of writing, there are a number of previews available that demonstrate some of the features that the DbContext API will gain in upcoming releases. This chapter shares available information about these future releases.

Conventions Used in This Book

The following typographical conventions are used in this book:

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Constant width

Used for program listings, as well as within paragraphs to refer to program elements such as variable or function names, databases, data types, environment variables, statements, and keywords.

Constant width bold

Shows commands or other text that should be typed literally by the user.

Constant width italic

Shows text that should be replaced with user-supplied values or by values determined by context.

Tip

This icon signifies a tip, suggestion, or general note.

Caution

This icon indicates a warning or caution.

Using Code Examples

This book is here to help you get your job done. In general, you may use the code in this book in your programs and documentation. You do not need to contact us for permission unless you’re reproducing a significant portion of the code. For example, writing a program that uses several chunks of code from this book does not require permission. Selling or distributing a CD-ROM of examples from O’Reilly books does require permission. Answering a question by citing this book and quoting example code does not require permission. Incorporating a significant amount of example code from this book into your product’s documentation does require permission.

We appreciate, but do not require, attribution. An attribution usually includes the title, author, publisher, and ISBN. For example: “Programming Entity Framework: DbContext by Julia Lerman and Rowan Miller (O’Reilly). Copyright 2012 Julia Lerman and Rowan Miller, 978-1-449-31296-1.”

If you feel your use of code examples falls outside fair use or the permission given above, feel free to contact us at [e-mail].

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Acknowledgments

We are grateful for the people who spent their precious free time reading through and even trying the walkthroughs in this book and providing feedback. Thanks to Rowan’s teammates, Arthur Vickers, Pawel Kadluczka, and Diego Vega, for their help in ensuring our accuracy throughout this book. Roland Civet, Mikael Eliasson, and Daniel Wertheim also provided invaluable feedback that helped us fine-tune our explanations and our code.

Thanks to Microsoft for making it possible for Rowan to participate in this project.

Thanks once again to O’Reilly Media, especially our editors, Meghan Blanchette and Rachel Roumeliotis, for their support, their copyediting, and their extreme patience as many schedule conflicts delayed our promised deadlines.