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The best way to learn what’s new in ASP.NET 4.5 Web Forms, is to work through the latest tutorial series by Erik Reitan: Getting Started with ASP.NET 4.5 Web Forms.
The 10 tutorials in the series progressively guide you through building a Web Forms application, which is a simplified example of a store front web site that sells items online. Each tutorial builds on the previous tutorial. When you’re finished, you’ll be familiar with common ASP.NET concepts, as well as many of the new features available in ASP.NET 4.5, including effective use of the new model binding features.
In addition, you’ll learn about, Entity Framework Code First, unobtrusive validation, the Anti-XSS library, HTML5, data annotations, Local DB, value providers, PayPal integration, ASP.NET Membership, URL routing, and more.
Here's an outline of the tutorial series:
Introduction and Overview
Create the Project
Create the Data Access Layer
UI and Navigation
Display Data Items and Details
Checkout and Payment with PayPal
Membership and Administration
ASP.NET Error Handling