Base class library changes
Many new APIs have been added around the framework to enable key scenarios. You will notice the following changes and additions:
Additional collections implement ReadOnlyCollection such as Queue and Stack.
Additional members support the task-based asynchronous pattern (TAP) such as Task.CompletedTask and NamedPipeClientStream.ConnectAsync.
The CultureInfo.CurrentCulture and CultureInfo.CurrentUICulture properties are now read-write rather than read-only. If you assign a new CultureInfo object to these properties, the current thread culture defined by the Thread.CurrentThread.CurrentCulture property and the current UI thread culture defined by the Thread.CurrentThread,CurrentUICulture properties also change.

64-bit JIT compiler for managed code
This release includes a new version of the 64-bit JIT compiler, which provides significant performance improvements over the existing 64-bit JIT compiler.

.NET Native
Windows Store apps written in C# that target the .NET Framework can now take advantage of a new technology that compiles apps to native code rather than IL. They produce apps characterized by faster startup and execution times. For more information, see Compiling Apps with .NET Native. For an overview of .NET Native that examines how it differs from both JIT compilation and NGEN and what that means for your code.
You can compile your apps to native code by configuring your project in Visual Studio.
To support debugging .NET Native apps, a number of new interfaces and enumerations have been added to the unmanaged debugging API.

ASP.NET 5
ASP.NET 5 is a lean .NET platform for building modern cloud-based apps. The platform is modular so you can include only those features that are needed in your application. It can be hosted on IIS or self-hosted in a custom process, and you can run apps with different versions of the framework on the same server. It includes a new environment configuration system that is designed for cloud deployment.
MVC, Web API, and Web Pages are unified into a single framework called MVC 6. You build ASP.NET 5 apps through the new tools in Visual Studio 14. Your existing applications will work on the new .NET framework; however to build an app that uses MVC 6 or SignalR 3, you must use the project system in Visual Studio 14.

Resizing in Windows Forms controls.
This feature has been expanded to include the DomainUpDown, NumericUpDown, DataGridViewComboBoxColumn, DataGridViewColumn and ToolStripSplitButton types.
This is an opt-in feature. To enable it, set the EnableWindowsFormsHighDpiAutoResizing element to true in the application configuration (app.config) file:




Support for code page encodings
.NET Core primarily supports the Unicode encodings and by default provides limited support for code page encodings. You can add support for code page encodings available in the .NET Framework but unsupported in .NET Core by registering code page encodings with the Encoding.RegisterProvider method.

Improvements to event tracing
An EventSource object can now be constructed directly and you can call one of the Write``1 methods to emit a self-describing event.

Open-source .NET Framework packages
Some great .NET packages such as the Immutable Collections and SIMD APIs are now available, open source, on GitHub.