Alternative implementations of parts of the framework

1. Microsoft's .NET Micro Framework is a .NET platform for extremely resource-constrained devices. It includes a small version of the .NET CLR and supports development in C# (though some developers were able to use VB.NET, albeit with an amount of hacking, and with limited functionalities) and debugging (in an emulator or on hardware), both using Microsoft Visual Studio. It also features a subset of the .NET base class libraries (about 70 classes with about 420 methods), a GUI framework loosely based on Windows Presentation Foundation, and additional libraries specific to embedded applications.

2. Monois an implementation of the CLI and the .NET Base Class Library (BCL), and provides additional functionality. It is dual-licensed under free software and proprietary software licenses. It includes support for ASP.NET, ADO.NET, and Windows Forms libraries for a wide range of architectures and operating systems. It also includes C# and VB.NET compilers.

3. Portable.NET(part of DotGNU) provides an implementation of the Common Language Infrastructure (CLI), portions of the .NET Base Class Library (BCL), and a C# compiler. It supports a variety of CPUs and operating systems.

4. Microsoft's Shared Source Common Language Infrastructure is a non-free implementation of the CLR component of the .NET Framework. However, the last version only runs on Microsoft Windows XP SP2, and was not updated since 2006, therefore it does not contain all features of version 2.0 of the .NET Framework.

5.CrossNet is an implementation of the CLI and portions of the .NET Base Class Library (BCL). It is free software using the open source MIT License.

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Drawbacks of .NET framework

1. The garbage-collector, which is integrated into the environment, can introduce unanticipated delays of execution over which the developer has little direct control, and it can cause runtime memory size to be larger than expected. "In large applications, the number of objects that the garbage collector needs to deal with can become very large, which means it can take a very long time to visit and rearrange all of them."

2. Newer versions of the framework (3.5 and up) are not pre-installed in versions of Windows below Windows 7 (although newer versions are available via Windows Update). For this reason, applications must lead users without the framework through a procedure to install it.

3. .NET platform is not cross platform, instead it supports only Windows

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JIT compilation in Dot Net Framework

The .NET Framework contains one or more JIT compilers that compile your IL code down to machine code, or code that is CPU-specific. This is done when the application is executed for the first time.

            You will notice this process after you build your first ASP.NET page. After you build any ASP.NET page, you compile the page down to IL. When you go to the browser and call the page by typing its URL in the address bar, you notice a slight pause of a few seconds as the computer seems to think about what it is doing. It is actually calling this IL code and converting it with a JIT compiler to machine code. This happens only the first time that someone requests the page. After the first time, you can hit F5 to refresh the page, and the page is immediately executed. The page has already been converted to machine code and is now stored in memory. The CLR knows the JIT compiler has already compiled the page. Therefore, it gets the output of the page from memory. If you later make a change to your ASP.NET page, recompile, and then run the page again, CLR detects that there was a change to the original file. It uses the JIT compiler once again to compile the IL code down to machine code.

            The JIT compiler, as it compiles to machine code, makes sure that the code is type safe. It does this to ensure that objects are separate, thereby making certain that objects won’t unintentionally corrupt one another.

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Features of Dot Net Framework

1. Interoperability -Because computer systems commonly require interaction between newer and older applications, the .NET Framework provides means to access functionality implemented in newer and older programs that execute outside the .NET environment. Access to COM components is provided in the System.Runtime.InteropServices and System.EnterpriseServices namespaces of the framework; access to other functionality is achieved using the P/Invoke feature.

2. Common Language Runtime engine - The Common Language Runtime (CLR) serves as the execution engine of the .NET Framework. All .NET programs execute under the supervision of the CLR, guaranteeing certain properties and behaviours in the areas of memory management, security, and exception handling.

3. Language independence - The .NET Framework introduces a Common Type System, or CTS. The CTS specification defines all possible datatypes and programming constructs supported by the CLR and how they may or may not interact with each other conforming to the Common Language Infrastructure (CLI) specification. Because of this feature, the .NET Framework supports the exchange of types and object instances between libraries and applications written using any conforming .NET language.

4. Base Class Library - The Base Class Library (BCL), part of the Framework Class Library (FCL), is a library of functionality available to all languages using the .NET Framework. The BCL provides classes that encapsulate a number of common functions, including file reading and writing, graphic rendering, database interaction, XML document manipulation, and so on. It consists of classes, interfaces of reusable types that integrates with CLR(Common Language Runtime).

5. Simplified deployment - The .NET Framework includes design features and tools which help manage the installation of computer software to ensure it does not interfere with previously installed software, and it conforms to security requirements.

6. Security - The design addresses some of the vulnerabilities, such as buffer overflows, which have been exploited by malicious software. Additionally, .NET provides a common security model for all applications.

7. Portability - While Microsoft has never implemented the full framework on any system except Microsoft Windows, it has engineered the framework to be platform-agnostic, and cross-platform implementations are available for other operating systems (see Silverlight and the Alternative implementations section below). Microsoft submitted the specifications for the Common Language Infrastructure (which includes the core class libraries, Common Type System, and the Common Intermediate Language), the C# language, and the C++/CLI language to both ECMA and the ISO, making them available as official standards. This makes it possible for third parties to create compatible implementations of the framework and its languages on other platforms.

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The Common Language Runtime

Many different languages and platforms provide a runtime, and the .NET Framework is no exception. You will find, however, that this runtime is quite different from most. 

            The Common Language Runtime (CLR) in the .NET Framework manages the execution of the code and provides access to a variety of services that will make the development process easier.

            The CLR has been developed to be far superior to previous runtimes, such as the VB runtime, by attaining the following:
            1. Cross-language integration
            2. Code access security
            3. Object lifetime management
            4. Debugging and profiling support

Code that is compiled and targeted to the CLR is known as managed code. Managed code provides metadata that is needed for the CLR to provide the services of multilanguage support, code security, object lifetime management, and memory management.

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