Usually you can't assign null to value types, such as int, bool, DateTime, etc. You can make them nullable by adding ? symbol after type declaration of variable/field/property:
int? nullableNumber = null;
But does nullableNumber become reference variable?
The .NET Framework Class Library is the foundation on which .NET Framework applications, components, and controls are built. The .NET Framework includes classes, interfaces, and value types that expedite and optimize the development process and provide access to system functionality. To facilitate interoperability between languages, most .NET Framework types are CLS-compliant and can therefore be used from any programming language whose compiler conforms to the Common Language Specification (CLS).
LINQ is a set of features that extends powerful query capabilities to the language syntax of C#. LINQ introduces standard, easily-learned patterns for querying and updating data, and the technology can be extended to support potentially any kind of data store. The .NET Framework includes LINQ provider assemblies that enable the use of LINQ with .NET Framework collections, SQL Server databases, ADO.NET Datasets, and XML documents.
JIT (just-in-time) compilation converts MSIL to native code on demand at application run time, when the contents of an assembly are loaded and executed. Because the common language runtime supplies a JIT compiler for each supported CPU architecture, developers can build a set of MSIL assemblies that can be JIT-compiled and run on different computers with different machine architectures.
The Common Language Runtime (CLR) is the virtual machine component of .NET framework. It manages the execution programs written with .NET languages. A process known as just-in-time compilation converts compiled IL (Intermediate Language) code into machine instructions which the computer's CPU then executes.
Plain Old CLR Object or POCO is a play on the term POJO, from the Java programming world (which was coined by Martin Fowler in 2000) and is used by developers targeting the Common Language Runtime of the .NET Framework. POCO is a concept that has been the source of many misconceptions because of the similarity with the concept of DTO (Data Transfer Object).
ADO.NET is a feature of Microsoft .NET Framework that you can use to access and manipulate data. It is a part of the base class library that is included with the .NET Framework. It is commonly used by programmers to access and modify data stored in relational database systems, though it can also access data in non-relational sources.
.NET Framework is a software framework developed by Microsoft, which significantly simplifies software development. It includes a large class library known as Framework Class Library (FCL) and provides language interoperability across several programming languages, which means each one of them can use code written in any of other languages. Programs written for .NET Framework execute in a software environment, known as Common Language Runtime (CLR), an application virtual machine. This unified framework offers a number of benefits to developers including code reuse, simplified debugging, and improved security. FCL and CLR together constitute .NET Framework.