The theory of object-oriented programming (OOP) has been used as a paradigm in the development of software engineering that has lasted over a few decades. Although software industry is changing rapidly with other languages and paradigm, OOP now is very much present in the design and architecture of present-day systems. An example of the survival of theoretical concepts is the .NET platform which started with early Common Language Runtime (CLR) and more recently modern ASP.NET Core framework and more up-to-date versions of the C# programming language, covering encapsulation, polymorphism, inheritance and abstraction.
This paper therefore seeks to address how the fundamental OOP concepts were used in creating the .NET ecosystem and how it mutated. It aims to understand what these principles are embodied in such aspects as runtime behaviors, language features, framework architecture, and design practice. The paper, using qualitative thematic synthesis on 30 peer-reviewed scholarly articles, theses, technical reports, and case-based assessments fuses theoretical framework with practice forms of implementation at various levels of .NET.
The findings reveal a consistent alignment between .NET's design philosophy and object-oriented theory. These values have been retained by key features like use of generics, dependency injection, interface programming, and popularization of design patterns. Additionally, more recent C# additions such as LINQ, immutable records, pattern matching, and async/ await reveal a practical shift to hybridization: merging the idea of functional programming performance and structure, with OO program modularity. Quantitative measurements indicated multi-threaded queries performed at 25%-35% higher level using PLINQ vs. the traditional LINQ, in multicore scenarios. The boxing overhead was minimized and memory consumption improved by up to 20% through the use of generic collections in .NET. Entity Framework queries with LINQ demonstrated an increase of up to 30% in readability and maintainability with no decrease in the performance at run time.
These findings indicate that OOP still offers sustainable and flexible model of regulating software complexity especially in large-sized enterprise systems. Although it is a subject of discussions regarding its theoretical limitations, the real-life experience of the evolution of .NET platforms clearly points to that the OOP is relevant in the development of scalable, maintainable, and robust applications.
The study comes to the conclusion that the OOP theory is not the one that is only historically important but the one that is actively used in designing the more recent programming platform, such as the .NET. With the current trend toward hybrid and multiparadigm languages, the interface of the OOP theory to such systems as the .NET platform would provide a great point of view in both educational and business spheres. The study confirms both the current relevance of OOP in the current software infrastructure and predisposes the chance to research the paradigm convergence, language design, and architectural resilience in the high-scale environments.

NET, Object-Oriented Programming, C#, Generics and Parametric Polymorphism

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