C# vs .NET vs the CLR: How C# Code Runs

"What is the difference between C# and .NET?" is often the very first question. The short answer: C# is the language you write. .NET is the platform that runs it.

The interview-ready answer

C# is a modern, object-oriented, type-safe programming language. .NET is the developer platform that provides the runtime, the base class library and the tools needed to build and run applications. The C# compiler turns your code into an intermediate form, and the .NET runtime turns that into machine code while the program runs.

From source code to a running program

 Program.cs  --(Roslyn compiler)-->  Program.dll  --(CLR + JIT)-->  machine code  -->  your OS
  (C# source)                       (IL + metadata)                 (runs natively)

1. You write C# source code (Program.cs).
2. The Roslyn compiler compiles it into Intermediate Language (IL), stored with metadata in an assembly (.dll or .exe).
3. When you run it, the CLR (Common Language Runtime) loads the assembly. The JIT (just-in-time) compiler turns IL into native machine code for the current CPU, method by method, the first time each method is needed.
4. The CLR then manages the running program: memory and garbage collection, threads, type safety and exception handling.

Because the output is IL, not machine code, the same assembly runs on Windows, Linux and macOS, as long as the right .NET runtime is installed. Modern .NET can also precompile ahead of time (ReadyToRun or Native AOT) to start faster.

The vocabulary

Managed code runs under the CLR, which handles memory and safety for you. Unmanaged code (native C or C++ libraries, operating system APIs) runs outside the CLR, so you manage its resources yourself, for example with .

C# keywords are aliases for .NET types

Keywords such as , and are C# names for types in the base class library: , and .

The first line prints the runtime you are using. The next two show that and are the same type, and that is just a friendlier name for .

.NET Framework versus modern .NET

New projects should use modern .NET. Releases alternate: the even-numbered ones (6, 8, 10) are long-term support.

Common wrong answers

• "C# is a Windows-only desktop tool and .NET is a library inside Visual Studio." Both are cross-platform and independent of any editor.
• "C# compiles directly to machine code." It compiles to IL first, and the JIT produces machine code later.
• "C# and .NET are the same thing." Other languages (F#, VB.NET) also run on .NET and can use the same libraries.

Follow-up questions

• Can code written in another language call C# code? Yes. F# and VB.NET also compile to IL, so assemblies can reference each other.
• What does the JIT do? It compiles IL to optimized machine code at run time. Modern .NET uses tiered compilation: quick code first, then a faster version for methods that run often.
• What is the difference between an assembly and a namespace? An assembly is a compiled file (deployment). A namespace is a logical grouping of type names (organization). One assembly can contain many namespaces, and a namespace can span assemblies.

Practise

1. Run the example above, then add and note the CLR version.
2. Build a project with and find the under . Open it in a decompiler such as ILSpy to see the IL.
3. Take the Types, Strings and OOP quiz after you finish the next two articles.