C sharp - .NET Compiler Platform (Roslyn)

Roslyn is the modern compiler platform for C# and VB.NET.
Unlike older compilers, Roslyn exposes the compiler itself as a set of APIs.

This means:
You can analyze, inspect, and even generate C# code programmatically.

1. What is Roslyn?

Before Roslyn:

  • The C# compiler was a black box.

  • It took code → produced IL.

  • Developers could not interact with its internal logic.

After Roslyn:

  • The compiler is written in C#.

  • It exposes APIs.

  • You can access syntax trees, semantic models, symbols, diagnostics.

Roslyn makes the compiler programmable.

2. Core Architecture

Roslyn has two main layers:

1. Syntax Layer

Represents code structure (parsing only).

Example:

int x = 5;

Is converted into a syntax tree like:

CompilationUnit
 └── VariableDeclaration
     ├── Type (int)
     ├── Identifier (x)
     └── Literal (5)

This is called a Syntax Tree.

2. Semantic Layer

Understands meaning of code.

Example:

  • Is int a valid type?

  • Is x defined?

  • Is there type mismatch?

  • Is method overloaded?

This is called the Semantic Model.

3. Syntax Tree Example

using Microsoft.CodeAnalysis;
using Microsoft.CodeAnalysis.CSharp;

string code = "int x = 5;";
var tree = CSharpSyntaxTree.ParseText(code);

var root = tree.GetRoot();
Console.WriteLine(root.Kind());

This parses C# code into structured objects.

4. Code Analysis with Roslyn

Roslyn enables:

  • Static code analysis

  • Custom linting rules

  • Code quality tools

  • Security scanning

  • Refactoring tools

Example:
You can detect:

  • Unused variables

  • Naming violations

  • Hard-coded strings

  • Performance issues

5. Creating a Code Analyzer

Roslyn allows creating analyzers that:

  • Inspect syntax

  • Report diagnostics

  • Suggest fixes

Example:
If variable name is single letter → show warning.

This is how Visual Studio shows:

  • Red squiggly errors

  • Suggestions

  • Quick fixes

6. Code Generation

Roslyn can generate C# code programmatically.

Example:
Auto-generate:

  • DTO classes

  • API clients

  • Builders

  • Mapping code

Instead of writing manually, you generate syntax trees and output code.

7. Source Generators

Introduced in modern .NET.

They:

  • Run at compile time

  • Generate additional C# files

  • Improve performance (no reflection needed)

Example use cases:

  • Auto property generation

  • Dependency injection wiring

  • JSON serializers

Source generators improve:

  • Startup time

  • Runtime performance

  • Type safety

8. Refactoring Engines

When you use:

  • Rename variable

  • Extract method

  • Convert to async

That feature is built using Roslyn APIs.

Roslyn enables deep refactoring safely.

9. Difference Between Reflection and Roslyn

Reflection Roslyn
Works at runtime Works at compile time
Inspects compiled assemblies Inspects source code
Slower More powerful for code analysis
Cannot modify source Can generate/transform code

Roslyn is more powerful for tooling.

10. Real-World Applications

Roslyn is used in:

  • Visual Studio

  • Rider IDE

  • Static analyzers (SonarQube)

  • Code formatters

  • Custom enterprise tooling

  • AI-assisted code refactoring tools

11. Why Roslyn is Important

Modern C# ecosystem relies on:

  • Code analyzers

  • Code fix providers

  • Source generators

  • Advanced IDE tooling

All powered by Roslyn.

If you build:

  • Large enterprise systems

  • Developer tools

  • Static analysis engines

Roslyn knowledge is extremely valuable.