ILSpy

ILSpy is a free, open-source tool that lets you look inside compiled .NET programs — software built with C# or other .NET languages — even when you don't have the original source code. Compiled programs are normally stored as binary files (called assemblies) that computers can run but humans can't easily read. ILSpy reverses that process (called decompilation) and reconstructs readable C# code from those binary files. This is useful for understanding how a library works, debugging a dependency, or inspecting a program when the source has been lost.

The tool works by reading .NET assembly files, parsing the low-level instructions (called Intermediate Language, or IL), and converting them back into human-readable C# code. It also supports browsing assembly metadata (information embedded in the file like type definitions and method signatures), navigating between types and methods with hyperlinks, and exporting entire projects worth of decompiled code. It handles modern .NET formats including ReadyToRun binaries, which are pre-compiled for faster startup.

You would use ILSpy when you need to understand what a third-party .NET library does internally, when you want to debug an issue in a dependency you don't have source for, or when you are auditing software for security or compliance. It runs cross-platform on Windows, Mac, and Linux, and integrates with Visual Studio and Visual Studio Code as an extension. The tech stack is C# targeting .NET 10.