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.NET Assembly Decompilation with ILSpy

ilspy-decompile

Understand implementation details of .NET code by decompiling assemblies. Use when you want to see how a .NET API works internally, inspect NuGet package source, view framework implementation, or understand compiled .NET binaries.

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SKILL.md

Full skill instructions

.NET Assembly Decompilation with ILSpy

Use this skill to understand how .NET code works internally by decompiling compiled assemblies.

Prerequisites

  • .NET SDK installed
  • ILSpy command-line tool available via one of the following:
    • dnx ilspycmd (if available in your SDK or runtime)
    • dotnet tool install --global ilspycmd

Both forms are shown below. Use the one that works in your environment.

Note: ILSpyCmd options may vary slightly by version.
Always verify supported flags with ilspycmd -h.

Quick start

# Decompile an assembly to stdout
ilspycmd MyLibrary.dll
# or
dnx ilspycmd MyLibrary.dll

# Decompile to an output folder
ilspycmd -o output-folder MyLibrary.dll

Common .NET Assembly Locations

NuGet packages

~/​.nuget/​packages/<package-name>/<version>/​lib/<tfm>/

.NET runtime libraries

dotnet --list-runtimes

.NET SDK reference assemblies

dotnet --list-sdks

Reference assemblies do not contain implementations.

Project build output

./​bin/​Debug/​net8.0/<AssemblyName>.dll
./​bin/​Release/​net8.0/​publish/<AssemblyName>.dll

Core workflow

  1. Identify what you want to understand
  2. Locate the assembly
  3. List types
  4. Decompile the target

Commands

Basic decompilation

ilspycmd MyLibrary.dll
ilspycmd -o ./​decompiled MyLibrary.dll
ilspycmd -p -o ./​project MyLibrary.dll

Targeted decompilation

ilspycmd -t Namespace.ClassName MyLibrary.dll
ilspycmd -lv CSharp12_0 MyLibrary.dll

View IL code

ilspycmd -il MyLibrary.dll

Notes on modern .NET builds

  • ReadyToRun images may reduce readability
  • Trimmed or AOT builds may omit code
  • Prefer non-trimmed builds

Legal note

Decompiling assemblies may be subject to license restrictions.