.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.