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davidfowl/dotnet-skillz/ilspy-decompile

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version: "1.0.1" name: ilspy-decompile description: Understand implementation details of .NET code by decompiling assemblies. Use when the user wants to see how a .NET API works internally, inspect NuGet package source, view framework implementation, or understand compiled .NET binaries. allowed-tools: Bash(dnx:*)

.NET Assembly Decompilation with ILSpy

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

Quick start

bash

# Decompile an assembly to stdout (no install needed with .NET 10+)
dnx ilspycmd MyLibrary.dll
 
# Decompile to an output folder
dnx ilspycmd -o output-folder MyLibrary.dll

Common .NET Assembly Locations

NuGet Packages

bash

# Windows
~/.nuget/packages/<package-name>/<version>/lib/<tfm>/
 
# Example: Newtonsoft.Json
~/.nuget/packages/newtonsoft.json/13.0.3/lib/netstandard2.0/Newtonsoft.Json.dll
 
# Example: Microsoft.Extensions.DependencyInjection
~/.nuget/packages/microsoft.extensions.dependencyinjection/8.0.0/lib/net8.0/Microsoft.Extensions.DependencyInjection.dll

.NET Runtime Libraries

bash

# Find .NET install location
dotnet --list-runtimes
 
# Windows typical paths
C:/Program Files/dotnet/shared/Microsoft.NETCore.App/<version>/
C:/Program Files/dotnet/shared/Microsoft.AspNetCore.App/<version>/
 
# Linux/macOS typical paths
/usr/share/dotnet/shared/Microsoft.NETCore.App/<version>/
/usr/share/dotnet/shared/Microsoft.AspNetCore.App/<version>/
 
# Example: System.Text.Json from runtime
C:/Program Files/dotnet/shared/Microsoft.NETCore.App/8.0.0/System.Text.Json.dll

.NET SDK Reference Assemblies

bash

# Find SDK location
dotnet --list-sdks
 
# Reference assemblies (design-time facades)
C:/Program Files/dotnet/packs/Microsoft.NETCore.App.Ref/<version>/ref/net8.0/

Project Build Output

bash

# Debug build
./bin/Debug/net8.0/<AssemblyName>.dll
 
# Published output
./bin/Release/net8.0/publish/<AssemblyName>.dll

Core workflow

Identify what you want to understand (API, class, method)
Locate the assembly containing that code
Use dnx ilspycmd -l class to find the exact type name
Decompile the specific type with -t

Commands

Basic Decompilation

bash

# Decompile to stdout
dnx ilspycmd MyLibrary.dll
 
# Decompile to output directory
dnx ilspycmd -o ./decompiled MyLibrary.dll
 
# Decompile as compilable project
dnx ilspycmd -p -o ./project MyLibrary.dll
 
# Decompile with nested namespace folders
dnx ilspycmd -p -o ./project --nested-directories MyLibrary.dll

Targeted Decompilation

bash

# Decompile a specific type
dnx ilspycmd -t Namespace.ClassName MyLibrary.dll
 
# Decompile with specific C# version
dnx ilspycmd -lv CSharp12_0 MyLibrary.dll
 
# Decompile with reference path for dependencies
dnx ilspycmd -r ./dependencies MyLibrary.dll

View IL Code

bash

# Show IL code instead of C#
dnx ilspycmd -il MyLibrary.dll
 
# Show IL for specific type
dnx ilspycmd -il -t Namespace.ClassName MyLibrary.dll

List Types

bash

# List all classes
dnx ilspycmd -l class MyLibrary.dll
 
# List interfaces
dnx ilspycmd -l interface MyLibrary.dll
 
# List structs
dnx ilspycmd -l struct MyLibrary.dll
 
# List enums
dnx ilspycmd -l enum MyLibrary.dll
 
# List delegates
dnx ilspycmd -l delegate MyLibrary.dll

Options

bash

# Show help
dnx ilspycmd -h
 
# Disable update check (useful for CI/automation)
dnx ilspycmd --disable-updatecheck MyLibrary.dll
 
# Remove dead code
dnx ilspycmd --no-dead-code MyLibrary.dll

Example: Understand how JsonSerializer works

bash

# Find System.Text.Json in the runtime
dotnet --list-runtimes
# Microsoft.NETCore.App 8.0.0 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
 
# List classes to find JsonSerializer
dnx ilspycmd -l class "C:/Program Files/dotnet/shared/Microsoft.NETCore.App/8.0.0/System.Text.Json.dll"
 
# Decompile JsonSerializer
dnx ilspycmd -t System.Text.Json.JsonSerializer "C:/Program Files/dotnet/shared/Microsoft.NETCore.App/8.0.0/System.Text.Json.dll"

Example: Inspect a NuGet package implementation

bash

# Decompile Polly's retry logic
dnx ilspycmd -t Polly.Retry.RetryPolicy ~/.nuget/packages/polly/8.0.0/lib/netstandard2.0/Polly.dll
 
# Decompile entire package to project for exploration
dnx ilspycmd -p -o ./polly-src ~/.nuget/packages/polly/8.0.0/lib/netstandard2.0/Polly.dll

Example: See how ASP.NET Core handles requests

bash

# Find the ASP.NET Core runtime
# C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App\8.0.0\
 
# Decompile the Kestrel server
dnx ilspycmd -l class "C:/Program Files/dotnet/shared/Microsoft.AspNetCore.App/8.0.0/Microsoft.AspNetCore.Server.Kestrel.Core.dll"
 
dnx ilspycmd -t Microsoft.AspNetCore.Server.Kestrel.Core.KestrelServer "C:/Program Files/dotnet/shared/Microsoft.AspNetCore.App/8.0.0/Microsoft.AspNetCore.Server.Kestrel.Core.dll"

Example: Compare C# and IL

bash

# View C# source
dnx ilspycmd -t MyNamespace.MyClass MyLibrary.dll
 
# View IL code for same type
dnx ilspycmd -il -t MyNamespace.MyClass MyLibrary.dll

C# Language Versions

Available versions for -lv option:

CSharp1 through CSharp12_0
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