Call a C# DLL from Python with pythonnet — Load a .NET Library

Demonstrates how to call a compiled C# / .NET library (DLL) from Python using pythonnet and its clr module.

Overview

This project shows cross-language interoperability between Python and .NET. A simple Calculator class is compiled as a C# class library (DLL), and Python loads it at runtime via the clr (Common Language Runtime) module from the pythonnet package. The Python script then calls the .NET methods directly as if they were native Python functions.

Features

  • Load and call a .NET assembly (.dll) from Python
  • Calls five static methods on the C# Calc class — a description plus four arithmetic operations:
    • About() — returns a description string
    • Addition(a, b)
    • Subtraction(a, b)
    • Multiplication(a, b)
    • Division(a, b)

Project Structure

LoadLibraries/
├── Calculator/
│   ├── Calc.cs               # C# source — static Calculator class
│   ├── Calculator.csproj     # .NET project file
│   └── Calculator.sln        # .NET solution file
├── Console_Code/
│   └── CalcCaller.py         # Python script that loads and calls the DLL
└── Library/
    └── Calculator.dll        # Compiled .NET class library

Requirements

pythonnet

Install the dependency:

pip install pythonnet

You also need a .NET runtime that can load the DLL. Calculator.csproj targets .NET 6 (net6.0).

Usage

The DLL is pre-compiled and located in Library/Calculator.dll. Update the path in CalcCaller.py if needed:

clr.AddReference(r'path\to\Library\Calculator.dll')

Then run the script:

python Console_Code/CalcCaller.py

Example Output

The script calls Addition(5, 5), Subtraction(10, 5), Multiplication(5, 5), and Division(10, 5), so it prints:

About: You're accessing .NET library

Addition: 10.0
Subtraction: 5.0
Multiplication: 25.0
Division: 2.0

Rebuilding the DLL

If you need to recompile the C# library:

cd Calculator
dotnet build --configuration Release

Copy the resulting .dll from the bin/Release/ output to Library/Calculator.dll.

Notes

  • The DLL path in CalcCaller.py is currently hardcoded. Update it to match your actual directory structure before running.
  • Calc is a public static class, so its methods are called directly on the type (Calc.Addition(5, 5)) without creating an instance.

Troubleshooting

  • The DLL fails to load on Windows. By default pythonnet 3 uses the .NET Framework runtime on Windows, while this DLL targets .NET 6. Select the .NET (Core) runtime before importing clr:

    from pythonnet import load
load("coreclr")
import clr

    The machine needs a .NET runtime that can run a net6.0 assembly, or rebuild the library for the runtime you have installed.

  • FileNotFoundException for the DLL — the path passed to clr.AddReference must point to the DLL on your machine.

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