Swi-cs-pl - A CSharp class library to connect .NET languages with SWI-Prolog
This document describes a CSharp interface to SWI-Prolog. The described interface provides a layer around the C-interface for natural programming from C#. The interface deals with automatic type-conversion to and from Prolog, mapping of exceptions and making queries to Prolog in an easy way. There is a call-back from Prolog to C#.
The first version of this Interface was more or less a port of the C++ interface. Now the naming is more '.Net' like and the interface provides a number of features that make queries to SWI-Prolog very easy and powerful. Using programmable type-conversion (casting), native data-types can be translated automatically into appropriate Prolog types. Automatic destruction deals with most of the cleanup required.
I would like to thank Jan Wielemaker for answering many questions and for his comments.
Also to Arne Skjærholt for the 64-Bit version (SwiPlCs64.dll).
Download binaries

Here is the link to download the latest binaries or older versions. Download page
There is also a copy on SWI-Prolog Twiki page and SWI-Prolog contrib page. The latter might be outdated.
At present I only publish the binaries including the documentation. The sources, which are now under LGPL 2, might be published later. If you like to see them or work on them don't hesitate to contact me via mail.


The latest version work with SWI-Prolog 5.8.0 and higher.

The AssemblyVersion number, e.g. 1.1.578.0, can be interpreted as follows:
  • 1 - major version
  • 1 - minor version
  • 578 - SWI-Prolog version 5.7.8 (test cases run against this prolog version)
  • 0 - patch level version
Getting started

Copy SwiPlCs.dll or SwiPlCs64.dll and SwiPlCs.XML where ever you want and add in your project a reference to SwiPlCs.dll.

After that intelligence and tool tips should be available.

screen shot

Make sure that swipl.dll and its dependencies could be found by the system. For the sample below it is swipl.dll and pthreadVC.dll. For a big application it could be a lot more.

TIP: For development add the SWI-prolog bin directory to the PATH environment variable.
NOTE: Don't forget to restart Visual Studio after that. VS must recognize the new environment for debugging.

Basically windows search first in the folder where the executable resist than in the windows system directory and at least in the directories that are listed in the PATH environment variable. For details see "Dynamic-Link Library Search Order"

If swipl.dll or one of its dependencies could not found you will recive an error like
System.IO.FileNotFoundException: Das angegebene Modul wurde nicht gefunden. (Ausnahme von HRESULT: 0x8007007E)

An other common error is:
     Could not find system resources]
Failed to release stacks

To fix this add the SWI_HOME_DIR environment variable as described in SWI-Prolog FAQ FindResources with a statment like this befor calling PlEngine.Initialize.

Environment.SetEnvironmentVariable("SWI_HOME_DIR", @"the_PATH_to_boot32.prc");
First program

A sample says more then I want to write here.

For further samples see the examples in and .

The class is the key to ask SWI-Prolog for proofs or solutions.

The plays a central role in conversion and operating on Prolog data.

Known Bugs

work *not* as expected if there are e.g. German umlauts in the parameters e.g. in the path or filename for a qlf file ( switch -x )
See marshalling in the source NativeMethods.cs