{"id":1770,"date":"2019-11-10T14:56:38","date_gmt":"2019-11-10T09:26:38","guid":{"rendered":"https:\/\/www.rangakrish.com\/?p=1770"},"modified":"2019-11-10T15:01:34","modified_gmt":"2019-11-10T09:31:34","slug":"calling-c-methods-from-sicstus-prolog-through-com-interface","status":"publish","type":"post","link":"https:\/\/www.rangakrish.com\/index.php\/2019\/11\/10\/calling-c-methods-from-sicstus-prolog-through-com-interface\/","title":{"rendered":"Calling C# Methods from Sicstus Prolog through COM Interface"},"content":{"rendered":"

Recently\u00a0<\/span>I needed to make use of a C#<\/strong><\/em> library (which I had implemented a few years ago) from Sicstus Prolog<\/strong><\/em>. Calling C\/C++<\/strong><\/em> functions from Sicstus Prolog<\/strong><\/em> is fairly well documented<\/em><\/a>. When it comes to C#\/.NET<\/strong><\/em>, the official documentation recommends the PrologBeans<\/strong><\/em><\/a> library. After going through the documentation, I felt that it would not work for me for the following two reasons:<\/p>\n

1) This library requires Prolog<\/strong><\/em> to be the server and C#\/.NET<\/strong><\/em> to be the client, and the primary communication is from the client to the server. But my requirement was the other way – calling C#<\/strong><\/em> from Prolog<\/strong><\/em>.<\/span><\/p>\n

2) The server and client run as different processes. This can be useful when we need to build a scalable, heavy-weight client-server system, but it was “too heavy” for my requirements. I needed an in-process mechanism for the communication.<\/span><\/p><\/blockquote>\n

I looked around the documentation and found that the library comclient<\/strong><\/em><\/a>\u00a0allows Sicstus Prolog<\/strong><\/em> to act as a COM<\/strong><\/em> client. This seemed like a workable solution; I have to provide a COM<\/strong><\/em> interface to my exisitng library and then it can be invoked from Sicstus Prolog<\/strong><\/em>. It is also an “in-process”<\/strong><\/em> interaction, and hence avoids inter-process communication overheads.<\/p>\n

And that is what I ended up doing. It worked pretty well.<\/p>\n

In this article, I wish to outline the process of implementing a simple COM<\/strong><\/em> object in C#<\/strong><\/em> and then show how to use it from Sicstus<\/strong><\/em>.<\/p>\n

I am using Visual Studio 2019 Ver 16.3.7<\/strong><\/em>, so there could be some differences between the screens I have shown here and yours (if you have a different version).\u00a0<\/span><\/p>\n

Step 1:<\/h3>\n

Make sure you launch Visual Studio in “Administrator Mode”<\/strong><\/em> (this is required for Step 4<\/strong><\/em> to work). Create a new project of type “Class Library (.NET Framework)”<\/strong><\/em>. See the following image.<\/p>\n

\"Creating<\/a>
Creating a New Project<\/strong><\/figcaption><\/figure>\n

Step 2:<\/h3>\n

Give a suitable name for the project and select an appropriate directory to save the solution.<\/p>\n

Step 3:<\/h3>\n

In the project’s “Build Settings”<\/strong><\/em>, enable “Register for COM Interop”<\/strong><\/em>. See below.<\/p>\n

\"Enable<\/a>
Enable COM Interop<\/strong><\/figcaption><\/figure>\n

Step 4:<\/h3>\n

Another important step is to register the generated DLL<\/strong><\/em> for use as COM<\/strong><\/em> object through the “RegAsm”<\/strong><\/em> utility. Since this is done after the project is built, we can include this command in the “Post-build”<\/strong><\/em> step in the Project settings.<\/p>\n

\"Registering<\/a>
Registering the COM Object<\/strong><\/figcaption><\/figure>\n

This applies to 64-bit DLL<\/strong><\/em> alone. The “\/tlb”<\/strong><\/em> option is not needed in our case, but would be required if you are, for example, implementing a COM<\/strong><\/em> client in C++<\/strong><\/em>.<\/p>\n

As I mentioned earlier, for this step to succeed, you must have started Visual Studio in “Administrator”<\/strong><\/em> mode.<\/p>\n

Step 5:<\/h3>\n

Define the class that should act as the COM<\/strong><\/em> object (server). Make sure you include the “System.Runtime.InteropServices”<\/strong><\/em> Namespace. My example class is quite simple. It has just 4 methods as shown in the following image:<\/p>\n

\"The<\/a>
The COM Class in C#<\/strong><\/figcaption><\/figure>\n

The first method “add”<\/strong><\/em> starts with a lowercase letter, which is not the recommended convention. I did this deliberately and will explain the reason later.<\/p>\n

The first two methods take two “int”<\/strong><\/em> arguments and return an “int”<\/strong><\/em>. The third method “Concat”<\/strong><\/em> takes two string args and returns a string.<\/p>\n

The last method “ProcID”<\/strong><\/em> returns the ID of the currently running process. I will be using this to show that both the Sicstus Prolog<\/strong><\/em> engine and C#<\/strong><\/em> run in the same process.<\/p>\n

In order to convert a class like this into a COM<\/strong><\/em> object, we have to decorate the class with additional attributes.<\/p>\n

The class requires a unique GUID<\/strong><\/em> that will be used from the client. Generating this is quite easy in Visual Studio.<\/p>\n

Select the “Create GUID”<\/strong><\/em> command from the “Tools”<\/strong><\/em> menu.<\/p>\n

\"Generating<\/a>
Creating GUID<\/strong><\/figcaption><\/figure>\n

This prompts you for the format of the GUID<\/strong><\/em>. Choose option (5)<\/strong><\/em>:<\/p>\n

\"Selecting<\/a>
Selecting GUID Format<\/strong><\/figcaption><\/figure>\n

Copy and paste the generated GUID<\/strong><\/em> in the GUID<\/strong><\/em> attribute of the class. Although ProgID<\/strong><\/em>\u00a0attribute is not needed, I recommend it because this gives a readable<\/strong><\/em> name to the COM<\/strong><\/em> object. In the Prolog<\/strong><\/em> client, we can use either the GUID<\/strong><\/em> or the ProgID<\/strong><\/em> to connect to the COM<\/strong><\/em> object.<\/p>\n

My build target is “x64”<\/strong><\/em> as the target CPU<\/strong><\/em>. The reason is my Sicstus Prolog<\/strong><\/em> is a 64-bit application running on Windows 10<\/strong><\/em>.<\/p>\n

Step 6:<\/h3>\n

Build the project. Here is the output of the Build process:<\/p>\n

\"Build<\/a>
Build Output<\/strong><\/figcaption><\/figure>\n

Files “Server.dll”<\/strong><\/em> and “Server.tlb”<\/strong><\/em> would have been created.\u00a0<\/span><\/p>\n

Using the COM Object from Prolog<\/b><\/h3>\n

I wrote a COM<\/strong><\/em> client in Prolog<\/strong><\/em>, adding convenient wrappers to call the C#<\/strong><\/em> functions. Here is my code:<\/p>\n

\"Prolog<\/a>
Prolog Client Code<\/strong><\/figcaption><\/figure>\n

The “open_com”<\/strong><\/em> predicate tries to connect to the COM<\/strong><\/em> object by passing its GUID<\/strong><\/em> or ProgID<\/strong><\/em>. When successful, it asserts a “fact”<\/strong><\/em> containing the object ID for subsequent use (removing any previously asserted IDs). Its counterpart “close_com”<\/strong><\/em> releases the COM<\/strong><\/em> object.<\/p>\n

The “call_fn”<\/strong><\/em> predicate facilitates the actual call. It uses the “comclient_invoke_method_fun”<\/strong><\/em> to invoke the C#<\/strong><\/em> function through the object handle and gets the corresponding result.<\/p>\n

If you look at the “add”<\/strong><\/em>, “mult”<\/strong><\/em>, “concat”<\/strong><\/em>, and “procid”<\/strong><\/em> predicates, you will notice that the names are in lowercase, even though the actual methods being called in C#<\/strong><\/em> are “add”<\/strong><\/em>, “Mult”<\/strong><\/em>, “Concat”<\/strong><\/em>, and “ProcID”<\/strong><\/em>. In Prolog<\/strong><\/em>, when you start a name in uppercase, it refers to a variable and not to the actual literal with that name. So we are forced to reference the C#<\/strong><\/em> functions in lowercase in Prolog<\/strong><\/em>, even though the actual function names might start in uppercase in C#<\/strong><\/em> (and that is the convention). It appears that the C#<\/strong><\/em> method names are lowercased by the “comclient<\/strong><\/em>” library in order to find the right match. I feel the\u00a0library could have handled this differently, but it is not a big issue.<\/p>\n

OK. Now let us look at the actual session in the Sicstus Prolog<\/strong><\/em> IDE.<\/p>\n

\"Prolog<\/a>
Prolog Session<\/strong><\/figcaption><\/figure>\n

You will notice that the Proc ID<\/strong><\/em>s from COM<\/strong><\/em> object as well as from the Prolog<\/strong><\/em> engine are the same. This confirms that the C#<\/strong> <\/em>code runs as part of the Prolog<\/strong><\/em> process.<\/p>\n

Everything works as expected. I am happy that COM<\/strong><\/em> solved my problem of connecting to C#<\/strong><\/em> from Prolog<\/strong><\/em>.\u00a0<\/span><\/p>\n

You can download the C#<\/strong><\/em> code<\/a> and Prolog<\/strong><\/em> code<\/a> from here.<\/p>\n

Have a nice weekend!<\/p>\n","protected":false},"excerpt":{"rendered":"

Recently\u00a0I needed to make use of a C# library (which I had implemented a few years ago) from Sicstus Prolog. Calling C\/C++ functions from Sicstus Prolog is fairly well documented. When it comes to C#\/.NET, the official documentation recommends the PrologBeans library. After going through the documentation, I felt that it would not work for […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"advanced_seo_description":"","jetpack_seo_html_title":"","jetpack_seo_noindex":false,"jetpack_post_was_ever_published":false,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":true,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2}},"categories":[17,147],"tags":[219,67,218,151],"class_list":["post-1770","post","type-post","status-publish","format-standard","hentry","category-programming","category-prolog","tag-net","tag-c","tag-com-interface","tag-sicstus-prolog"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p9OLnF-sy","jetpack-related-posts":[{"id":1004,"url":"https:\/\/www.rangakrish.com\/index.php\/2018\/08\/05\/sicstus-prolog-building-a-windows-dll\/","url_meta":{"origin":1770,"position":0},"title":"Sicstus Prolog – Building a Windows DLL","author":"admin","date":"August 5, 2018","format":false,"excerpt":"Last week I upgraded to the latest version (4.4.1) of Sicstus Prolog\u00a0for Windows.\u00a0Since the Prolog engine can be embedded (royalty free) in other applications, it is useful to understand how to create a DLL (on Windows) for this purpose. In today's article, I would like to outline this process in\u2026","rel":"","context":"In "C++"","block_context":{"text":"C++","link":"https:\/\/www.rangakrish.com\/index.php\/category\/c\/"},"img":{"alt_text":"Creating Prolog Program Image","src":"https:\/\/i0.wp.com\/www.rangakrish.com\/wp-content\/uploads\/2018\/08\/Saving-Prolog-Image.png?resize=350%2C200","width":350,"height":200,"srcset":"https:\/\/i0.wp.com\/www.rangakrish.com\/wp-content\/uploads\/2018\/08\/Saving-Prolog-Image.png?resize=350%2C200 1x, https:\/\/i0.wp.com\/www.rangakrish.com\/wp-content\/uploads\/2018\/08\/Saving-Prolog-Image.png?resize=525%2C300 1.5x"},"classes":[]},{"id":1025,"url":"https:\/\/www.rangakrish.com\/index.php\/2018\/08\/19\/sicstus-prolog-building-a-windows-executable\/","url_meta":{"origin":1770,"position":1},"title":"Sicstus Prolog – Building a Windows Executable","author":"admin","date":"August 19, 2018","format":false,"excerpt":"In my previous post, I showed how to build a C-based Windows DLL to execute Prolog predicates in the Sicstus Prolog\u00a0engine. 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Since we can control how the terms are indexed in the database, it is possible to store and retrieve a large amount of Terms\u00a0(the limit is 2^32-1) fairly efficiently. 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