Skip to main content

Roslyn shopping cart DSL - Part 1


Lately, a lot of people where asking on mailing lists and users groups, how to dynamically load and execute C# code. This code maybe be stored as expressions in the database or will be entered by the user in some sort editor or whatever they want to use, but the important thing is that the app should compile and execute the code at runtime. Unlike Ruby or Python, C# does not support this kind of stuff right out of the box. Fortunately, the Redmond guys had implemented a nice library called Roslyn, which allows us to do this and a lot of other crazy stuff.

Whereas this was possible in the past by using reflection or the CodeDomProvider, it were a lot of work (complex and error prone work), so, very few people did it.

In this post I’m going to show a little app that uses Roslyn to compile and execute C# code at runtime.

Instead of work with expressions like 2+2 or a>b (a.k.a. the classic “hello world!” example), I’ll work with something more close to a DSL. This DSL will evaluate expressions against business objects and it'll move values back and forth. The idea is provide a more real world example, not just a lab test.

Now suppose we are working on a shopping cart DSL that allows sales people enter business rules to modify the order processing pipeline. In order to keep things consistent, it would make sense to use the same domain objects that the company’s ERP uses. As it turns out, that is a really easy thing to do with Roslyn.


This is how it looks the syntax of our DSL:





Something that worth to mention is with Roslyn we must provide valid C# or VB code. Unlike the Boo compiler, Roslyn does not allow us to extend the language syntax (and this is by design). Obviously this is no good enough for sales people, who ain't gonna learn C# in order to work with our DSL, but it’s a great starting point for us, because with a little bit of source to source translation we can get the job done (We don’t have to worry about semantic analysis, operator’s precedence, etc., etc… all the heavy lifting will done by the Roslyn compiler).

You can get the source code from github using this link

By reading the unit tests you will understand how it works, there are just eleven of them, so is not a big deal. In future post I’ll cover in more detail some aspects of the Roslyn API internals, but I think this example will give you an easy to read overview, on how to build a DSL on top of the Roslyn APIs.



In the next post I'll be revising some design considerations that you may want to have in mind while designing your DSL syntax as well as the business rule editor.

Comments

Popular posts from this blog

How to create MS Word documents from Office templates using C#

The OpenXML SDK allows you to do pretty much anything you want with office files such as Excel, Word, etc… While many people like this library, I found it complex, unintuitive and poorly documented, not to mention the awful xml format that uses under the hood to represent the documents, styles, etc. So I decided not to use it and build my own solution. If you, like me, don’t like that library, you will find in this post an alternative approach to build word documents from templates using c#.
A neat trick to work with Office is to use the macro recorder to understand how things work. The macro recorder allows you to start a macro, do something by hand, stop it, and then take a look at the generated VBA code. Once you do this, you are pretty much set.
This is how it looks the template I’am going to use.


Note: save the file as a Word template (.dotx)
This is the code to create Word documents from C#:

By running the code, you should get a document that looks like this.


Note that the font, forma…

Printing html using the embedded web browser control

In this post I’ll try to answer some questions about the web browser control and provide some workarounds for known issues involved in the printing process.
I'm assuming that you have some experience with the web browser control and basic knowledge of COM and hosting APIs. So I’m not going to cover those topics.
At the bottom of this page I’ve added the links to download a small library I wrote that takes care of printing HTML and a demo app so you can try it out without having to write any code by yourself.
Using the code The HtmlPrinter class will allow you to print html from an URL or just passing the html as string, you can also specify the title and the number of copies you want to print. The code may look something like this: Now that we know how to use the API let get answer some questions.
Why my app crashes when I try to print multiple copies of a page? Well, apparently when you send a lot of print commands to the web browser control, there is a lot of COM crap in between that l…

WinForms, paging the DataGridView the right way

I know this may sound like old history, but in the enterprise world there is still a lot of WinForms development. Just a couple of days ago, I had to implement a custom DataGridView capable to work over a butt load of data (100K+ records) and keep responses times acceptables.
I thought paging will be a good way to go, and as WinForms is pretty old nowadays, I supposed it will be easy to find a couple examples on the web.
While in fact I found examples, all of them were incompletes and/or they wouldn't perform well in real world apps... So I decided to roll my own component and post it online. Hopefully, someone else will find it useful ;).


The bread and butter of this solution relies on LINQ and deferred execution. As LINQ takes care of all complicated work, it was quite easy to implement.
This component also supports conditional format, sorting and some search capabilities, but in this post I will concentrate on paging only (I'll cover the rest of the features in future posts)…