Quick tutorial about creating xlsx Excels with C# and the ClosedXML nuget package.

The blog posts only covers the most important functionality. More extensive code examples can be found in the GitHub project.

Last month, I had the opportunity to attend DDD Europe 2025 in the Queen Elisabeth Hall in Antwerp — a few inspiring days full of insights, ideas, and engaging discussions around Domain-Driven Design, software quality, and architecture. Here are some of my key takeaways:

You’ve developed the most fancy application and are running it on your Synology in a Docker container.

But you really don’t want that ugly “Not secure” when opening it. This series goes over your options, starting with the easiest one to setup: using the Synology nginx.

dotnet/machinelearning : ML.NET is an open source and cross-platform machine learning framework for .NET.

microsoft/LightGBM : A fast, distributed, high performance gradient boosting (GBT, GBDT, GBRT, GBM or MART) framework based on decision tree algorithms, used for ranking, classification and many other machine learning tasks.

After writing some code to fetch and mold the data, I was ready to train “my” AI, dubbed “PongRank” which was easy enough and all was good until I added a small WebApi to automate future syncing, retraining and predictions.

While moving some todos from one location to another I came across “Check Site Inspector” and thought, why not run it against this website right now – how hard can it be.

siteinspector/siteinspector :

Not that hard it turns out.

For our bootcamp we try to get everyone out of their comfort zone and, like many of our sessions, try to do something hands-on.

This year the topic of choice was “Home Automation”. The day was divided into two parts:

• Solder your own “smart socket” (and take it home with you!)
• Integrate it into one of the “smart houses” with Home Assistant

JamesNK/Newtonsoft.Json

System.Text.Json (STJ) is about twice as fast as Newtonsoft.Json while only consuming 50% as much memory. A legacy project might still be using Newtonsoft but chances are serialization isn’t a bottleneck so it’s probably not worth making the switch.

For new applications you’ll want to stick with STJ however. Only add the extra nuget if you really need one of the advanced Newtonsoft features (ex: Linq to Json, polymorphic serialization, circular reference handling).

Adding the [AllowAnonymous] or [Authorize("policy")] whenever a new Controller Action Method is added, it’s something that is easily forgotten.

… As I noticed when I was looking at some of our controllers 😵

Of course I have written this test many times before. These days it can be quickly re-created with AI but to avoid having to debug its code, here is the copy pasta version, with some possible variations.