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How it works: Selenium


I've done quite a bit of work with Selenium on and off, mainly for browser automation tests. There are many high quality official libraries for Selenium covering a range of languages. It's an amazing technology and a real joy to work with; however sometimes stuff can go wrong and it's useful to know what's going on under the hood in order to debug where the problem lies.

For this reason I present my very-probably-wrong guide to Selenium.

Gecko, drivers!? What's going on?

There are 3 components in code that interacts with Selenium (ignoring RemoteWebDriver use cases):

  1. Your code
  2. A driver
  3. The browser

For each browser that supports Selenium, the browser vendor provides a "driver". This is usually a small executable (.exe) program tailored for a specific browser:

This driver is the part your code actually interacts with. The driver then communicates with the browser using whatever magic the vendors design. In order to change browsers while using the same code, the drivers expose interfaces/endpoints that implement the WebDriver specification.

Put simply, this means the drivers are a simple RESTful API.


To demonstrate this I'll use the ChromeDriver version 2.29 which is the latest version at the time of writing. Drivers can only talk to the versions of the browser they are compatible with. In this demo I'm running Chrome on version 58.


Writing a custom debug visualizer for Visual Studio 2015


Often at work I'll be debugging some incredibly complex logic with lists that can be hundreds, if not thousands, of items long. While the ability to inspect these lists using the normal debugging tools is useful, it gets annoying trying to scroll to the 700th item in a list and then accidentally moving the mouse focus away and having to start again.

For this reason I wanted to look into the possiblity of creating a custom display for certain objects while debugging in Visual Studio. For those of you who have worked with the DataTable you might be familiar with this screen:

data table visualizer

This can be accessed by hovering over the object when paused in the debugger and clicking the magnifying glass icon as shown here:


Reading from a COM port


I was recently researching how to read from a COM (Serial) port in order to communicate with an old embedded system. If you're not familiar with them that's one of these:

a serial port has a 9 pin connection

C# provides the SerialPort class for just this purpose which is hugely helpful. However there's one problem with this class which is that it's just not... very good. I didn't know about this but this post by Ben Voigt should leave you in no doubt that it has some problems.


In order to test the code on my machine without access to the actual device which was elsewhere, I installed Eltima's Virtual Serial Port Driver which would allow me to test with emulated serial ports. Once I had set up a pair of ports I could send data from one end using PowerShell.

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