Communicating with IDTech Devices requires the following library to be referenced by the project:

IDTechSDK.dll

The best way to obtain this library, and keep in sync with the latest updates, is to import the library from NuGet. The NuGet package is "IDTechSDK_STD". This will also install any other dependencies needed for the particular project you are building for.

Alternately, you can use the IDTechSDK.dll included with the SDK distribution. This is a legacy version that doesn't have any other dependencies, and will work with a minimum .Net Framework version 4.5. If you use this SDK, you would add the reference as you would any .NET managed library reference. The most direct method would be right-click on the "References" in the Solution Explorer for the project, select "Add Reference...", click "Browse..." and locate IDTechSDK.dll.