EPLAN API was created, using the Microsoft® .Net  4.7.2 technology. 

To develop applications with the EPLAN API, we recommend Microsoft Visual Studio 2019 or 2017. However it should work also with other development environments for the Microsoft .Net framework  4.7.2. The EPLAN API however was not tested with other IDEs, than the one mentioned above. 

To start a new EPLAN API project in Microsoft Visual Studio 2019, you select the Ribbon "File" > "New...". In the "New Project" dialog, you first select the programming language, you want to use (C#, Visual Basic, or C++). 

Next you decide, which type of application you want to write:

• For an add-in you either select a "Class Library" project or you use the add-in wizard EPLAN provides. How to proceed using the add-in wizard is described in the topic "Creating an add-in in VisualStudio". If you want to create an add-in without the wizard, just add the references to the API assemblies. You at least have to reference Eplan.EplApi.AF.dll and Eplan.EplApi.Base.dll. Read more about creating add-ins in the topics below Add-ins. In you project settings under "Configuration Properties>Debugging" add the W3u.exe as startup program (or Eplan.exe with 'Variant' parameter) and make sure the created dll is copied into the bin folder after building it.
• For an offline-program you for example create a new Windows Application project and proceed like described in the topic "Using EPLAN API assemblies in other processes". Make sure a path is set to the EPLAN bin directory or the working directory of your code project is set respectively, otherwise depending dlls may not be found.

Working with an older EPLAN API, requires appropriate environment for this version of the framework, 

For example version 2.8 of EPLAN API were developed for Microsoft .Net Framework 4.5.2. 

Make sure you don't mix EPLAN API assemblies, which use different frameworks.