In .NET/WinForms/C# I wrote a small program for "viewing" PDF files. It just opens a real PDF viewer and does some more things (not important for the following problem).
However, under Windows 10 I cannot set my program as the default for opening PDF files.
If I open a PDF file via "Open with..." and select my program and activate the option "Always open with...", next time I open a PDF Windows will use the default PDF viewer again.
In the case that I try to set the default program via the default apps settings, Windows will not even accept my program as a default for PDF files. It only shows other viewers like Acrobat or Chrome.
How can I set my self-coded program to be the default viewer for PDF files?