Visual Studio 1.51.1, connecting Arduino Board MKR1000

I use Visual Studio 1.51.1. at OS: Windows_NT x64 10.0.19042.
I want to do a sketch (blink) to get to know the program.
When I load the sketch onto the board, I get an error message:

No device found on COM3
*** [upload] Error 1

The board is not connected to com3, but to com6. It seems that the port will change from com6 to com3 during the upload

There seem to be two ports, port COM3 bootloader and COM6 MKR1000.

I would be grateful for an explanation of the phenomenon.
Regards Jurgen