Visual Micro and Visual Studio do not have any connection to how the code is compiled. We simply run the respective tool chain .exe to compile each file.

So this sounds like you are using a different or older tool chain. The information requested in the yellow box above will show you the compilation so you can see the .exe that runs the compile for each file.

Many versions of Gcc does not appear to take any notice of the command. Please use the syntax you see when creating .cpp/.h file using the "vMicro>Add Code" menu. That is the syntax that Arduino uses.

#ifndef _MY_HEADER_h
#define _MY_HEADER_h

	#include "arduino.h"

	//more code here


#endif 

Replace MY_HEADER with the name of the header file.

Paul I am unsure what you are saying. The #pragma once is not something Visual Micro manages or has control over.

Thanks. I think this means you have updated your toolchain in board manager and that the toolchain previously supported #unknown statements. 

It always helps to post your verbose build output with the build properties enabled then we can be more specific in our answers which often depend on the toolchain you have installed for each type of board.

The output would also help us knowif you still have a build failure or not. 

Yes, there have been a few changes in the past year to ensure we are arduino compatible by default. There have also been huge changes in arduino and tool chains. The ability to configure compiler paths has been added to visual micro in the past year because the std. arduino way of doing things does not provide that functionality and we needed to be compatible with arduino out of the box. 

All changes were added to the relase notes of the respective vendors but I know that doesn't help people who come back to things after a lengthy break. 

Sometimes it's best to re-start existing code with same versions of tool chain, arduino ide and visual micro that you used previously. This is so that you can prove things still work before updating.

A couple of years later I hit the same issue with #pragma once on a new project & version 2022.905.2207 of Visual Micro. 

It looks like this problem occurs because Visual Micro's path include path options add the project source folder, not the build folder, to the path when compiling source files. 

I imagine this confuses gcc's logic for matching header files when it processes the #pragma once options. Turning off the flags and adding -I"{build.path}" to the Extra cpp flags option still resolves this problem.