'Unity project, Visual Studio 2019 included file disappears when reopen

I have an unity project with structure like this:

Scripts
 |--csFile1.cs
 |
Resources
 |--XML
 |   |----xmlFile1.xml
 |
 |--JSON
 |   |----jsonFile1.json

The jsonFile1.json is not included in the vs solution. What I did is in the Solution Explorer click on Show All Files, then on the JSON folder or on the json file name right click then choose Include In Porject, then the json files are included properly.

However every time I reopen the vs project, the json files are not in the solution.

The XML files are included properly but I don't remember how I managed to do it.

Anyone knows how to properly include a file into the solution in a Unity project? Thanks!



Solution 1:[1]

====Updates====

Sorry, it seems the following way is still not the solution. After restarting my Visual Studio, the json files are excluded again.

====Original Answer====

After almost 3 years, I accidentally bumped into the solution: In the File menu, just choose "move 'file name' to xx Assembly". Usually you want to move it into the "Assembly-CSharp" in my case.

Sources

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Source: Stack Overflow

Solution Source
Solution 1