[I don’t know if this is related to Monogame or not (since I’m only doing projects with Monogame)]
Hoping someone knows something about this on some corner of the internet. No luck so far.
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I notice that my older builds have the blue icon and run when you click on the executable and are of type: “Executable”, but new builds which should be identical are now producing (executable only from command prompt using: ./MyGame) files of “Shared Library” type. If you try to click the binary file (ie: MyGame), it says something about a MIME is needed or some installed application needed to run “Shared Library” files.
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I’m hoping to find a way to force it to build it as Type: Executable instead of Type: Shared Library - but so far no luck. Any ideas?
I’ve been searching this problem for days now. 
On another fresh install of Linux, fresh install of VS code, net core 3.1 (not 5 - doesn’t work with mgcb), monogame, etc…
Again, possible to make it run in command prompt. Doesn’t work under any Files explorers I’ve tried.
It seems this is a relatively new problem that people aren’t aware of that comes with newer versions of software.
After experimenting, it seems the most likely cause is changes to the new version of VS Code.
Another message it may say is: “The file has no known programs associated with it.”
Basically only experienced users using command line will be able to run the program… which is made for children. . .
I’ll keep hunting for a solution and update with one if I find one - otherwise I’ll have to find a way to get older version of VS Code I guess… if that doesn’t work I suppose I may have to abandon Linux for now.