Trying to use a C# application that calls System.PrivateFontCollection.AddMemoryFont on Linux leaves a bunch of unused font files in the tmp folder, even after calling Dispose.

Most functions in Mono's PrivateFontCollection are simply wrappers to libgdiplus calls, so I guess the bug originates from here.

Digging through the source code, it seems that this piece of code here is responsible:

#ifdef WIN32
	f = CreateTempFile (fontfile);
	if (!f)
		return FileNotFound;

	if (fwrite(memory, sizeof(BYTE), length, f) != length) {
		fclose (f);
		return FileNotFound;
	}

	fclose (f);
#else
	strcpy ((char *) fontfile, "/tmp/ffXXXXXX");
	f = mkstemp ((char *) fontfile);
	
	if (f == -1)
		return FileNotFound;

	if (write (f, memory, length) != length) {
		close (f);
		return FileNotFound;
	}
	close (f);
#endif

	FcConfigAppFontAddFile (fontCollection->config, fontfile);
	/* FIXME - May we delete our temporary font file or does 
	   FcConfigAppFontAddFile just reference our file?  */
	/* unlink(fontfile); */

	return Ok;

After research, at least on my Ubuntu 20.04 machine, FcConfigAppFontAddFile simply references that file, deleting the font makes the app crash when a new Form or Control is drawn. However, the function does not seem to save the temp files anywhere.