Backend(,Backend.Tests): improve marshalling to fix crash #2952

Job	Run time
windows--dotnet6-and-legacyFramework	5m 54s
macOS--mono-only	8m 10s
macOS--dotnet6-and-mono	6m 51s
macOS--dotnet6-only	6m 13s
linux22-github--dotnet-and-mono	4m 2s
linux22-github--dotnet-and-newmono	3m 47s
windows--dotnet6-only	5m 21s
windows--legacyFramework-only	12m 7s
linux22-vanilla--stockdotnet6-and-newmono	5m 27s
linux22-vanilla--stockdotnet6-and-stockmono	4m 47s
linux22-vanilla--stockdotnet6-only	4m 51s
linux22-vanilla--stockmono-only	4m 49s
linux20-github--dotnet-and-mono	3m 40s
linux20-github--dotnet-and-newmono	3m 31s
linux20-vanilla--stockmono-only	4m 43s
linux22-vanilla--newmono-only	6m 8s
linux20-vanilla--newmono-only	5m 26s
conventions	1m 17s
snap_pkg	0s
	1h 37m 4s

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