DotNet Community: Terrarium (emphasis below is my own)
Interestingly, at the time of development, the DirectX SDK had not been implemented for managed, or .NET Framework-based, code. As a result, the developers of Terrarium took advantage of COM Interop, a .NET Framework service used to tie into existing unmanaged code from within a .NET Framework-based application. The ability to call unmanaged code is a powerful one and can enable developers to leverage existing, legacy components as they go forward in their application development.
So now we know what .NET is (we knew it all along but were afraid to ask) -- just another plaform from Microsoft with just enough gaps in functionality to tie it to Windows. Really makes that CLR look useful, huh?