While it won't support .NET 4.0 directly, the .NET distributions are cumulative in nature, so .NET 4.0 has everything that .NET 2.0 had. LV 8.6 supports .NET 2.0. Thus, if you don't use anything that's .NET 4.0 specific, .NET code should work OK, for the most part. There are bound to be glitches, but it's next to impossible to guess what they might be. For example, I have .NET 1.1, 2.0, 3.5, and 4.0 on my machine. Of the .NET examples that ship with LabVIEW, the ".NET Meetings Calendar" is the only one that doesn't work properly - when some of the .NET nodes try to execute the CPU usage goes to 100%. Also, if you are using a private assembly, I'd bet on the "not working" part.
