mongodb/mongo-csharp-driver — comment on commit 4c1b6d2a80cf1b9b87157b9d758ebe63d864b9ef
File:  line 0
By rstam:

I like the suggestion, but ConcurrentDictionary was introduced in .NET Framework 4 and we are currently only requiring .NET Framework 3.5. Any suggestion on when it would be OK to raise the minimum requirement to .NET Framework 4?