cgSudoku
Sudoku engine for .NET 4.0 and up
...Source is currently being developed in Visual Studio 2015, although it does not rely heavily on C# 6.0 features and could probably be converted to an earlier version with little trouble.
External references include NUnit 2.6.4 and Moq 4.2 (used in the test library projects only) and Ninject 3.2 (could be written out easily).