cgSudoku
Sudoku engine for .NET 4.0 and up
...Also includes a custom control to drop on forms to take care of the display and basic key handling, and a sample WinForms application to demonstrate the basic functionality of the library and control.
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).