For most new projects we recommend using the declarations for .NET, available to download as source. These are usable with Framework 4 and also current .NET Core releases such as .NET 5. To use the .NET declarations you will need the NAG Library installed on your computer.
.NET includes classes that provide declarations that allow managed .NET code to access functions from a native compiled library such as the NAG Library.
The .NET declarations are available in C# and VB.NET and are usable in other .NET languages, see, for example, the F# examples.