Excel-DNA Add-ins

link - docs.excel-dna.net
The name Excel-DNA stands for "Excel DotNet for Applications".
This is an open source project that lets you create native (.xll) add-ins for Excel using C#, VB.Net or F#.
The Excel C API (introduced in 1995) is not directly accessible from .NET code.
The Excel-DNA runtime allows managed assemblies to integrate with this low-level Windows API.
Excel-DNA lets you create high performance user-defined functions and requires no installation or registration.
Allowing a user to install and run on a machine without admin access can be extremely useful.

User Defined Functions

Also known as Custom Functions
There are three different types of User Defined Functions you can create:

Synchronous - Not Thread Safe or Thread Safe
A function that is not thread safe will lock the Excel session and will only use ONE calculation thread.
A function that is thread safe will lock the Excel session and can use ALL the available calculation threads.

Asynchronous - Not Thread Safe or Thread Safe
A function that is not thread safe will lock the Excel session and can use ALL the available calculation threads.
A function that is thread safe will lock the Excel session and can use ALL the available calculation threads.

RTD Functions
These are Real Time Data functions that update automatically.
An RTD function doesn't have to keep updating Excel, it can just notify Excel once, when the calculation is complete.
link - learn.microsoft.com/en-us/office/vba/excel/concepts/excel-performance/excel-performance-and-limit-improvements

Versions

Nuget and GitHub Version History

Links

link - excel-dna.net/blog/ 
link - nuget.org/packages/Excel-Dna 
link - nuget.org/packages/ExcelDna.Integration/ 
link - github.com/Excel-DNA/Samples 
link - github.com/Excel-DNA/ExcelDna 
link - groups.google.com/g/exceldna 
link - youtube.com/watch?v=UEIO2Q8HyD8 
link - youtube.com/watch?v=0-Rb0StiJgM 
link - bitbucket.org/andysprague44/excel-dna-examples/src/master/