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/ 

ExcelDna.Integration.ExcelIntegration 
ExcelDna.Integration.ExcelFunction 
ExcelDna.Integration.XlCall 
ExcelDna.Integration.ExcelError  (enumeration) 

Excel C API

The Excel C API is documented in the Excel XLL Software Development Kit, available from Microsoft here:
link - http://msdn.microsoft.com/en-us/library/office/bb687883.aspx
link - docs.excel-dna.net/excel-c-api/
Direct access to the C API is provided by Excel-DNA via two classes:

ExcelReference

ExcelDna.Integration.ExcelReference 

The ExcelReference class is a thin wrapper around the C API sheet reference information, and refers to a region (or multiple regions) on a specific sheet.
ExcelReference has helper methods to get and set the values in the region, but for additional information, specific C API calls must be made.
Using C API calls it is possible to get the address of an ExcelReference - calling XlCall.Excel(XlCall.xlfReftext, myReference, true) - and from there a COM Range object may be contructed.

XlCall

ExcelDna.Integration.XlCall 

Excel
xlfActiveCell
xlfCaller
xlfGetWorkspace
xlfSelection
xlcEcho
xlcFormatNumber
xlcFormulaGoto
xlcSelect
xlcWorkbookSelect
xlSheetNm

Other Objects

ExcelDna.Integration.IExcelAddIn 

ExcelDna.Integration.DnaLibrary 
ExcelDna.Integration.ExcelArgument 
ExcelDna.Integration.ExcelArgumentAttribute 
ExcelDna.Integration.ExcelAsyncHandle 

ExcelDna.Integration.ExcelComAddIn 
ExcelDna.Integration.ExcelComAddInHelper 
ExcelDna.Integration.ExcelCommandAttribute 

ExcelDna.Integration.ExcelDnaUtil 
ExcelDna.Integration.ExcelDnaUtil.WindowHandle 

Application - the correct Application root object
ExcelLimits
ExcelVersion
MainManagedThreadId
SupportsDynamicArrays
XllPath
XllPathInfo

ExcelDna.Integration.ExcelEmpty 

ExcelDna.Integration.ExcelError 

ExcelErrorNA

ExcelDna.Integration.ExcelErrorUtil 

ExcelDna.Integration.ExcelFunctionAttribute 

Description = ""
Name = ""
IsMacroType = false

ExcelDna.Integration.ExcelIntegration 

RegisterUnhandledExceptionHandler

ExcelDna.Integration.ExcelLimits 
ExcelDna.Integration.ExcelMissing 
ExcelDna.Integration.ExcelReference 
ExcelDna.Integration.ExcelSynchronizationContext 
ExcelDna.Integration.ExternalLibrary 
ExcelDna.Integration.IExcelObservable 
ExcelDna.Integration.IExcelObserver 
ExcelDna.Integration.Image 
ExcelDna.Integration.Integration 
ExcelDna.Integration.Project 
ExcelDna.Integration.Reference 
ExcelDna.Integration.SourceItem 

ExcelDna.Integration.XlCallException 
ExcelDna.Integration.XlLibrary 

public static object MyFunction(string code) 
{ 
  if (ExcelDnaUtil.IsInFunctionWizard() == true) 
   { 
    return "Uncalculated"; 
   } 

   var results = method_ReturnTheValue(); 

   return  results ?? ExcelDna.Integration.ExcelEmpty.Value; 
}