Object Model

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; 
}