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COM Server Support

Functions and macros created in an Excel-DNA add-in can be called directly from Excel VBA by using Application.Run(...). However, .NET also supports creating rich object models that are exported as COM libraries, which can be Tools->Referenced in VBA. Excel-DNA has some advanced support to host COM-exported objects from Excel-DNA add-ins, giving some advantages over the regular .NET ‘Register for COM interop’ hosting approach:

  • COM objects that are created via the Excel-DNA COM server support will be active in the same AppDomain as the rest of the add-in, allowing direct shared access to static variables, internal caches etc.

  • COM registration for classes hosted by Excel-DNA does not require administrative access (even when registered via RegSvr32.exe).

  • Everything needed for the COM server can be packed in a single-file .xll add-in, including the type library used for IntelliSense support in VBA.

In addition to the description below, there is a sample project and a step-by-step instructions in the Excel-DNA Samples repository. Note that these techniques would works equally well with code written in VB.NET, allowing you to port VB/VBA libraries to VB.NET with Excel-DNA and then use these from VBA.

Mikael Katajamäki has written some detailed tutorial posts on his Excel in Finance blog that explore this Excel-DNA feature, with detailed explanation, step-by-step instructions, screen shots, and further links. Note that these project descriptions do not use the Excel-DNA Nuget packages. Therefore, the project layout is not current however, the implementation might still be instructive:

COM visible classes in ExternalLibrary tags marked ComServer="true", and COM visible classes that implement IRtdServer can be activated through the .xll directly. Even if the add-in is not loaded in Excel, such objects can be created in VBA.

These classes are (persistently) registered by calling regsvr32 <MyAddin>.xll or dynamically by the add-in (for example in an AutoOpen method) by calling ComServer.DllRegisterServer(), and unregistered by regsvr32 /u <MyAddin>.xll or by ComServer.DllUnregisterServer().

Following are short examples both in C-Sharp and VB, these only demonstrate the unreferenced (late-bound) technique:

using ExcelDna.Integration;
using ExcelDna.ComInterop;
using System.Runtime.InteropServices;

public class AccessibleFunctions
public double add(double x, double y)
return x + y;

class ExcelAddin : IExcelAddIn
public void AutoOpen()
public void AutoClose()
<DnaLibrary Name="ComAddin" RuntimeVersion="v4.0">
<ExternalLibrary Path="ComAddin.dll" ComServer="true" />

Usage in VBA:

Option Explicit

Sub tester()
Dim lib As Object: Set lib = CreateObject("ComAddin.FunctionLibrary")
Debug.Print lib.Add(12, 13)
Set lib = Nothing
End Sub

The VB.Net example is also available in the Samples repository ComServerVB after building it, start Excel by opening bin/release/ComAddin.xll or ComAddin64.xll (depending on your bitness) and enter the code under "Usage in VBA:" somewhere.

Such classes can be accessed directly as RTD servers or from VBA using CreateObject("MyServer.ItsProgId"), and will be loaded in the add-in's AppDomain. (The add-in need not be loaded for registered classes to be accessed through COM.)

A type library (.tlb) can be created for the assembly using tlbexp.exe, and will be registered if available (if the .tlb is found next to the .dll). If the assembly is packed in the .xll, the type library will be packed too. An example for this can be found in DnaComServer.