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c# – Call non-registered .NET dll from Excel VBA

Posted by: admin March 9, 2020 Leave a comment


I need to write a .NET dll that can be called directly from an Excel’s VBA Module such that the dll and .xls are simply deployed in the same directory without any COM registration.

Purpose of dll is to run algorithms implemented in C.

How should I proceed? Is this possible at all?

How to&Answers:

You don’t explain what the role of .NET would be in your scenario. You can indeed call many C libraries directly from VBA using ‘Declare Function’.

If you find .NET useful, and want to call your .NET library functions as user-defined functions (UDFs) from Excel, you can use Excel-DNA. It gives you an .xll add-in library that integrates your .NET library into Excel. You’d still have to open the .xll add-in somehow – either by File -> Open, by adding it as an Excel add-in, or automatically from some VBA in your workbook. But it needs no other registration.

From the .NET library you can call the C .dll functions using P/Invoke, add categories, function and argument descriptions to integrate into the function wizard etc.

(Disclaimer: I’m the developer of Excel-DNA.)


Have you seen this MSDN article? Basically you register functions from a DLL, not a DLL itself. I don’t know if it is put very clear in the article, but the syntax is:

[Public | Private] Declare Function name Lib "libname" [Alias "aliasname"] [([arglist])] [As type]

where “libname” can contain full path e.g. “C:\tmp\algo.dll” or only a name e.g. “algo.dll”. In the second case the local path will be searched for a matching binary.


If the DLL contains C functions then you should avoid .net and the CLR. That’s a needless overhead.

Instead use a C compiler, MSVC for example, to build the DLL and export the functions you need from that DLL. Then import the DLL functions into VBA with Declare statements.

When you build the C functions into a DLL make sure that you use the __stdcall calling convention since that is the only option with Declare. You may also need to use a .def file when building the DLL to avoid name decoration.

A very simple example:


int __stdcall add(int a, int b)
    return a+b;


Public Declare Function add Lib "mylib.dll" (ByVal a As Long, ByVal b As Long) As Long