Introduction

If you’ve already developed COM APIs with .Net, typically in C# with VBA as the consuming language, you’ve probably leveraged two powerful features:

• by-reference parameter passing that allows the API to change the input object itself, not only its content.
  This is marked with ref in C#:

  void LoadData(ref string data)

• optional parameters that allow the caller to omit them when calling into the API:

  void SaveData(string data = "DEFAULT")

Sometimes you want to have both for a parameter: the ability to omit it and to change it.
This is something COM supports but unfortunately this is not supported by C#.
Recently one of my customer needed precisely that for a COM API developed in C# migrated from VB6 where it was plug-and-play.
The original developers of the API had done a quick study and concluded that it was not possible with the new C# API and indeed it was not documented at all.
From experience I know that COM is not used a lot today and that resources are scarce, so, to be sure, I’ve done my due diligence and reexamined the issue, digging a little deeper and testing undocumented stuff.
I’ve finally found a solution that I would describe as a “trick”, because it’s not documented, and that I’ll present in detail in this article.
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