What is the difference between Excel add-ins and COM add-ins

🔍 Difference Between Excel Add-ins and COM Add-ins

The difference between Excel add-ins and COM add-ins lies primarily in how they’re built, deployed, and integrated with Excel.

🔹 Excel Add-ins (aka Office Add-ins or JavaScript Add-ins)

These are modern add-ins built using web technologies like HTML, CSS, and JavaScript.

✅ Key Features:

• Run across Excel on Windows, Mac, Web, and mobile.
• Use the Office JavaScript API.
• Installed via Office Add-in Store, sideloading, or admin tools.
• UI-focused (task panes, buttons, etc.).
• Run in a sandboxed environment (limited access to system resources).

🛠️ Example Use Cases:

• Fetch live data from a REST API.
• Show custom task panes with forms.
• Automate Excel tasks via Office JS.

💡 Example Code: JavaScript Excel Add-in

Let’s say we want to write “Hello World” into the currently selected Excel cell:

taskpane.js

Office.onReady(() => {
  document.getElementById("run").onclick = writeHelloWorld;
});

async function writeHelloWorld() {
  await Excel.run(async (context) => {
    const range = context.workbook.getSelectedRange();
    range.values = [["Hello World from JavaScript!"]];
    await context.sync();
  });
}

HTML Button UI:

<button id="run">Write to Excel</button>

📌 To test this, you’d sideload the add-in using Office Add-in Manifest or publish it via AppSource.

🔸 COM Add-ins (Component Object Model Add-ins)

These are legacy add-ins written in compiled languages like C# or VB.NET, typically built using Visual Studio.

✅ Key Features:

• Windows Desktop only.
• Built on the VSTO (Visual Studio Tools for Office) platform or directly via COM interfaces.
• Full access to Excel’s object model.
• Can read/write system resources.

🛠️ Example Use Cases:

• Custom ribbon buttons that execute macros or business logic.
• Import/export data to files or databases.
• Deep integration with Windows-based applications (like Outlook or Access).

💡 Example Code: COM Add-in Using C# (VSTO)

This example shows how to write “Hello from COM Add-in” to the selected cell when a ribbon button is clicked.

1. Ribbon Button Code (Ribbon1.cs)

private void button1_Click(object sender, RibbonControlEventArgs e)
{
    Excel.Application app = Globals.ThisAddIn.Application;
    Excel.Range cell = app.ActiveCell;
    cell.Value2 = "Hello from COM Add-in!";
}

2. Ribbon Designer (Ribbon1.xml or via Visual Studio Designer)

<customUI xmlns="http://schemas.microsoft.com/office/2009/07/customui">
  <ribbon>
    <tabs>
      <tab id="customTab" label="My Add-in">
        <group id="group1" label="Demo">
          <button id="button1" label="Say Hello" onAction="button1_Click"/>
        </group>
      </tab>
    </tabs>
  </ribbon>
</customUI>

3. Deployment:
You would build this using Visual Studio and deploy it via an installer (.msi) or ClickOnce.

🆚 Quick Comparison Table

🧠 When to Use Which?