Upgrading Inventor Add-ins to .NET 8: Image Handling & Interop Best Practices

With Autodesk Inventor 2026 and modern development tools like Visual Studio 2022 and .NET 8, many developers are upgrading and modernizing their Inventor add-ins. This migration brings clear benefits — improved performance, future compatibility, and simplified deployment — but it also introduces new patterns for COM interop, image handling, and ribbon UI resources.

One of the most common issues developers encounter during migration is:

Inventor fails to load the add-in whenever ribbon button images are used.
(But loads perfectly when using text-only buttons.)

In this post, we’ll explore why this happens, how to fix it, and what has changed for .NET 8–based Inventor add-ins.

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The Challenge: When Images Break Your Add-in

Developers migrating from older Inventor versions (using .NET Framework 4.7) have reported the following behaviour in Inventor 2026.1:

• ✔ The add-in loads correctly when using text-only ribbon buttons
• ✖ The add-in fails to load as soon as a custom icon is assigned

This issue did not occur in Inventor 2020–2024 but became visible after switching to:

• Inventor 2026.1
• .NET 8
• The new COM interop model

The root cause?
❗ Changes in how .NET 8 handles image → COM IPictureDisp conversion.

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The Fix: Safe Image Conversion Using IPictureDisp

To reliably load images in Inventor 2026+, icons must be converted to Inventor.IPictureDisp using a safe helper that leverages a hidden AxHost wrapper.

Below is the recommended utility method for .NET 8 add-ins.

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Utility Class: Image → IPictureDisp Conversion

using stdole;
using System.Drawing;
using System.Windows.Forms;

public static class ImageConverterUtil
{
    public static IPictureDisp ConvertImageToPictureDisp(Image image)
    {
        return (IPictureDisp)AxHostWrapper.GetIPictureDispFromPicture(image);
    }

    private class AxHostWrapper : AxHost
    {
        private AxHostWrapper() : base("") { }

        public static object GetIPictureDispFromPicture(Image image)
        {
            return GetIPictureDispFromPicture(image);
        }
    }
}

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Usage Example — Creating a ButtonDefinition

var icon = Properties.Resources.MyCustomIcon; // .png or .ico

var largeIcon = ImageConverterUtil.ConvertImageToPictureDisp(icon);
var smallIcon = ImageConverterUtil.ConvertImageToPictureDisp(icon);

_buttonDef = invApp.CommandManager.ControlDefinitions.AddButtonDefinition(
    "MyDrawingDocButton",
    "MyCompany.DrawingDocButton",
    CommandTypesEnum.kShapeEditCmdType,
    "{YOUR-GUID-HERE}",
    "Button tooltip text",
    "Button description",
    smallIcon,
    largeIcon,
    ButtonDisplayEnum.kAlwaysDisplayText
);

✔ Once this conversion method is implemented, icons load correctly
✔ Add-ins no longer fail during startup
✔ Works reliably across Inventor 2026 and future releases

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What’s New for .NET 8–Based Inventor Add-ins

Migrating from Inventor 2024 (.NET Framework) to Inventor 2026 (.NET 8) involves several important updates.

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1. Use Inventor.IPictureDisp — No Need for stdole NuGet

Before (.NET Framework 4.7)	Now (.NET 8)
Developers referenced stdole via NuGet	No longer necessary
Manual interop alignment required	Inventor.IPictureDisp is provided directly in Autodesk.Inventor.Interop.dll

✔ Cleaner project setup
✔ Fewer dependencies

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2. Updated Interop Reference Settings

Correct reference settings for .NET 8:

Setting	.NET Framework (≤ 2024)	.NET 8 (Inventor 2026+)
Embed Interop Types	True	False
Copy Local	False	True

Why the change?

• .NET 8 uses a different COM activation model
• Interop DLLs must be shipped alongside your add-in
• Embedded types prevent correct COM binding

✔ Set Embed Interop Types = False
✔ Set Copy Local = True

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Key Takeaways for a Smooth Migration

Here’s a consolidated checklist for developers modernizing their Inventor add-ins:

Image & Ribbon Icons

• ✔ Use safe conversion: AxHost → IPictureDisp
• ✔ Avoid direct .NET → COM image casting in .NET 8

Interop

• ✔ Use Autodesk.Inventor.Interop.dll
• ✔ Embed Interop Types = False
• ✔ Copy Local = True

Ribbon Control Definitions

• ✔ Always convert icons before passing them to Inventor

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Conclusion

Migrating to .NET 8 and Inventor 2026+ brings many benefits — smoother deployment, improved performance, and long-term compatibility. However, add-ins using ribbon icons must update their image conversion approach to avoid loading failures.

By implementing the IPictureDisp conversion method and adjusting interop settings, developers can ensure:

• Stable add-in loading
• Proper icon rendering
• Fully compatible .NET 8–based Inventor extensions