Docs: Custom Task Panes section omits the .NET 6+ COM default-interface requirement

[webdjoe]

Summary

src/pages/reference-various.md → Custom Task Panes documents the CTP API but not the COM attribution a UserControl needs on .NET 6+. Without it, CustomTaskPaneFactory.CreateCustomTaskPane fails with a bare E_FAIL (0x80004005) and no diagnostic text.

This is already known — you answered it in issue #558 and in the group thread CustomTaskPaneFactory.CreateCustomTaskPane throws 'Unable to create specified ActiveX control':

You now need an explicit 'default interface' for your type - this will cause the class to implement IDispatch (if it is ComVisible).

The reference page doesn't carry that, so it's still reachable by reading the docs alone.

What I measured

Standalone add-in, no other dependency, ExcelDna.AddIn 1.9.0, SDK 10.0.400, Excel 16.0.20326.20112 x64 Click-to-Run Current Channel, Windows 11 25H2 26200.9168, not elevated, Defender only, Trust Center at defaults.

Control under test:

[ComVisible(true)]
public sealed class FixedLabelControl : UserControl
{
    public FixedLabelControl() => Controls.Add(new Label { Text = "Repro Pane", AutoSize = true });
}

Target	Runtime	Result
net10.0-windows	10.0.11	COMException, HRESULT -2147467259 (0x80004005)
net7.0-windows	7.0.7	COMException, HRESULT -2147467259 (0x80004005)

Byte-identical source both times; only the TFM differed. Adding the documented attribution and changing nothing else:

[ComVisible(true)]
[Guid("...")]
public interface IFixedLabelControl { }

[ComVisible(true)]
[Guid("...")]
[ComDefaultInterface(typeof(IFixedLabelControl))]
public sealed class FixedLabelControl : UserControl, IFixedLabelControl { ... }

net10.0-windows then completed the full lifecycle — create, show, hide, delete, dispose — with no HRESULT.

Because the requirement splits .NET Framework from .NET Core rather than one .NET version from another, a net7-vs-net10 comparison can't discriminate it: both fail identically. That cost me a fair amount of bisecting before I found #558, which is really the motivation for this request.

Suggested addition

After the CreateCustomTaskPane example in the Custom Task Panes section:

.NET 6 and later: the control needs an explicit COM default interface. .NET Framework generated a class interface automatically, so a [ComVisible(true)] UserControl had a usable IDispatch; .NET Core and later do not, and CreateCustomTaskPane then fails with E_FAIL / "Unable to create specified ActiveX control".

public interface IMyUserControl { }

[ComVisible(true)]
[Guid("....")]
[ComDefaultInterface(typeof(IMyUserControl))]
public class MyUserControl : UserControl, IMyUserControl { }

Happy to open a PR against src/pages/reference-various.md if that's easier.

Separately, if it's ever cheap to do

ExcelCustomTaskPane.CreateCustomTaskPane already logs a helpful message for the UnauthorizedAccessException registry case. A comparable hint when ICTPFactory.CreateCTP throws E_FAIL — pointing at the default-interface requirement — would make this self-diagnosing. Entirely optional; the docs change is the ask.

[govert]

Thank you for creating the detailed issue. I understand and agree with the suggestion and will have a closer look.