[DesignOnly] attribute not supported for extender properties

[Lydia-Shi]

• .NET Core Version:
  5.1.100.rc.1.20371.1

• Have you experienced this same bug with .NET Framework?:
  No

Problem description:
Extra '(Name)' property appears in Property Browser window for 'splitContainer.panel'

Expected behavior:
No (Name) property appears in Property Browser window.

Minimal repro:

1. Create a Winforms .NET Core project
2. Drag and drop a SplitContainer control to Form Designer
3. Select the splitContainer1.Panel1, observe its Property Browser window

[merriemcgaw]

As described by @DustinCampbell

The symptom that is appearing in the WinForms Designer is small, but the real issue is significant. The problem is that every extender property does not get a [DesignOnly] attribute in .NET Core, when it does in .NET Framework. In .NET Framework, there is a specific code path to make sure this attribute is added specially, so it's a clear .NET regression. This is a fundamental bug that needs to be fixed because it can affect any tools that process extender properties and rely on this attribute being present. The WinForms designer is not the only consumer of the TypeDescriptor/PropertyDescriptor API.

Hence migrating this to the dotnet core runtime repo

[Dotnet-GitSync-Bot]

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[DustinCampbell]

The problem is a bit of code missing on .NET Core here:

runtime/src/libraries/System.ComponentModel.TypeConverter/src/System/ComponentModel/ReflectTypeDescriptionProvider.cs

Lines 1105 to 1114 in 746fbba

	// Now that we have our extended properties we can build up a list of callable properties. These can be
	// returned to the user.
	//
	properties = new PropertyDescriptor[extendedProperties.Length];
	for (int idx = 0; idx < extendedProperties.Length; idx++)
	{
	ReflectPropertyDescriptor rpd = extendedProperties[idx];
	properties[idx] = new ExtendedPropertyDescriptor(rpd, rpd.ExtenderGetReceiverType(), provider, null);
	}

On NetFx, that looks like so:

            // Now that we have our extended properties we can build up a list of callable properties.  These can be 
            // returned to the user.
            //
            properties = new PropertyDescriptor[extendedProperties.Length];
            for (int idx = 0; idx < extendedProperties.Length; idx++)
            {
                Attribute[] attrs = null;
                IComponent comp = provider as IComponent;
                if (comp == null || comp.Site == null)
                {
                    attrs = new Attribute[] {DesignOnlyAttribute.Yes};
                }
 
                ReflectPropertyDescriptor  rpd = extendedProperties[idx];
                ExtendedPropertyDescriptor epd = new ExtendedPropertyDescriptor(rpd, rpd.ExtenderGetReceiverType(), provider, attrs);
                properties[idx] = epd;
            }

Essentially, in should be adding an extra property for sited components, but it doesn't.

I can make this fix if that helps.

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[danmoseley]

We welcome PR's 😊

[safern]

This has been the behavior since 3.1 so marking as future for now given the timeframe, but a PR is welcome if we want to get this in earlier 😄

[Lydia-Shi]

Verified this issue with Dev16 16.10.0 Preview 2.0 [31023.296.main], this issue is fixed that no (Name) property appears in Property Browser window for 'splitContainer.panel', results as below: