fix: support .NET Core in winforms activation design mode check#4358
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glennawatson merged 3 commits intoMay 17, 2026
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* Delay the check. Controls are not yet sited while their constructor is running. * Use Control.IsAncestorSiteInDesignMode to support arbitrarily deeply nested controls.
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Might be about a week before we release btw. Just waiting on validation of a new code signing certificate. |
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What kind of change does this PR introduce?
Control.IsAncestorSiteInDesignMode(polyfilled on older targets) to support deeply nested controls.LicenseManager.UsageModecheck because it doesn't work on .NET Core and is no longer needed on .NET Framework after the other fixes.Form.What is the current behavior?
Design mode detection is broken on .NET Core.
The detection is done too early. Controls are not yet sited in their constructor, so
control.Site?.DesignModeis never true.LicenseManageris not supported on .NET Core.LicenseManager.UsageMode == LicenseUsageMode.Designtimeis always false.Design mode detection does not support deeply nested controls.
Only the current control and its parent are checked for design mode.
What is the new behavior?
Design mode detection works both on .NET Framework and .NET Core and supports deeply nested controls.
What might this PR break?
This shouldn't change anything at runtime, only design time.
Checklist
mainbranchAdditional information
Manually tested on
net481andnet10.0-windows10.0.19041.0.There are still more issues with the Winforms Designer.
Unless you inherit your controls from a base class that ensures ReactiveUI is initialized in its constructor,
WhenActivatedfails to resolveIActivationForViewFetcher.I don't know how to fix this in a way that can be upstreamed.