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IMsoComponent support is opt-in

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A change was made to avoid Windows Forms threads always registering with existing IMsoComponentManager instances. However, you can opt in to register existing IMsoComponentManager instances to your Windows Forms threads.

Version introduced

.NET 9 Preview 2

Previous behavior

Previously, Windows Forms threads always registered with existing IMsoComponentManager instances.

New behavior

Starting in .NET 9, Windows Forms threads don't automatically integrate with process-registered IMsoComponentManagers. To get the previous behavior back, set the switch Switch.System.Windows.Forms.EnableMsoComponentManager.

Change category

This change is a behavioral change.

Reason for change

This change was made for performance and efficiency. The previous behavior presented a lot of overhead as it used COM, and not all developers need this behavior.

If you wish to revert to the previous behavior, you can opt in to IMsoComponent support using a switch in the runtimeconfig.json file or as a RuntimeHostConfigurationOption item in the project file.

runtimeconfig.json file:

{
  "configProperties": {
    "Switch.System.Windows.Forms.EnableMsoComponentManager": true
 }
}

Project file:

<ItemGroup>
  <RuntimeHostConfigurationOption Include="Switch.System.Windows.Forms.EnableMsoComponentManager" Value="true" />
</ItemGroup>

Affected APIs