CancelAsync Race condition

It would be helpful to not just point out that this can occur, but to give example(s) of the best ways of handling it. The Managed Threading Best Practices article does not really do this.

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#### Document Details

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* ID: 426f70a3-bedc-c1ff-06f2-13d81d269397
* Version Independent ID: 36a9a55e-d5ee-8180-2ff0-3163d675cbad
* Content: [BackgroundWorker.CancelAsync Method (System.ComponentModel)](https://docs.microsoft.com/en-us/dotnet/api/system.componentmodel.backgroundworker.cancelasync?view=netframework-4.7.2#System_ComponentModel_BackgroundWorker_CancelAsync)
* Content Source: [xml/System.ComponentModel/BackgroundWorker.xml](https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System.ComponentModel/BackgroundWorker.xml)
* Product: **dotnet-api**
* GitHub Login: @dotnet-bot
* Microsoft Alias: **dotnetcontent**