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Nullness issue - Can't cleanly implement INotifyPropertyChanged by publishing an Event #18361

@marklam

Description

@marklam

Issue description

A method of implementing INotifyPropertyChanged by publishing an Event now seems impossible to make compile cleanly (?)

namespace Nullness

open System.ComponentModel

type XViewModel() =
    let propertyChanged = Event<PropertyChangedEventHandler, PropertyChangedEventArgs>()
    //let propertyChanged = Event<PropertyChangedEventHandler|null, PropertyChangedEventArgs>()

    interface INotifyPropertyChanged with
        [<CLIEvent>]
        member this.PropertyChanged = propertyChanged.Publish

generates this warning:

1>C:\git\temp\Nullness\Nullness\XViewModel.fs(10,39): warning FS3261: Nullness warning: The types 'System.Delegate' and 'System.Delegate | null' do not have compatible nullability.. See also C:\git\temp\Nullness\Nullness\XViewModel.fs(10,38)-(10,61).

And there doesn't seem to be a way to avoid this.

Choose one or more from the following categories of impact

  • Unexpected nullness warning (false positive in nullness checking, code uses --checknulls and langversion:preview).
    Missing nullness warning in a case which can produce nulls (false negative, code uses --checknulls and langversion:preview).
    Breaking change related to older null constructs in code not using the checknulls switch.
    Breaking change related to generic code and explicit type constraints (null, not null).
    Type inference issue (i.e. code worked without type annotations before, and applying the --checknulls enforces type annotations).
    C#/F# interop issue related to nullness metadata.
    Other (none of the categories above apply).

Operating System

Windows (Default)

What .NET runtime/SDK kind are you seeing the issue on

.NET SDK (.NET Core, .NET 5+)

.NET Runtime/SDK version

Net SD 9.0.200

Reproducible code snippet and actual behavior

namespace Nullness

open System.ComponentModel

type XViewModel() =
    let propertyChanged = Event<PropertyChangedEventHandler, PropertyChangedEventArgs>()
    //let propertyChanged = Event<PropertyChangedEventHandler|null, PropertyChangedEventArgs>()

    interface INotifyPropertyChanged with
        [<CLIEvent>]
        member this.PropertyChanged = propertyChanged.Publish

Possible workarounds

No response

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Area-NullnessIssues related to handling of Nullable Reference TypesBug

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@marklam@abonie@T-Gro

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    Nullness issue - Can't cleanly implement INotifyPropertyChanged by publishing an Event · Issue #18361 · dotnet/fsharp