Show how to coerce a value for a custom dependency property

[michael-hawker]

UWP doesn't support the coercion method that WPF did, but there is a pattern the WinUI team uses:

https://github.com/microsoft/microsoft-ui-xaml/blob/3d2d5a82eea26475c154e1fdf7c589e97365eeef/dev/ProgressRing/ProgressRing.cpp#L147-L164

It'd be great to have a concrete C++ and C# example to show how to do this common pattern within the docs, as it's important to do in the property change callback and not the property setter (as per the docs calling out that SetValue/property setter is short-cuted by the XAML parser).

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Document Details

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• ID: cf035b98-e0c9-50c2-45ad-1dd37b4f31b0
• Version Independent ID: 4434d33d-7a43-8181-26f2-414ac194c272
• Content: Custom dependency properties - UWP applications
• Content Source: windows-apps-src/xaml-platform/custom-dependency-properties.md
• Product: uwp
• Technology: platform
• GitHub Login: @jwmsft
• Microsoft Alias: jimwalk

[michael-hawker]

This is a link for more about the finally construct used in the C++ code-base of WinUI: https://docs.microsoft.com/cpp/code-quality/c26448

Not sure if there's a better way than just setting flag back in C# land... e.g.

private bool _isUpdating;

private static void PropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
    if (d is MyClass thing && !thing._isUpdating)
    {
        thing._isUpdating = true;

        thing.CoerceValues();
        
        thing._isUpdating = false;
    }
}

private void CoerceValues()
{
    if (Value < Minimum)
    {
        Value = Minimum;
    }
    else if (Value > Maximum)
    {
        Value = Maximum;
    }
}

@codendone any suggestions for this pattern?

[codendone]

I think that is the best pattern I've seen. @MikeHillberg, are you aware of any better patterns?