MvvmCross双向绑定问题：nint与enum类型转换异常

Let's break down what's going wrong and fix it quickly:

The Root Cause #

Your exception happens because the generic MvxValueConverter<TFrom, TTo> base class tries to perform a forced cast on a boxed value before your custom ConvertBack runs.

UISegmentedControl's SelectedSegment is a nint, but during binding, it gets boxed as an int (a common quirk in Xamarin.iOS bindings). Your converter uses MvxValueConverter<MyViewModel.Selection, nint>, so the base class tries to cast the boxed int directly to nint—which throws an InvalidCastException because you can't unbox an int directly to a nint struct. That's why your ConvertBack code never even gets called.

Solution 1: Use int Instead of nint in the Converter #

Since your enum values are small (0,1,2), int and nint are compatible across 32/64-bit systems. This is the simplest fix:

Update your SelectionValueConverter to target int instead of nint:

public class SelectionValueConverter : MvxValueConverter<MyViewModel.Selection, int>
{
    protected override int Convert(MyViewModel.Selection value, Type targetType, object parameter, CultureInfo culture)
    {
        return (int)value;
    }

    protected override MyViewModel.Selection ConvertBack(int value, Type targetType, object parameter, CultureInfo culture)
    {
        return (MyViewModel.Selection)value;
    }
}

Why this works:

• When updating the UI from the ViewModel: The enum-to-int conversion works, and int implicitly converts to nint for UISegmentedControl.
• When updating the ViewModel from the UI: The boxed int from the control is correctly cast by the base class, so your ConvertBack runs as expected.

Solution 2: Use Non-Generic MvxValueConverter (For Strict nint Handling) #

If you need to strictly work with nint (e.g., for large segment counts), skip the generic base class and handle all type conversions yourself:

public class SelectionValueConverter : MvxValueConverter
{
    public override object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        if (value is MyViewModel.Selection selection)
        {
            return (nint)selection;
        }
        return 0;
    }

    public override object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        int intValue;
        // Handle both boxed int and nint cases
        if (value is nint nintVal)
        {
            intValue = (int)nintVal;
        }
        else if (value is int intVal)
        {
            intValue = intVal;
        }
        else
        {
            throw new ArgumentException($"Unsupported value type: {value.GetType().Name}");
        }

        return (MyViewModel.Selection)intValue;
    }
}

This bypasses the generic base class's problematic casting step and lets you handle all input types explicitly.

Your Binding Code Stays the Same #

No changes needed here—your existing binding setup will work with either converter:

var set = this.CreateBindingSet<MyView, MyViewModel>();
set.Bind(SegmentedControl).To(vm => vm.SelectedSegment).WithConversion(new SelectionValueConverter());
set.Apply();

内容的提问来源于stack exchange，提问作者Ivan Mir