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SegmentedControl SelectedIndex issue - NullReferenceException

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SegmentedControl

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answered on 26 Nov 2018, 08:16 PM

Hello Martin,

This is expected behavior as you cannot set an index that is out of the available range (the exception should be an OutOfRangeException instead).

To clarify about the -1 value, is the typical default SelectedIndex when nothing is selected. Such is the case when there are no items yet. If you have a view model property two-way bound to the control, you should always initialize that property to -1 instead of 0. The SegementedControl will change it to 0 after the ItemsSource is set.

private mySelectedIndex = -1;

public MySelectedIndex

{

get => mySelectedIndex;

set => SetProperty(ref mySelectedIndex, value);

}

On the other side of things, you should always check the item count before setting an index:

var indexToSet = 5;

if(indexToSet > myItems.Count - 1)

{

indexToSet = myItems.Count - 1;

}

SelectedIndex = indexToSet;

Feature Request

Although the Xamarin.Forms Picker lets you set an out of bound value, and it shifts to the next available value, this is not the typical behavior of items controls. If there isn't an index available, it should not select an invalid location instead (first or last item).

In any case, I have submitted a feature request to development team to consider adding a Picker-like behavior: SegmentedControl SelectedIndex Bounds. Please take a moment to up-vote/follow the item and leave a comment if you have any other info you'd like to add.

I hope this info was helpful.

Regards,
Lance | Tech Support Engineer, Sr.
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answered on 27 Nov 2018, 09:37 AM

Hello Lance,

thank you for your message. You are right, SelectedIndex is databound to my view model too and I initialized it with 0, but the ItemsSource was loaded async via Task in my view model. That's why ItemsSource was still null when Segmented Control read the databound SelectedIndex. I fixed my view model immediately. I think the NullReferenceExceptions is thrown from the underlying framework, because Segemented Control accesses it's ItemsSource without null-check, that's why I was writing here.

What I know from my experience with WPF, controls do not throw exceptions on problematic values coming from databinding. I think they see databinding more or less as a lightweight or loose connection, which does not throw hard exceptions, but ignores.

The example with setting SelectedIndex to an out of bounds value was just a test I ran to see what Segmented Control does and what Framework controls do, it was not from my programming.

That's all, thank you and have a nice day.

Regards

Martin