Xamarin.forms - 具有 3 种不同更改方法的 3 个选择器会感到困惑

Question

I have these 3 Pickers:

<Picker x:Name="CommunityPicker" Title="Select Community" WidthRequest="400" ItemDisplayBinding="{Binding Description}" SelectedIndexChanged="communityChange" />
<Picker x:Name="LotPicker" Title="Select Lot" WidthRequest="400" ItemDisplayBinding="{Binding Lot}" SelectedIndexChanged="lotChange" />
<Picker x:Name="ModelPicker" Title="Select Model" WidthRequest="400" ItemDisplayBinding="{Binding Description}" SelectedIndexChanged="modelChange" />

and I have these 3 changes methods:

private void communityChange(object sender, EventArgs e) {
     community = ((sender as Picker).SelectedItem as AreaClass).Area;
}

private void lotChange(object sender, EventArgs e) {
     lot = ((sender as Picker).SelectedItem as LotClass).Lot;
}

private void modelChange(object sender, EventArgs e) {
     model = ((sender as Picker).SelectedItem as ModelClass).Model;
}

When I go to use the Pickers for the first time everything works, but when I goto pick another option, I get the following error:

Object reference not set to an instance of an object

On the lines where I am getting the selected value....and for some reason I am getting this error on the wrong method...IE when I goto change the value of CommunityPicker, I am getting the method for LotPicker...

What am I doing wrong?

Answer 1

I don't know the other codes of your app. But I reproduced this question by a simple demo.You can check it here .

And I resolved this question by determining whether the value is null and then process it accordingly, you can refer to it:

 private void RegionPicker_SelectedIndexChanged(object sender, EventArgs e)
    {
        Region region =  ((sender as Picker).SelectedItem as Region);

        if (region != null)
        {
            StreetPicker.ItemsSource = region.Streets;
        }
        else {
            StreetPicker.ItemsSource = null;
        }
    }

So you can try to do like this(The same goes for other pickers):

LotClass temp = (sender as Picker).SelectedItem as LotClass);

if(temp!=null){

}else{

}

Hope it can help you.