radio button not pre-selecting on screen loading - xamarin.forms

Question

I feel this is weird. i can't get the Radio button to pre-select a saved value and it's driving me mad. I have this xaml:
<StackLayout Orientation="Horizontal" RadioButtonGroup.GroupName="Parities"
RadioButtonGroup.SelectedValue="{Binding Parity}">
<RadioButton Value="1" Content="Income" />
<RadioButton Value="-1" Content="Expense" />
<RadioButton Value="0" Content="Neutral" />
</StackLayout>
Furthermore, even if I replace SelectedValue with a hard coded literal value "1" (for Income), the radio button still show up blank. The only way that works is by setting IsChecked on each of the 3 options to have the them pre-selected.
What am I missing?

Answer 1

Based on your code ,I created a simple demo, but I couldn't reproduce this problem. It just works properly.
You can refer to the following code:
MyPage.xaml
<ContentPage.BindingContext>
<radiobuttondemos:MyViewModel></radiobuttondemos:MyViewModel>
</ContentPage.BindingContext>
<StackLayout>
<StackLayout Orientation="Horizontal" RadioButtonGroup.GroupName="{Binding GroupName}"
RadioButtonGroup.SelectedValue="{Binding Parity}">
<RadioButton Value="1" Content="Income" />
<RadioButton Value="-1" Content="Expense" />
<RadioButton Value="0" Content="Neutral" />
</StackLayout>
</StackLayout>
The MyViewModel.cs
public class MyViewModel : INotifyPropertyChanged
{
string groupName;
object parity;
public object Parity
{
get => parity;
set
{
parity = value;
OnPropertyChanged(nameof(Parity));
}
}
public MyViewModel () {
GroupName = "Parities";
Parity = "1";
}
public string GroupName
{
get => groupName;
set
{
groupName = value;
OnPropertyChanged(nameof(GroupName));
}
}
public event PropertyChangedEventHandler PropertyChanged;
void OnPropertyChanged(string propertyName)
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
}
Note:
In the constructor of MyViewModel, I initialize the value of variable Parity as follows:
Parity = "1";
And if we initialize a value as follows, the UI will not pre-select the saved value :
Parity = 1;