Xamarin.Forms DynamicResource doesn't seem to be working - xaml

Question

I can't seem to get a DynamicResource working correctly.
Here's the XAML in App.xml:
<Application.Resources>
<ResourceDictionary>
<Thickness x:Key ="DefaultInsets"
Bottom="4"
Top ="4"
Left ="8"
Right ="8" />
</ResourceDictionary>
</Application.Resources>
And here's what I'm trying to do inside another file's ContentView:
<StackLayout
VerticalOptions="Center"
HeightRequest="225"
Margin="{DynamicResource DefaultInsets}" >
<Entry
x:Name="nameControl"
Placeholder="Full Name"
Margin="{DynamicResource DefaultInsets}"
/>
<Entry
x:Name="passwordControl"
IsPassword="True"
Placeholder="Password"
Margin="{DynamicResource DefaultInsets}" />
</StackLayout>
It seems like this should make consistent insets for the StackLayout from the ContentView, and for the Entry fields from the StackLayout. But I don't see any of that.
What have I done wrong?
Update : I have also tried this with StaticResource, and still no luck.

Answer 1

I use following type with ResourceDictionary. For distinguish, I set the top_margin to 40, here is my screenshot.
Here is my ResourceDictionary.
<Application.Resources>
<ResourceDictionary>
<OnPlatform x:Key="OuterPadding" x:TypeArguments="Thickness">
<On Platform="Android">8 ,40,8,4</On>
<On Platform="iOS">20</On>
<On Platform="WinPhone">24</On>
</OnPlatform>
<!-- left, top, right, and bottom-->
</ResourceDictionary>
</Application.Resources>
Here is my layout.
<StackLayout
VerticalOptions="Center"
HeightRequest="225"
Margin="{DynamicResource OuterMargin}" >
<Entry
x:Name="nameControl"
Placeholder="Full Name"
Margin="{DynamicResource OuterMargin}"
/>
<Entry
x:Name="passwordControl"
IsPassword="True"
Placeholder="Password"
Margin="{DynamicResource OuterMargin}" />
</StackLayout>
Update
I found you layout issue is related to the VerticalOptions="Center" in the StackLayout(If you set the VerticalOptions="Center", then set the margin value is not work).
If I delete it and use StaticResource.
Here is running screenshot(For testing, I change the value of margin top to 20)

Answer 2

Other answers may be valid for some people, but in the end here's what I was doing wrong:
InitializeComponent() was never getting called inside App.xaml.cs.
I had an if-then statement that was always hitting a return before ever getting to InitializeComponent().
When that doesn't get called, App.xaml is inaccessible to the rest of the app, and nothing defined there is available as a DynamicResource.
So I moved InitializeComponent() to happen before the if-then statement, and now it all works like gangbusters.
I hope this helps someone!