How set OnPLatform value of "nothing" - xamarin.forms

Question

I see threads about default values for OnPlatform, but I don't want a default value, I want a value of "nothing."
Here is my code, with a ToolbarItem for UWP and "nothing" for Android. Unfortunately, it throws an exception. How do I code an On Platform value of "nothing"? I can get rid of the exception by coding <ToolbarItem Text=""/> but it takes up Toolbar real estate that gets in the way of other controls.
<ContentPage.ToolbarItems>
<OnPlatform x:TypeArguments="ToolbarItem">
<On Platform="UWP">
<ToolbarItem IconImageSource="MyIcon.png"/>
</On>
<On Platform="Android">
</On>
</OnPlatform>
...

Answer 1

It appears you cannot do this in XAML. OnPlatform must provide a TypeArguments object for each platform on which it executes, or an exception will be thrown. Seems to me like a shortcoming in the XAML design, but no one asked me.
The solution, and in fewer lines of code than the XAML above, is in the page constructor:
if (Device.RuntimePlatform == Device.UWP)
{
ToolbarItem tbi = new ToolbarItem();
tbi.IconImageSource = "MyIcon.png";
this.ToolbarItems.Insert(0, tbi);
}
You could vary the 0 in the Insert depending on where you wanted the ToolbarItem to appear (untested). I wanted it first of the ToolbarItems, hence 0.

Answer 2

You could use the code below:
<ContentPage.ToolbarItems>
<OnPlatform x:TypeArguments="ToolbarItem">
<On Platform="UWP">
<ToolbarItem IconImageSource="MyIcon.png"/>
</On>
<On Platform="Android">
<ToolbarItem IconImageSource=""></ToolbarItem>
</On>
</OnPlatform>
</ContentPage.ToolbarItems>

Answer 3

Keep the ToolbarItem in XAML ResourceDictionary (e.g. in <ContentPage.Resources>):
<ResourceDictionary>
<OnPlatform x:Key="toolbarItem" x:TypeArguments="ToolbarItem">
<On Platform="iOS">
<ToolbarItem
...
<On Platform="UWP">
<ToolbarItem
...
<!-- Android: No ToolbarItem -->
Add it in the code behind page constructor using the resource key "toolbarItem":
InitializeComponent();
...
if (Resources.TryGetValue(key: "toolbarItem", out object value) &&
value is OnPlatform<ToolbarItem> onPlatform)
{
IEnumerable<ToolbarItem> items = GetElement(onPlatform);
if (items.Any() &&
items.FirstOrDefault() is ToolbarItem toolbarItem)
{
ToolbarItems.Add(toolbarItem);
GetElement example implementation:
IEnumerable<T> GetElement<T>(OnPlatform<T> onPlatform) where T : Element
{
foreach (var platforms in onPlatform.Platforms)
{
foreach (string platform in platforms.Platform)
{
if (platform == Device.RuntimePlatform)
{
yield return platforms.Value as T;
}
}
}
}