Using method from App class in separate form xamarin - xamarin

Question

I am a beginner, so this should be an easy fix for someone with experience.
I am a veterinarian trying to build an app for vets to use. I am trying to learn via youtube and previous posts online, but I cannot find a solution for my roadblock:
I am trying to call a method already written in the App class:
Settings form binds a volume of onhand penicillin.
Usage form displays that volume, and asks the user to enter a volume used.
On button click I need the onhand volume to be 'updated' but i can't call the method in the App class (which has a get and set)
I have simplified for easier evaluation:
App code behind:
using Xamarin.Forms;
using Xamarin.Forms.Xaml;
[assembly: XamlCompilation(XamlCompilationOptions.Compile)]
namespace methods
{
public partial class App : Application
{
private const string penicillin = "penicillin";
public App()
{
InitializeComponent();
MainPage = new NavigationPage(new MainPage());
}
public string Penicillin
{
get
{
if (Properties.ContainsKey(penicillin))
{
return Properties[penicillin].ToString();
}
return "";
}
set
{
Properties[penicillin] = value;
}
}
}
}
Settings Form:
<?xml version="1.0" encoding="UTF-8"?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms" xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" x:Class="methods.Settings">
<ContentPage.Content>
<StackLayout>
<Label Text="Penicillin Onhand" ></Label>
<Entry Text="{Binding Penicillin}" Keyboard="Numeric" ></Entry>
</StackLayout>
</ContentPage.Content>
</ContentPage>
Settings Code behind:
using Xamarin.Forms;
namespace methods
{
public partial class Settings : ContentPage
{
public Settings()
{
InitializeComponent();
BindingContext = Application.Current;
}
}
}
Usage form:
<?xml version="1.0" encoding="UTF-8"?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms" xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" x:Class="methods.Usage">
<ContentPage.Content>
<StackLayout>
<Label Text="Penicillin Held" ></Label>
<Label x:Name="Held" Text="{Binding Penicillin}" ></Label>
<Label Text="Penicillin Used" ></Label>
<Entry x:Name="Using" Placeholder="ml" Keyboard="Numeric" ></Entry>
<Button Text="Use" Clicked="Used_Clicked"></Button>
</StackLayout>
</ContentPage.Content>
</ContentPage>
Usage Code behind:
using Xamarin.Forms;
namespace methods
{
public partial class Usage : ContentPage
{
public Usage()
{
InitializeComponent();
BindingContext = Application.Current;
}
void Used_Clicked(object sender, System.EventArgs e)
{
var VUsed = double.Parse(Using.Text);
var VHeld = double.Parse(Held.Text);
var Remaining = VHeld - VUsed;
//how do i code the following? Error:'App' does not contain definition for Penicillin????
App.Penicillin.value = Remaining;
}
}
}

Answer 1

You don't need to create a new instance of App - Xamarin Forms already maintains a reference to the current instance
((App)Application.Current).Penicillin = Remaining;