Using Visual Studio 2019 and Pixel 2 Pie 9.0 - API 28 (Android 9.0 - API 28) to create a simple Celsius / Fahrenheit converter mobile application.

Enter the temperature and select [Convert to Celsius] if you're using Fahrenheit and [Convert to Fahrenheit] if you're using Celsius. Simple.

MainPage.xaml

<?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="Temperature.MainPage">

    <StackLayout>
        <Grid Padding="20">
            <Grid.RowDefinitions>
                <RowDefinition Height="50" />
                <RowDefinition Height="50" />
                <RowDefinition Height="50" />
                <RowDefinition Height="50" />
                <RowDefinition Height="50" />
                <RowDefinition Height="50" />
                <RowDefinition Height="50" />
            </Grid.RowDefinitions>
            <Grid.ColumnDefinitions>
                <ColumnDefinition/>
            </Grid.ColumnDefinitions>
            <Label x:Name="lblPrompt" Text="Enter Temperature (degrees):" Grid.Row="0" Grid.Column="0" FontSize="20"/>
            <Entry x:Name="initialTemp" Grid.Row="1" Grid.Column="0" FontSize="30"/>
            <Label x:Name="lblResult" Text="Converted Temperature:" Grid.Row="2" Grid.Column="0" FontSize="20"/>
            <Label x:Name="convertedTemp" Grid.Row="3" Grid.Column="0" FontSize="30"/>

            <Button x:Name="BtnToCelsius" Text="Convert to Celsius" Grid.Row="4" Grid.Column="0" FontSize="20" Clicked="BtnToCelsius_Clicked" />
            <Button x:Name="BtnToFahrenheit" Text="Convert to Fahrenheit" Grid.Row="5" Grid.Column="0" FontSize="20" Clicked="BtnToFahrenheit_Clicked" />
            <Button x:Name="BtnReset" Text="Reset" Grid.Row="6" Grid.Column="0" FontSize="20" Clicked="BtnReset_Clicked" />
        </Grid>
    </StackLayout>

</ContentPage>

MainPage.xaml.cs

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using Xamarin.Forms;

namespace Temperature
{
    public partial class MainPage : ContentPage
    {
        public MainPage()
        {
            InitializeComponent();
        }

        //parse user input of degrees into a double
        //public double initTemp;

        private void BtnToCelsius_Clicked(object sender, EventArgs e)
        {
            try
            {
                //parse user input of degrees into a double
                double initTemp = Double.Parse(initialTemp.Text);
                //use formula of converting fahrenheit to celsius
                double TempToCelsius = (initTemp - 32) * 5 / 9;

                //output - convert to string
                convertedTemp.Text = TempToCelsius.ToString() + " °C";

                //disable both conversion buttons to avoid more than one conversion at a time
                disableControls();
            }
            catch (Exception)
            {
                //if user does not enter a number, this error message will be displayed
                convertedTemp.Text = "Please enter valid input.";
            }
        }

        private void BtnToFahrenheit_Clicked(object sender, EventArgs e)
        {
            try
            {
                //parse user input of degrees into a double
                double initTemp = Double.Parse(initialTemp.Text);

                //use formula of converting celsius to fahrenheit
                double TempToFahrenheit = (initTemp * 1.8) + 32;

                //output - convert to string
                convertedTemp.Text = TempToFahrenheit.ToString() + " °F";

                //disable both conversion buttons to avoid more than one conversion at a time
                disableControls();
            }
            catch (Exception)
            {
                //if user does not enter a number, this error message will be displayed
                convertedTemp.Text = "Please enter valid input.";
            }

        }

        //reset fields and enable buttons
        private void BtnReset_Clicked(object sender, EventArgs e)
        {
            initialTemp.Text = String.Empty;
            convertedTemp.Text = String.Empty;
            enableControls();
        }

        //method that disables interactive controls
        private void disableControls()
        {
            initialTemp.IsEnabled = false;
            BtnToCelsius.IsEnabled = false;
            BtnToFahrenheit.IsEnabled = false;
        }

        //method that enables interactive controls
        private void enableControls()
        {
            initialTemp.IsEnabled = true;
            BtnToCelsius.IsEnabled = true;
            BtnToFahrenheit.IsEnabled = true;
        }
    }
}

Running