XamarinForms NET Standard 2.0

FileExtensions.cs

public static class FileExtensions
{
    private static readonly string LocalFolder;

    static FileExtensions()
    {
        // Gets the target platform's valid save location
        LocalFolder = Environment.GetFolderPath(Environment.SpecialFolder.LocalApplicationData);
    }

    // Byte[] extension methods

    public static async Task<string> SaveToLocalFolderAsync(this byte[] dataBytes, string fileName)
    {
        return await Task.Run(() =>
        {
                // Use Combine so that the correct file path slashes are used
                var filePath = Path.Combine(LocalFolder, fileName);

            if (File.Exists(filePath))
                File.Delete(filePath);

            File.WriteAllBytes(filePath, dataBytes);

            return filePath;
        });
    }

    public static async Task<byte[]> LoadFileBytesAsync(string filePath)
    {
        return await Task.Run(() => File.ReadAllBytes(filePath));
    }
    
    // Stream extension methods

    public static async Task<string> SaveToLocalFolderAsync(this Stream dataStream, string fileName)
    {
        // Use Combine so that the correct file path slashes are used
        var filePath = Path.Combine(LocalFolder, fileName);

        if (File.Exists(filePath))
            File.Delete(filePath);

        using (var fileStream = File.OpenWrite(filePath))
        {
            if (dataStream.CanSeek)
                dataStream.Position = 0;

            await dataStream.CopyToAsync(fileStream);

            return filePath;
        }
    }

    public static async Task<Stream> LoadFileStreamAsync(string filePath)
    {
        return await Task.Run(() =>
        {
            using (var fileStream = File.OpenRead(filePath))
            {
                return fileStream;
            }
        });
    }
}

StartPage.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"
             xmlns:telerikDataControls="clr-namespace:Telerik.XamarinForms.DataControls;assembly=Telerik.XamarinForms.DataControls"
             xmlns:viewModels="clr-namespace:NetStandardTest.UpdatedPortable.ViewModels;assembly=NetStandardTest.UpdatedPortable"
             x:Class="NetStandardTest.Forms.StartPage">

    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition />
            <RowDefinition Height="Auto" />
        </Grid.RowDefinitions>
        
        <Image x:Name="BackgroundImage" 
               Grid.RowSpan="2"/>

        <Frame BackgroundColor="White"
               HorizontalOptions="Center"
               VerticalOptions="Center"
               Margin="20">
            <Label x:Name="OutputLabel"
                   LineBreakMode="CharacterWrap"/>
        </Frame>
        
        <Button Text="Save File"
                Clicked="Button_OnClicked"
                Margin="20"
                Grid.Row="1" />
    </Grid>
</ContentPage>

StartPage.xaml.cs

[XamlCompilation(XamlCompilationOptions.Compile)]
public partial class StartPage : ContentPage
{
    private readonly HttpClient client;

    private string imageFileName = "downloadedImage.jpg";

    public StartPage()
    {
        InitializeComponent();
        client = new HttpClient();
    }

    private async void Button_OnClicked(object sender, EventArgs e)
    {
        await DownloadAndSaveImage();
    }

    private async Task DownloadAndSaveImage()
    {
        try
        {
            using (var response = await client.GetStreamAsync("https://picsum.photos/200/300/?random&blur"))
            {
                var filePath = await response.SaveToLocalFolderAsync(imageFileName);

                // Using FileImageSource just to confirm the image was saved as a file
                BackgroundImage.Source = new FileImageSource { File = filePath };

                OutputLabel.Text = $"Image file saved to\r\n\n{filePath}";
            }
        }
        catch (Exception ex)
        {
            Debug.WriteLine($"DownloadAndSaveImage Exception: {ex}");
        }
    }
}