LogInPage.xaml.cs

using Coursework1.Tables;
using SQLite;
using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

using Xamarin.Forms;
using Xamarin.Forms.Xaml;

namespace Coursework1.Views
{
    [XamlCompilation(XamlCompilationOptions.Compile)]
    public partial class LogInPage : ContentPage
    {
        public LogInPage()
        {
            InitializeComponent();
        }

        async void Button_Clicked(object sender, EventArgs e)
        {
            await Navigation.PushModalAsync(new RegisterPage());
        }

        void Button_Clicked_1(object sender, EventArgs e)
        {
            var dbpath = Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments), "UserDatabase.db");
            var db = new SQLiteConnection(dbpath);
            var myquery = db.Table<RegisteredUsers>().Where(u => u.Username.Equals(EntryUsername.Text) && u.Password.Equals(EntryPassword.Text)).FirstOrDefault();

            if (myquery != null)
            {
                App.Current.MainPage = new NavigationPage(new MainPage());
            }

            else 
            {
                Device.BeginInvokeOnMainThread(async () =>
                {
                    var result = await this.DisplayAlert("Log In Failed", "Check both Username and Password", "Okay", "Cancel");
                    if (result)
                    await Navigation.PushModalAsync(new LogInPage());
                });
            }

        }
    }
}