DataService.cs

using System;
using System.Collections.Generic;
using System.IO;
using System.Runtime.CompilerServices;
using System.Text;
using System.Threading.Tasks;
using App3.Models;
using App3.Services;
using SQLite;
using Xamarin.Essentials;

namespace App3.Services
{
    public class DataService : IDataService

    {

        SQLiteAsyncConnection db;
        async Task Init()
        {
            if (db != null)
                return;
            var databasePath = Path.Combine(FileSystem.AppDataDirectory, "MyData.db");
            db = new SQLiteAsyncConnection(databasePath);

            await db.CreateTableAsync<User>();

        }

        public async Task AddUser(string Username, string Password)
        {
            await Init();
            var User = new User
            {
                UserName = Username,
                Password = Password,

            };

            var id = await db.InsertAsync(User);
        }

        public async Task RemoveUser(int id)
        {
            await Init();

            await db.DeleteAsync<User>(id);
        }

        public async Task<IEnumerable<User>> GetUser()
        {
            await Init();

            var User = await db.Table<User>().ToListAsync();
            return User;

        }

        public Task AddUser(object username, object password)
        {
            throw new NotImplementedException();
        }

        public Task AddUser(string username, object password)
        {
            throw new NotImplementedException();
        }
    }
}