我按照本指南将SQLite添加到我的Xamarin.Forms共享资产项目中。它不是PCL。我正在使用XAML和Code Behind。
我似乎在所有正确的地方都拥有所有代码。我有一个SQLite.cs，它包含我想要制作的所有数据库调用，并且还定义了数据库。
我在一个我知道可以工作的类中定义了一个接口，并且可以访问它，因为其中正在使用其他函数。
然后我在每个平台项目中都有一个类（我在droid示例下面显示），它使用该接口获取本地文件路径。
最后在我的共享资产项目中，我定义并（应该）实例化（我认为）我的数据库。
所有这一切，在我的一个ViewModel中，我尝试执行数据库调用。但是，内部异常表明我收到了对象引用错误。基本上我的数据库＆＃34;对象在尝试执行database.MyMethod()时为null。
我无法弄清楚为什么它没有被实例化。有什么想法吗？
它执行的行是
var dbtimings = TechsportiseData.GetTimingsAsync().Result.ToList();
那种方法是......
public static Task<List<Timing>> GetTimingsAsync()
{
return database.Table<Timing>().ToListAsync();
}
因为database为空而绊倒。
SQLite.cs（共享资产项目）
using System;
using System.Collections.Generic;
using System.Text;
using Xamarin.Forms;
using static TechsportiseApp.Helpers.GlobalFunctions;
using TechsportiseApp.Models;
using System.Threading.Tasks;
using SQLite;
namespace TechsportiseApp.Data
{
public class TechsportiseData
{
readonly SQLiteAsyncConnection database;
public TechsportiseData(string dbPath)
{
database = new SQLiteAsyncConnection(dbPath);
database.CreateTableAsync<Scan>().Wait();
database.CreateTableAsync<Timing>().Wait();
}
public static Task<List<Timing>> GetTimingsAsync()
{
return database.Table<Timing>().ToListAsync();
}
public static Task<List<Timing>> GetTimingsNotUploadedAsync()
{
return database.QueryAsync<Timing>("SELECT * FROM [Timing] WHERE [Uploaded] = 0");
}
public static Task<Timing> GetTimingAsync(int id)
{
return database.Table<Timing>().Where(i => i.Id == id).FirstOrDefaultAsync();
}
public static Task<int> SaveTimingAsync(Timing timing)
{
if (timing.Id != 0)
{
return database.UpdateAsync(timing);
}
else
{
return database.InsertAsync(timing);
}
}
public static Task<int> DeleteTimingAsync(Timing timing)
{
return database.DeleteAsync(timing);
}
public static Task DeleteAllTimingsAsync()
{
database.DropTableAsync<Timing>().Wait();
return database.CreateTableAsync<Timing>();
}
public static Task<List<Scan>> GetScansAsync()
{
return database.Table<Scan>().ToListAsync();
}
public static Task<List<Scan>> GetScansNotUploadedAsync()
{
return database.QueryAsync<Scan>("SELECT * FROM [Scan] WHERE [Uploaded] = 0");
}
public static Task<Scan> GetScanAsync(int id)
{
return database.Table<Scan>().Where(i => i.Id == id).FirstOrDefaultAsync();
}
public static Task<int> SaveScanAsync(Scan scan)
{
if (scan.Id != 0)
{
return database.UpdateAsync(scan);
}
else
{
return database.InsertAsync(scan);
}
}
public static Task<int> DeleteScanAsync(Scan scan)
{
return database.DeleteAsync(scan);
}
public static Task DeleteAllScansAsync()
{
database.DropTableAsync<Scan>().Wait();
return database.CreateTableAsync<Scan>();
}
public static Task<RaceClock> GetRaceClockAsync()
{
return database.Table<RaceClock>().FirstOrDefaultAsync();
}
public static Task<int> SaveRaceClockAsync(RaceClock clock)
{
return database.InsertAsync(clock);
}
public static Task DeleteRaceClockAsync()
{
database.DropTableAsync<RaceClock>().Wait();
return database.CreateTableAsync<RaceClock>();
}
}
}
App.xaml.cs
using Xamarin.Forms;
using Xamarin.Forms.Xaml;
using System.Collections.Generic;
using TechsportiseApp.API;
using TechsportiseApp.Views;
using TechsportiseApp.ViewModels;
using TechsportiseApp.Models;
using TechsportiseApp.Helpers;
using TechsportiseApp.Data;
using static TechsportiseApp.Helpers.GlobalFunctions;
//[assembly: XamlCompilation(XamlCompilationOptions.Compile)]
namespace TechsportiseApp
{
public partial class App : Application
{
static TechsportiseData database;
public App()
{
InitializeComponent();
Application.Current.Properties["APIServer"] = "https://www.techsportise.com/";
//Application.Current.Properties["APIServer"] = "http://localhost:52693/";
if (Application.Current.Properties.ContainsKey("ShowHelpOnStartup") == false)
{
Application.Current.Properties["ShowHelpOnStartup"] = true;
}
if (Application.Current.Properties.ContainsKey("ShowCompletedRaces") == false)
{
Application.Current.Properties["ShowCompletedRaces"] = false;
}
//MainPage = new NavigationPage(new ResultsProcess());
//MainPage = new NavigationPage(new Scanning());
//If the token is present and they are remembered
if ((Application.Current.Properties.ContainsKey("Token")) && (Application.Current.Properties.ContainsKey("IsRemembered")))
{
//And is blank/null
if (((string)Application.Current.Properties["Token"] == "") || ((GlobalFunctions.PropertyToBool(Application.Current.Properties["IsRemembered"].ToString()) != true)))
{
//They need to login
MainPage = new NavigationPage(new Login());
}
//Otherwise they can go straight to their page
else
{
if (GlobalFunctions.PropertyToBool(Application.Current.Properties["ShowHelpOnStartup"].ToString()) == true)
{
MainPage = new StartupHelp(true);
}
else
{
MainPage = new MainMenuMasterDetail();
}
}
}
else
{
MainPage = new NavigationPage(new Login());
}
}
public static TechsportiseData Database
{
get
{
if (database == null)
{
database = new TechsportiseData(DependencyService.Get<IFileHelper>().GetLocalFilePath("Techsportise.db3"));
}
return database;
}
}
protected override void OnStart()
{
// Handle when your app starts
}
protected override void OnSleep()
{
// Handle when your app sleeps
}
protected override void OnResume()
{
// Handle when your app resumes
}
}
}
界面（共享资产项目）
using System;
using System.IdentityModel.Tokens.Jwt;
using System.Linq;
using System.Net;
using Xamarin.Forms;
namespace TechsportiseApp.Helpers
{
public class GlobalFunctions
{
public GlobalFunctions()
{
}
public interface IFileHelper
{
string GetLocalFilePath(string filename);
}
public static bool CheckForInternetConnection()
{
try
{
HttpWebRequest request = (HttpWebRequest)WebRequest.Create("http://www.google.com");
request.Timeout = 5000;
request.Credentials = CredentialCache.DefaultNetworkCredentials;
HttpWebResponse response = (HttpWebResponse)request.GetResponse();
if (response.StatusCode == HttpStatusCode.OK)
return true;
else
//return false;
return true;
}
catch
{
//return false;
return true;
}
}
public static bool HasPremium()
{
var handler = new JwtSecurityTokenHandler();
var token = handler.ReadToken(Application.Current.Properties["Token"].ToString()) as JwtSecurityToken;
var premium = token.Claims.FirstOrDefault(claim => claim.Type == "premium").Value.ToString();
var premiumbool = PropertyToBool(premium);
return premiumbool;
}
}
}
平台类（本例中为Droid）
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using Android.App;
using Android.Content;
using Android.OS;
using Android.Runtime;
using Android.Views;
using Android.Widget;
using Xamarin.Forms;
using TechsportiseApp.Droid;
using static TechsportiseApp.Helpers.GlobalFunctions;
using System.IO;
[assembly: Dependency(typeof(FileHelper))]
namespace TechsportiseApp.Droid
{
public class FileHelper : IFileHelper
{
public string GetLocalFilePath(string filename)
{
string path = System.Environment.GetFolderPath(System.Environment.SpecialFolder.Personal);
return Path.Combine(path, filename);
}
}
}
答案 0 :(得分：2)
您的数据库类不一致。初始化在实例构造函数中完成，而数据库调用是静态的，即使实例尚未初始化，也会引导您调用静态函数。
将函数更改为实例1（从函数中删除static限定符）或在静态构造函数上初始化数据库类。