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我有一个非常奇怪的问题，我无法弄清楚。我无法将新项目插入到我的 Sqlite 数据库中。我不断收到此错误：-

System.NullReferenceException: Object reference not set to an instance of an object.

我真的不知道为什么并且有完全相同的代码在应用程序中工作并保存其他表。

这是我的代码：-

用户.cs

public class User
{
    [PrimaryKey, AutoIncrement]
    public int ID { get; set; }
    public string Name { get; set; }
    public string EmailAddress { get; set; }

    public User()
    {
        this.Name = " ";
        this.EmailAddress = " ";
    }
}

用户管理器.cs

public class UserManager 
{
    UserRepository repository; 

    public UserManager(SQLiteConnection conn)
    {
        repository = new UserRepository (conn);
    }

    public User GetDriver(int id)
    {
        return repository.GetDriver(id);
    }

    public IList<User> GetDrivers()
    {
        return new List<User>(repository.GetDrivers());
    }

    public int SaveDriver(User item)
    {
        return repository.SaveDriver(item);
    }

    public int DeleteDriver(int id)
    {
        return repository.DeleteDriver(id);
    }
}

用户存储库.cs

public class UserRepository
{
    UserDatabase db = null;

    public UserRepository(SQLiteConnection conn)
    {
        db = new UserDatabase(conn);
    }

    public User GetDriver(int id)
    {
        return db.GetDriverItem(id);
    }

    public IEnumerable<User> GetDrivers()
    {
        return db.GetAllDriverItems();
    }

    public int SaveDriver(User item)
    {
        return db.SaveDriverItem(item);
    }

    public int DeleteDriver(int id)
    {
        return db.DeleteDriverItem(id);
    }
}

用户数据库.cs

public class UserDatabase
{
    static object locker = new object();
    public SQLiteConnection database;
    public string path;

    public UserDatabase(SQLiteConnection conn)
    {
        database = conn;
        database.CreateTable<User>();
    }

    public IEnumerable<User> GetAllDriverItems () {
        lock (locker) {
            return (from i in database.Table<User>() select i).ToList();
        }
    }

    public User GetDriverItem (int id) 
    {
        lock (locker) {
            return database.Table<User>().FirstOrDefault(x => x.ID == id);
        }
    }

    public int SaveDriverItem (User item) 
    {
        lock (locker) {
            if (item.ID != 0) {
                database.Update(item);
                return item.ID;
            } else {
                return database.Insert(item);
            }
        }
    }

    public int DeleteDriverItem(int id) 
    {
        lock (locker) {
            return database.Delete<User>(id);
        }
    }
}

Application.cs（在 Android 中）

public class UserApp : Application
{
    public static UserApp Current { get; private set; }

    public UserManager UserManager { get; set; }

    SQLiteConnection conn;

    public UserApp(IntPtr handle, global::Android.Runtime.JniHandleOwnership transfer)
        : base(handle, transfer)
    {
        Current = this;
    }

    public override void OnCreate()
    {
        base.OnCreate();

        var sqliteFilename = "UserItemDB.db3";
        string libraryPath = Environment.GetFolderPath(Environment.SpecialFolder.Personal);
        var path = Path.Combine(libraryPath, sqliteFilename);
        conn = new SQLiteConnection(new SQLite.Net.Platform.Generic.SQLitePlatformGeneric(), path);

        UserManager = new UserManager(conn);

        User testUser= new User();
        testUser.Name = "Dave";
        testUser.EmailAddress = "Dave@Gmail.com";
        UserApp.Current.UserManager.SaveDriver(testUser);
    }

这是我的堆栈跟踪：-

System.NullReferenceException: Object reference not set to an instance of an object.
08-09 17:07:32.423 E/mono    (31670):   at     System.Reflection.MonoProperty.GetValue (System.Object obj, System.Object[]     index) [0x00031] in <filename unknown>:0 
08-09 17:07:32.423 E/mono    (31670):   at     SQLite.Net.TableMapping+Column.GetValue (System.Object obj) [0x00000] in     <filename unknown>:0 
08-09 17:07:32.423 E/mono    (31670):   at     SQLite.Net.SQLiteConnection.Insert (System.Object obj, System.String extra,     System.Type objType) [0x00098] in <filename unknown>:0 
08-09 17:07:32.423 E/mono    (31670):   at     SQLite.Net.SQLiteConnection.Insert (System.Object obj) [0x00012] in <filename     unknown>:0 
08-09 17:07:32.423 E/mono    (31670):   at     Project.DataLayer.UserDatabase.SaveDriverItem (Project.BusinessLayer.Models.User     item) [0x00038] in     C:\Users\phill\Desktop\Work\ProjectTEST\Project\Project.Shared\DataLayer\UserDat    abase.cs:42 
08-09 17:07:32.423 E/mono    (31670):   at     Project.DataAccessLayer.UserRepository.SaveDriver     (Project.BusinessLayer.Models.User item) [0x00001] in     C:\Users\phill\Desktop\Work\ProjectTEST\Project\Project.Shared\DataAccessLayer\U    serRepository.cs:30 
08-09 17:07:32.423 E/mono    (31670):   at     Project.BusinessLayer.Managers.UserManager.SaveDriver     (Project.BusinessLayer.Models.User item) [0x00001] in     C:\Users\phill\Desktop\Work\ProjectTEST\Project\Project.Shared\BusinessLayer\Man    agers\UserManager.cs:33 
08-09 17:07:32.423 E/mono    (31670):   at Project.Droid.UserApp+    <getDriverDetails>d__16.MoveNext () [0x000e2] in     C:\Users\phill\Desktop\Work\ProjectTEST\Project\Project.Droid\Application.cs:75
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1 回答 1

1

已解决 - 经过长时间的头痛后，我终于找到了问题所在。

好的，所以 NullObject 是指数据库文件本身。由于 Xamarin 中的某些设置，无法更改数据库的结构，并且我的应用程序无法创建新数据库。不是 100% 确定其中哪一个是确切的答案，但我的项目终于在不久之后工作了......

在 VS 中，转到工具 > 选项 > Xamarin > Android > 勾选“保留应用程序数据”

我认为这是主要问题^^。我也检查过/未检查过其他人。

Project > Project.Properties > Android Options > 勾选 Shared Runtime & Fast Deployment 然后下一个选项卡 Linker = None。

不确定以上哪一项解决了我的问题，但它运行良好

于 2016-08-11T06:49:27.827 回答