页面加载时出现 NullReferenceException

Question

当我的页面加载时，我从SQLite 数据库加载一个列表，有时当它加载时我得到NullReferenceException，错误提示为Object reference not set to an instance of an object.

它在 SQLite 类文件中中断了这段代码

public TableMapping GetMapping (Type type)
{
    if (_mappings == null) {
        _mappings = new Dictionary<string, TableMapping> ();
    }
    TableMapping map;
    if (!_mappings.TryGetValue (type.FullName, out map)) {
        map = new TableMapping (type);
        _mappings [type.FullName] = map; //null here
    }
    return map;
}

这就是我在页面加载时所做的事情

public MainPage()
{
    this.InitializeComponent();
    Loaded += MainPage_Loaded;
}

void MainPage_Loaded(object sender, RoutedEventArgs e)
{
    createDatabase();
    getBowlers();
}

private async void createDatabase()
{
    SQLiteAsyncConnection conn = new SQLiteAsyncConnection(BOWLERS_DATABASE);
    await conn.CreateTableAsync<Bowler>();
    conn = new SQLiteAsyncConnection(GAMES_DATABASE);
    await conn.CreateTableAsync<Games>();
}

private async void getBowlers()
{
    SQLiteAsyncConnection conn = new SQLiteAsyncConnection(BOWLERS_DATABASE);

    var query = conn.Table<Bowler>();
    itemListView.DataContext = await query.ToListAsync();
}

我是页面生命周期的新手，但似乎我正试图从数据库中提早提取？

编辑

堆栈跟踪

System.NullReferenceException was unhandled by user code
HResult=-2147467261
Message=Object reference not set to an instance of an object.
Source=mscorlib
StackTrace:
   at System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add)
   at System.Collections.Generic.Dictionary`2.set_Item(TKey key, TValue value)
   at SQLite.SQLiteConnection.GetMapping(Type type) in c:\Users\Jeff\Dropbox\Tournament Director Windows\Tournament Director Windows\SQLite.cs:line 231
   at SQLite.TableQuery`1..ctor(SQLiteConnection conn) in c:\Users\Jeff\Dropbox\Tournament Director Windows\Tournament Director Windows\SQLite.cs:line 2129
   at SQLite.SQLiteConnection.Table[T]() in c:\Users\Jeff\Dropbox\Tournament Director Windows\Tournament Director Windows\SQLite.cs:line 616
   at SQLite.SQLiteAsyncConnection.Table[T]() in c:\Users\Jeff\Dropbox\Tournament Director Windows\Tournament Director Windows\SQLiteAsync.cs:line 260
   at Tournament_Director_Windows.MainPage.<getBowlers>d__c.MoveNext() in c:\Users\Jeff\Dropbox\Tournament Director Windows\Tournament Director Windows\MainPage.xaml.cs:line 116
InnerException: 

Answer 1

试试这个：

if (!_mappings.ContainsKey(type.FullName))
{
    _mappings.Add(map);
}

_mappings [type.FullName] = map;

Answer 2

根据异常和提供的堆栈跟踪，我认为您的问题是 here 所描述的。

注意：

1. 异常是从字典代码中引发的，而不是从您的（用户）代码中引发的
2. 异常是 NullReferenceException，而不是 ArgumentNullException，所以不仅仅是参数为 null

Dictionary 不是线程安全的。这意味着如果它被多个线程访问，则对它的访问应该是同步的。

更新 1

在 SQLite 异步处理中看起来像 there is a bug。

Answer 3

原始代码的问题在于 GetMapping 方法不是线程安全的。在下面添加一个锁，强制它是线程安全的并解决了问题。

根据答案和cmets，这里是代码，把你SQLite.cs文件中的GetMapping方法改成这个

private Object thisLock = new Object(); // add lock obj
public TableMapping GetMapping(Type type, CreateFlags createFlags = CreateFlags.None)
{
    // include original code in the lock block
    lock (thisLock)
    {
        if (_mappings == null)
        {
            _mappings = new Dictionary<string, TableMapping>();
        }
        TableMapping map;
        if (!_mappings.TryGetValue(type.FullName, out map))
        {
            map = new TableMapping(type, createFlags);
            _mappings[type.FullName] = map;
        }
        return map;
    }
}