如何从 Azure 数据库中读取数据 table

Question

您好，我正在尝试读取 table 中名字为 "John" 的用户。

 async private void GetUsers()
        {
            Initialize();
            SyncCoffee();
            try
            {
              await coffeeTable.PullAsync("user", coffeeTable.Where(ct => ct.FirstName == "John" ));
            }

        catch (Exception er)
        {
            await DisplayAlert("Alert", "the error: "+er, "ok");
        }

    }

初始化方法

public async Task Initialize()
    {
            var mainPage = new MainPage();
            if (isInitialised)
            {
                return;
            }

            MobileServiceClient client = new MobileServiceClient("database link");

            const string path = "syncstore.db";

        var store = new MobileServiceSQLiteStore(path);
        store.DefineTable<User>();
        await MobileService.SyncContext.InitializeAsync(store, new MobileServiceSyncHandler());

        coffeeTable = MobileService.GetSyncTable<User>();
            isInitialised = true;

       }

同步方法

public async Task SyncCoffee()
        {
            try
            {
                await coffeeTable.PullAsync("allusers", coffeeTable.CreateQuery());
                await MobileService.SyncContext.PushAsync();


            }
            catch (Exception ex)
            {
                Debug.WriteLine("Unable to sync coffees, that is alright as we have offline capabilities: " + ex);
            }
        }

我不断收到的错误是

System.NullReferenceException: 对象引用未设置为对象的实例。

欢迎任何帮助，因为我对此很陌生。

Answer 1

await coffeeTable.PullAsync("user", coffeeTable.Where(ct => ct.FirstName == "John" ));

您对特定用户的拉取操作是正确的。正如 Jason 所说，您需要在 GetUsers 方法中等待 Initialize()、SyncCoffee()。 coffeeTable 对象可能是 null 并且 coffeeTable.PullAsync 会在 Initialize 执行之前失败。对于类似的问题，您可以通过 VS 调试并在本地单步执行代码以缩小范围。此外，我建议您阅读 adrian hall 关于 An Offline Client and An Online Client 的书。

如何从 Azure 数据库中读取数据 table

How to read data from an azure database table

azure

xamarin.android

xamarin

azure-mobile-services

xamarin.forms