等待 IObservable 时出现 System.ArgumentNullException答案

【问题标题】：System.ArgumentNullException when awaiting IObservable等待 IObservable 时出现 System.ArgumentNullException
【发布时间】：2018-10-01 20:52:57
【问题描述】：

我正在尝试等待一个 Observable 并且我得到一个

System.ArgumentNullException

值不能为空。
参数名称：来源

堆栈跟踪：

System.ArgumentNullException: Value cannot be null.
Parameter name: source
  at System.Reactive.Linq.Observable.GetAwaiter[TSource] (System.IObservable`1[T] source) [0x00003] in <370f6a6bb34048878534065376a195cb>:0
  at Lynkd.Models.Peripherals.RPHPeripheral+<readMacAddress>d__13.MoveNext () [0x00012] in /Users/ericbowman/Documents/Development/Mobile/LYNKD/Lynkd/Models/Peripherals/RPHPeripheral.cs:24
  at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw () [0x0000c] in /Library/Frameworks/Xamarin.iOS.framework/Versions/11.12.0.4/src/Xamarin.iOS/mcs/class/referencesource/mscorlib/system/runtime/exceptionservices/exceptionservicescommon.cs:152
  at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess (System.Threading.Tasks.Task task) [0x00037] in /Library/Frameworks/Xamarin.iOS.framework/Versions/11.12.0.4/src/Xamarin.iOS/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:187
  at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification (System.Threading.Tasks.Task task) [0x00028] in /Library/Frameworks/Xamarin.iOS.framework/Versions/11.12.0.4/src/Xamarin.iOS/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:156
  at System.Runtime.CompilerServices.TaskAwaiter.ValidateEnd (System.Threading.Tasks.Task task) [0x00008] in /Library/Frameworks/Xamarin.iOS.framework/Versions/11.12.0.4/src/Xamarin.iOS/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:128
  at System.Runtime.CompilerServices.TaskAwaiter.GetResult () [0x00000] in /Library/Frameworks/Xamarin.iOS.framework/Versions/11.12.0.4/src/Xamarin.iOS/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:113
  at Lynkd.Models.Peripherals.RPHGunbox+<InitializeAsync>d__1.MoveNext () [0x00020] in /Users/ericbowman/Documents/Development/Mobile/LYNKD/Lynkd/Models/Peripherals/RPHGunbox.cs:12
  at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw () [0x0000c] in /Library/Frameworks/Xamarin.iOS.framework/Versions/11.12.0.4/src/Xamarin.iOS/mcs/class/referencesource/mscorlib/system/runtime/exceptionservices/exceptionservicescommon.cs:152
  at System.Runtime.CompilerServices.AsyncMethodBuilderCore+<>c.<ThrowAsync>b__6_0 (System.Object state) [0x00000] in /Library/Frameworks/Xamarin.iOS.framework/Versions/11.12.0.4/src/Xamarin.iOS/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/AsyncMethodBuilder.cs:1018
  at UIKit.UIKitSynchronizationContext+<>c__DisplayClass1_0.<Post>b__0 () [0x00000] in /Library/Frameworks/Xamarin.iOS.framework/Versions/11.12.0.4/src/Xamarin.iOS/UIKit/UIKitSynchronizationContext.cs:24
  at Foundation.NSAsyncActionDispatcher.Apply () [0x00000] in /Library/Frameworks/Xamarin.iOS.framework/Versions/11.12.0.4/src/Xamarin.iOS/Foundation/NSAction.cs:125
  at at (wrapper managed-to-native) UIKit.UIApplication.UIApplicationMain(int,string[],intptr,intptr)
  at UIKit.UIApplication.Main (System.String[] args, System.IntPtr principal, System.IntPtr delegate) [0x00005] in /Library/Frameworks/Xamarin.iOS.framework/Versions/11.12.0.4/src/Xamarin.iOS/UIKit/UIApplication.cs:79
  at UIKit.UIApplication.Main (System.String[] args, System.String principalClassName, System.String delegateClassName) [0x0002c] in /Library/Frameworks/Xamarin.iOS.framework/Versions/11.12.0.4/src/Xamarin.iOS/UIKit/UIApplication.cs:63
  at Lynkd.iOS.Application.Main (System.String[] args) [0x00001] in /Users/ericbowman/Documents/Development/Mobile/LYNKD/Lynkd.iOS/Main.cs:17

这是我的来源：

protected async Task readMacAddress()
{
    await (Device.ReadCharacteristic(BleService.DeviceInformation, BleCharacteristic.MacAddress).Subscribe(
        onNext: Result =>
        {
            MacAddress = BitConverter.ToString(Result.Data).Replace("-", string.Empty);
        },
        onError: Error =>
        {
            throw new BleException("Cannot Retrive MAC Address, Error Reading MAC Address");
        },
        onCompleted: () =>
        {
            return;

    }) as IObservable<CharacteristicGattResult>);
}

有什么想法吗？

【问题讨论】：

你能提供一个minimal reproducible example吗？
我不知道该怎么做，因为它正在从我们自己的内部外围设备中读取蓝牙特性。我也许可以使用很多人拥有的其他一些通用外围设备来做到这一点？这可能是必要的，但它需要几个小时的工作，所以我现在要推迟了

标签： asynchronous xamarin system.reactive

【解决方案1】：

我不知道 API，但我敢说值的类型不是 CharacteristicGattResult。

可能是 BluetoothGattCharacteristic`？

【讨论】：

猜对了；但这是正确的类型。我将它作为一个问题发布在 GitHub 页面上，供蓝牙库的作者使用。他表示我的代码完全错误，我要求澄清。

【解决方案2】：

事实证明，我对 RX 的使用想得太多了。您可以像这样简单地等待原始函数调用：

protected async Task readMacAddress()
{
    var macResult = await Device.ReadCharacteristic(BleService.DeviceInformation, BleCharacteristic.MacAddress);
    MacAddress = BitConverter.ToString(macResult.Data).Replace("-", string.Empty);
}

非常感谢艾伦·里奇（Allan Ritchie）引导我朝着正确的方向前进。（我正在使用的方便的 Xamarin BLE 库的作者）https://github.com/aritchie/bluetoothle

【讨论】：