System.InvalidOperationException: '无法分配没有元素的本机控件；

Question

我正在做一个项目"CoManga"，我想在其中添加广告。在 UWP 上实施广告似乎很简单，例如 Android 和 iOS。但是，我现在卡住了。

无论如何，我跟着this tutorial by James Montemagno添加了所有内容。我什至看到了测试广告，这很棒。但是，当我尝试离开该页面（当我按 "BACK Button" 时）并转到上一页时，出现错误。

这是错误：

Setting up AdControlView in UWP throws System.InvalidOperationException: 'Cannot assign a native control without an Element; Renderer unbound and/or disposed. Please consult Xamarin.Forms renderers for reference implementation of OnElementChanged.'.

它被抛出在 line number 50，我设置 SetNativeControl(adView); 的地方。我现在已经把它注释掉了，但是一旦我取消注释，我就会看到这个错误。

谁能帮我解决这个问题。

Answer 1

Setting up AdControlView in UWP throws System.InvalidOperationException: 'Cannot assign a native control without an Element; Renderer unbound and/or disposed. Please consult Xamarin.Forms renderers for reference implementation of OnElementChanged.

原因是xamarin元素已经释放但是SetNativeControl再次调用导致页面返回时原生控件找不到匹配的xamarin元素。所以你可以设置一个标志（isRegist）来记录注册的广告。

public class AdViewRenderer : ViewRenderer<AdControlView, AdControl>
{
    string bannerId = "test";
    AdControl adView;
    string applicationID = "3f83fe91-d6be-434d-a0ae-7351c5a997f1";
    bool isRegist = false;

    protected override void OnElementChanged(ElementChangedEventArgs<AdControlView> e)
    {
        base.OnElementChanged(e);

        if (Control == null && isRegist != true)
        {
            CreateNativeAdControl();
            SetNativeControl(adView);
            isRegist = true;
        }
    }
    private void CreateNativeAdControl()
    {
        if (adView != null)
            return;

        var width = 300;
        var height = 50;
        if (AnalyticsInfo.VersionInfo.DeviceFamily == "Windows.Desktop")
        {
            width = 728;
            height = 90;
        }
        // Setup your BannerView, review AdSizeCons class for more Ad sizes. 
        adView = new AdControl
        {
            ApplicationId = applicationID,
            AdUnitId = bannerId,
            HorizontalAlignment = Windows.UI.Xaml.HorizontalAlignment.Center,
            VerticalAlignment = Windows.UI.Xaml.VerticalAlignment.Bottom,
            Height = height,
            Width = width
        };
    }
}

System.InvalidOperationException: '无法分配没有元素的本机控件；

System.InvalidOperationException: 'Cannot assign a native control without an Element;

c#

custom-renderer

xamarin

xamarin.forms

uwp