IAsyncOperation<StoreProductResult> 不包含 GetAwaiter 的定义

.net 标准 2.0 上有一个关于 Xamarin Forms 的项目。在通用项目中声明了一个接口：

public interface IIAPService
{
    Task<string> GetAppInfoAsync();
}

在 UWP 项目中实施时（取自 Microsoft 网站的示例：

public class IAPService : IIAPService
{
    private StoreContext context = null;

    public IAPService() { }

    public async Task<string> GetAppInfoAsync()
    {
        if (context == null)
        {
            context = StoreContext.GetDefault();
            // If your app is a desktop app that uses the Desktop Bridge, you
            // may need additional code to configure the StoreContext object.
            // For more info, see https://aka.ms/storecontext-for-desktop.
        }

        // Get app store product details. Because this might take several moments,   
        // display a ProgressRing during the operation.
        StoreProductResult queryResult = await context.GetStoreProductForCurrentAppAsync();

        if (queryResult.Product == null)
        {
            // The Store catalog returned an unexpected result.
            //textBlock.Text = "Something went wrong, and the product was not returned.";

            // Show additional error info if it is available.
            if (queryResult.ExtendedError != null)
            {
                //textBlock.Text += $"\nExtendedError: {queryResult.ExtendedError.Message}";
            }

            return;
        }

        // Display the price of the app.
        //textBlock.Text = $"The price of this app is: {queryResult.Product.Price.FormattedBasePrice}";
    }
}

一个错误：Ошибка CS4036 'IAsyncOperation<StoreProductResult>" не содержит определения для "GetAwaiter", и не удалось найти метод расширения "GetAwaiter", принимающий тип "IAsyncOperation<StoreProductResult>" в качестве первого аргумента (возможно, пропущена директива using для "System").

在 Google 的帮助下，我只找到了一个解决方案：使用 .net framework 4.5 而不是 4.0，但问题是我使用的是 .net standard 2.0