应用程序内购买的问题。当前不存在名称“DependencyService”

我尝试如何在我的项目中实现可消耗的应用内购买。

教程：

购买耗材

保护应用内购买

我已经阅读了这些教程，但我不知道 DependencyService、Debug 和 Device 是什么。我收到以下错误消息：

错误 CS0103：当前上下文中不存在名称“DependencyService”（CS0103） 错误 CS0103：当前上下文中不存在名称“设备”（CS0103） 错误 CS0103：当前上下文中不存在名称“调试”上下文 (CS0103)

另外，我不明白这行代码是什么意思。为什么我们要检查 Device.RuntimePlatform == Device.iOS ？如果是真的我该怎么办，如果是假的我该怎么办？

 if(Device.RuntimePlatform == Device.iOS)
            return;

这就是我在我的项目中实现代码的方式：

 string payload = "apppayload";
    public async Task<bool> PurchaseItem(string productId, string payload)
    {
        if (!CrossInAppBilling.IsSupported)
            return false;

        var billing = CrossInAppBilling.Current;
        try
        {
            var connected = await billing.ConnectAsync(ItemType.InAppPurchase);
            if (!connected)
            {
                //we are offline or can't connect, don't try to purchase
                return false;
            }

            var verify = DependencyService.Get<IInAppBillingVerifyPurchase>();

            //check purchases
            var purchase = await billing.PurchaseAsync(productId, ItemType.InAppPurchase, payload, verify);

            //possibility that a null came through.
            if (purchase == null)
            {
                //did not purchase
            }
            else if (purchase.State == PurchaseState.Purchased)
            {
                //purchased, we can now consume the item or do it later

                //If we are on iOS we are done, else try to consume the purchase
                //Device.RuntimePlatform comes from Xamarin.Forms, you can also use a conditional flag or the DeviceInfo plugin
                if (Device.RuntimePlatform == Device.iOS)
                    return false;

                var consumedItem = await CrossInAppBilling.Current.ConsumePurchaseAsync(purchase.ProductId, purchase.PurchaseToken);

                if (consumedItem != null)
                {
                    //Consumed!!
                }
            }
        }
        catch (InAppBillingPurchaseException purchaseEx)
        {
            //Billing Exception handle this based on the type
            Debug.WriteLine("Error: " + purchaseEx);
        }
        catch (Exception ex)
        {
            //Something else has gone wrong, log it
            Debug.WriteLine("Issue connecting: " + ex);
        }
        finally
        {
            await billing.DisconnectAsync();
        }

        return false;
    }