问题的根源在于包含IndexOf()方法的部分.它是在主线程上的始祖执行之前执行的，索引始终为-1.通过更改来解决:

The root of problem was the part that contains IndexOf() method. It was executing before the ancedent execution on Main Thread and index was always -1. Fixed by changing:

Device.BeginInvokeOnMainThread(() => ... );

作者

await Device.InvokeOnMainThreadAsync(() => { ... } );