Xamarin spinner adapter

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Xamarin

have a question

var spn = new ArrayAdapter<String>(this, Android.Resource.Layout.SimpleSpinnerItem, lstCS);
            spn.SetDropDownViewResource(Android.Resource.Layout.SimpleSpinnerDropDownItem);
            var spinner = FindViewById<Spinner>(Resource.Id.spinner);
            spinner.Adapter = spn;

i have two axml layout main.axml and child.axml
in child.axml contain Spinner with id "spinner" and i want to put thatspinner together in main.axml
the problem is spinner always return null

What I have tried:

that is no problem if i put spinner in main.xml directly