Xamarinで画像ピッカーの開き方

Xamarinど素人です。
TCPでファイル(画像)を送受信する～ファイル転送アプリを作った話～
このサイトを参考にXamarin.FormsのAndroidで画像ピッカーを開くプログラムを作ろうと思ったのですが思うようにできません...

C#
1public class ImagePicker
2{
3    static int REQ_GALLERY = 10;
4    public void GetStreamByPicker(Action&lt;string[], Stream[]> result)
5    {
6        ActivityHost.OnResult = (x) =>
7        {
8            string[] fileNames;
9            Stream[] stms = GetStreamByURI(x, out fileNames);
10            result(fileNames, stms);
11        };
12        var intent = new Intent();
13        intent.SetType("image/*");
14        intent.PutExtra(Intent.ExtraAllowMultiple, true);
15        intent.SetAction(Android.Content.Intent.ActionGetContent);
16        ActivityHost.activity.StartActivityForResult(intent, REQ_GALLERY);
17    }
18
19    public Stream[] GetStreamByURI(Android.Net.Uri[] uris, out string[] fileNames)
20    {
21        Android.Content.Context context = Android.App.Application.Context;
22        Stream[] streams = new Stream[uris.Length];
23        fileNames = new string[uris.Length];
24
25        for (int i = 0; i &lt; uris.Length; i++)
26        {
27            streams[i] = context.ContentResolver.OpenInputStream(uris[i]);
28
29            ICursor returnCursor = context.ContentResolver.Query(uris[i], null, null, null, null);
30            int nameIndex = returnCursor.GetColumnIndex(OpenableColumns.DisplayName);
31            returnCursor.MoveToFirst();
32            fileNames[i] = returnCursor.GetString(nameIndex);
33        }
34        return streams;
35    }
36}
37/* 以下はAndroidプロジェクト内に追加 */
38public class MainActivity 
39{
40    protected override void OnCreate(Bundle bundle)
41    {
42        base.OnCreate(bundle);
43        ActivityHost.activity = this; //追加
44    }
45
46    protected override void OnActivityResult(int requestCode, Result resultCode, Intent data)
47    {
48        base.OnActivityResult(requestCode, resultCode, data);
49
50        if (requestCode == REQ_GALLERY &amp;&amp; resultCode == Result.Ok)
51        {
52            int cnt = 0;
53            bool isSingle = false;
54
55            if (data.ClipData != null) cnt = data.ClipData.ItemCount;
56            else isSingle = true;
57
58            Android.Net.Uri[] uris = new Android.Net.Uri[cnt];
59            for (int i = 0; i &lt; cnt; i++)
60            {
61                uris[i] = data.ClipData.GetItemAt(i).Uri;
62            }
63
64            if (isSingle)
65            {
66                uris = new Android.Net.Uri[1];
67                uris[0] = data.Data;
68            }
69
70            ActivityHost.OnResult?.Invoke(uris);
71        }
72    }
73}
74
75public class ActivityHost{
76    public static Activity activity;
77    public static Action&lt;Android.Net.Uri[]> OnResult;
78}