如何让用户在他的图库中选择图片？ （xamarin表格）

Question

我尝试打开设备的图库，以便用户选择一张照片，然后我得到它的路径...但是我的Stream总是返回null，然后，我有一个例外，因为... 我不知道出了什么问题 有人能帮我吗？ 它是一个xamarin表单项目，现在我正在使用android进行测试。

我的主要活动

 [Activity(Label = "neoFly_Montana", Icon = "@drawable/icon", Theme = "@style/MainTheme", MainLauncher = false, ConfigurationChanges = ConfigChanges.ScreenSize | ConfigChanges.Orientation)]
public class MainActivity : global::Xamarin.Forms.Platform.Android.FormsAppCompatActivity
{
    protected override void OnCreate(Bundle bundle)
    {
        TabLayoutResource = Resource.Layout.Tabbar;
        ToolbarResource = Resource.Layout.Toolbar;

        base.OnCreate(bundle);

        global::Xamarin.Forms.Forms.Init(this, bundle);

        //inicializa imageCircle
        ImageCircleRenderer.Init();

        //inicializa o mapa
        global::Xamarin.FormsMaps.Init(this, bundle);

        LoadApplication(new App());
    }

    // Field, property, and method for Picture Picker
    public static readonly int PickImageId = 1000;

    public TaskCompletionSource<Stream> PickImageTaskCompletionSource { set; get; }

    protected override void OnActivityResult(int requestCode, Result resultCode, Intent intent)
    {
        base.OnActivityResult(requestCode, resultCode, intent);

        if (requestCode == PickImageId)
        {
            if ((resultCode == Result.Ok) && (intent != null))
            {
                Android.Net.Uri uri = intent.Data;
                Stream stream = ContentResolver.OpenInputStream(uri);

                // Set the Stream as the completion of the Task
                PickImageTaskCompletionSource.SetResult(stream);
            }
            else
            {
                PickImageTaskCompletionSource.SetResult(null);
            }
        }
    }
}

在我的android项目中我也有这个类： （在此 PicturePickerImplementation：IPicturePicker 不起作用，IPicturePicker无法识别...我不知道为什么

using System;
using System.Collections.Generic;
using System.Linq;
 using System.Text;

 using Android.App;
 using Android.Content;
 using Android.OS;
 using Android.Runtime;
 using Android.Views;
 using Android.Widget;
 using System.Threading.Tasks;
 using System.IO;
 using Xamarin.Forms;
 using neoFly_Montana.Droid;

 [assembly: Dependency(typeof(PicturePickerImplementation))]
 namespace neoFly_Montana.Droid
 {
public class PicturePickerImplementation : IPicturePicker
{
    public Task<Stream> GetImageStreamAsync()
    {
        // Define the Intent for getting images
        Intent intent = new Intent();
        intent.SetType("image/*");
        intent.SetAction(Intent.ActionGetContent);

        // Get the MainActivity instance
        MainActivity activity = Forms.Context as MainActivity;

        // Start the picture-picker activity (resumes in MainActivity.cs)
        activity.StartActivityForResult(
            Intent.CreateChooser(intent, "Select Picture"),
            MainActivity.PickImageId);

        // Save the TaskCompletionSource object as a MainActivity property
        activity.PickImageTaskCompletionSource = new TaskCompletionSource<Stream>();

        // Return Task object
        return activity.PickImageTaskCompletionSource.Task;
    }

}
 }

我在pcl中的界面：

public interface IPicturePicker
{
    Task<Stream> GetImageStreamAsync();
}