您正面临权限问题.

首先，您必须添加AndroidManifest:

First, you will have to add in your AndroidManifest:

<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />

并且由于Android Marshmallow，您需要向用户询问权限，因此，我建议使用软件包权限插件

And since Android Marshmallow, you need to ask the user for the permissions, so I advise to use the package Permissions.Plugin

并添加您的MainActivity:

And add in your MainActivity:

public override void OnRequestPermissionsResult(int requestCode, string[] permissions, [GeneratedEnum] Android.Content.PM.Permission[] grantResults)
{
    PermissionsImplementation.Current.OnRequestPermissionsResult(requestCode, permissions, grantResults);
    base.OnRequestPermissionsResult(requestCode, permissions, grantResults);
}

您可以通过以下方式签入运行时是否具有权限:

You can check in runtime if you have the permissions by:

var status = await CrossPermissions.Current.CheckPermissionStatusAsync(Permission.Storage);
    if (status != PermissionStatus.Granted)
    {
        if(await CrossPermissions.Current.ShouldShowRequestPermissionRationaleAsync(Permission.Storage))
        {
            await DisplayAlert("Need storage, "Request storage permission", "OK");
        }

        var results = await CrossPermissions.Current.RequestPermissionsAsync(Permission.Storage);
        //Best practice to always check that the key exists
        if(results.ContainsKey(Permission.Storage))
            status = results[Permission.Storage];
    }

有关更多信息，您可以查看此博客文章，其中解释了Android中的所有权限- https://devblogs.microsoft.com/xamarin/requesting-runtime-permissions-in-android-marshmallow/

For further information you can check this blog post explaining all the permissions in Android - https://devblogs.microsoft.com/xamarin/requesting-runtime-permissions-in-android-marshmallow/