Android 项目中不需要创建新的 ITextRecognition 接口，你应该使用您在 Xamarin.forms 项目中创建的界面.

You don't need to create a new ITextRecognition interface in Android project, you should use the interface which you created in the Xamarin.forms projet.

另外，***不要在 FoodMenu 类中创建接口.

Also, it's better not to create the interface inside the FoodMenu class.

代码示例如下:

在 Xamarin.Forms 中:

In Xamarin.Forms:

namespace App384
{
    [XamlCompilation(XamlCompilationOptions.Compile)]
    public partial class FoodMenu : ContentPage
    {
        public FoodMenu()
        {
            InitializeComponent();
        }

        public void Food_CameraEntry(object sender, EventArgs e)
        {
            Xamarin.Forms.DependencyService.Register<ITextRecognition>();
            DependencyService.Get<ITextRecognition>().LaunchActivityInAndroid();

        }
    }

    public interface ITextRecognition
    {
        void LaunchActivityInAndroid();
    }
}

在 Xamarin.Android 项目中:

In Xamarin.Android project:

[assembly: Xamarin.Forms.Dependency(typeof(TextRecognition))]
namespace App384.Droid
{
    [Preserve(AllMembers = true)]
    [Activity(Label = "TextRecognition", Theme = "@style/Theme.AppCompat.Light.NoActionBar", MainLauncher = true)]
    public class TextRecognition : AppCompatActivity, ITextRecognition
    {
        protected override void OnCreate(Bundle savedInstanceState)
        {
            base.OnCreate(savedInstanceState);
        }

        public void LaunchActivityInAndroid()
        {
            Console.WriteLine("LaunchActivityInAndroid");
        }

    }
}

我已经上传了这里的测试演示，您可以查看.

I have uploaded test demo here and you can check.

参考:Xamarin.Forms依赖服务介绍