Как я могу сохранить, получить и использовать растровое изображение в Xamarin.Android?

Поскольку изображение - это большой файл, я предлагаю вам сохранить его на внешнем хранилище Здесь вы можете узнать, что такое внешнее хранилище, по пути вроде: /storage/sdcard/Pictures, я сделал для вас демо:

using Android.App;
using Android.Widget;
using Android.OS;
using Android.Util;
using Android.Graphics;
using System.IO;

namespace SaveBitmap
{
    [Activity(Label = "SaveBitmap", MainLauncher = true)]
    public class MainActivity : Activity
    {
        string bitmapPath;
        protected override void OnCreate(Bundle savedInstanceState)
        {
            base.OnCreate(savedInstanceState);
            // Set our view from the "main" layout resource
            SetContentView(Resource.Layout.Main);
            bitmapPath = getPath("bitmap.png");
            Bitmap bitmap = BitmapFactory.DecodeResource(Resources, Resource.Drawable.capture);
            //save bitamp
            saveBitmap(bitmap);
            ImageView iv = FindViewById<ImageView>(Resource.Id.iv);
            //load bitmap
            Bitmap bi = BitmapFactory.DecodeFile(bitmapPath);
            iv.SetImageBitmap(bi);
            
        }
        public string getPath(string fileName) {
            var sdCardPath = Environment.GetExternalStoragePublicDirectory(Environment.DirectoryPictures);
            Log.Error("lv", sdCardPath.AbsolutePath);
            string filePath = System.IO.Path.Combine(sdCardPath.AbsolutePath, fileName);
            return filePath;

        }
        public void saveBitmap(Bitmap bitmap ) {
            FileStream stream = new FileStream(bitmapPath, FileMode.Create);
            bitmap.Compress(Bitmap.CompressFormat.Png, 100, stream);
            stream.Close();
         
        }
    }
}

Не забудьте добавить это в свой манифест:

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

Если вы запускаете приложение на более поздней версии Android 6.0, вам потребуется разрешение на выполнение.

Обновлять:

Я добавил разрешение на выполнение.

using Android.App;
using Android.Widget;
using Android.OS;
using Android.Util;
using Android.Graphics;
using System.IO;
using System;
using Android;
using Android.Content.PM;
using Android.Runtime;

namespace SaveBitmap
{
    [Activity(Label = "SaveBitmap", MainLauncher = true)]
    public class MainActivity : Activity
    {
        string bitmapPath;
        protected override void OnCreate(Bundle savedInstanceState)
        {
            base.OnCreate(savedInstanceState);
            // Set our view from the "main" layout resource
            SetContentView(Resource.Layout.Main);
            bitmapPath = getPath("bitmap.png");
            permissionCheck();
        }

        private void permissionCheck()
        {
            if (CheckSelfPermission(Manifest.Permission.ReadExternalStorage) != Permission.Granted)
            {
                // Should we show an explanation? 
                if (ShouldShowRequestPermissionRationale(Manifest.Permission.ReadExternalStorage))
                {
                    // Show an expanation to the user *asynchronously* -- don't block 
                    // this thread waiting for the user's response! After the user 
                    // sees the explanation, try again to request the permission. 
                }
                else
                {
                    // No explanation needed, we can request the permission. 
                    RequestPermissions(new String[] { Manifest.Permission.ReadExternalStorage }, 1);
                    // MY_PERMISSIONS_REQUEST_READ_CONTACTS is an 
                    // app-defined int constant. The callback method gets the 
                    // result of the request. 
                }
            }
            else {
                saveAndLoadBitmap();
            }
        }

        public string getPath(string fileName) {
            var sdCardPath = Android.OS.Environment.GetExternalStoragePublicDirectory(Android.OS.Environment.DirectoryPictures);
            Log.Error("lv", sdCardPath.AbsolutePath);
            string filePath = System.IO.Path.Combine(sdCardPath.AbsolutePath, fileName);
            return filePath;

        }
        public void saveBitmap(Bitmap bitmap ) {
            FileStream stream = new FileStream(bitmapPath, FileMode.Create);
            bitmap.Compress(Bitmap.CompressFormat.Png, 100, stream);
            stream.Close();
         
        }

        public void saveAndLoadBitmap() {
            Bitmap bitmap = BitmapFactory.DecodeResource(Resources, Resource.Drawable.capture);
            saveBitmap(bitmap);
            ImageView iv = FindViewById<ImageView>(Resource.Id.iv);
            Bitmap bi = BitmapFactory.DecodeFile(bitmapPath);
            iv.SetImageBitmap(bi);
        }
        public override void OnRequestPermissionsResult(int requestCode, string[] permissions, [GeneratedEnum] Permission[] grantResults)
        {
            switch (requestCode)
            {
                case 1:
                    {
                        // If request is cancelled, the result arrays are empty. 
                        if (grantResults.Length > 0 && grantResults[0] == Permission.Granted)
                        {
                            // permission was granted, yay! Do the 
                            // contacts-related task you need to do. 
                            saveAndLoadBitmap();
                        }
                        else
                        {
                            
                            // permission denied, boo! Disable the 
                            // functionality that depends on this permission. 
                        }
                        return;
                    }

                    // other 'case' lines to check for other 
                    // permissions this app might request 
            }
        }
    }
}