<?xml version="1.0" encoding="utf-8" ?>
<resources>
  <declare-styleable name="RoundImageView">
    <attr name="border_width" format="dimension" />
    <attr name="border_color" format="color" />
  </declare-styleable>
</resources>

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

using Android.App;
using Android.Content;
using Android.Graphics;
using Android.OS;
using Android.Runtime;
using Android.Views;
using Android.Widget;
using Android.Util;
using Java.Util.Jar;
using Android.Graphics.Drawables;
using static Android.Graphics.PorterDuff;
using Android.Content.Res;

namespace RoundImageViewDemo
{
    /// <summary>
    /// 圆角
    /// </summary>
    public class RoundImageView : ImageView
    {
        private int mBorderWidth = 10;

        private Bitmap mask;
        private Paint paint;
        private Color mBorderColor = Color.ParseColor("#CCCCCC");

        private Context mContext;
        public RoundImageView(Context context) : base(context)
        {
            mContext = context;
        }

        public RoundImageView(Context context, IAttributeSet attrs) : base(context, attrs)
        {
            mContext = context;
            GetAttributes(context, attrs);
        }

        public RoundImageView(Context context, IAttributeSet attrs, int defStyle) : base(context, attrs, defStyle)
        {
            mContext = context;
            GetAttributes(context, attrs);
        }

        /// <summary>
        /// 获取自定义属性
        /// </summary>
        /// <param name="context"></param>
        /// <param name="attrs"></param>
        private void GetAttributes(Context context, IAttributeSet attrs)
        {
            TypedArray t_attrs = context.ObtainStyledAttributes(attrs, Resource.Styleable.RoundImageView);
            mBorderColor = t_attrs.GetColor(Resource.Styleable.RoundImageView_border_color, mBorderColor);
            int defalut = (int)(2 * context.Resources.DisplayMetrics.Density + 0.5f);
            mBorderWidth = t_attrs.GetDimensionPixelOffset(Resource.Styleable.RoundImageView_border_width, defalut);
            t_attrs.Recycle();
        }

        protected override void OnDraw(Canvas canvas)
        {
            Drawable localDrawable = Drawable;
            if (localDrawable == null)
                return;
            if (localDrawable is NinePatchDrawable)
                return;

            if (this.paint == null)
            {
                PorterDuff.Mode localMode = PorterDuff.Mode.DstIn;

                Paint localPaint = new Paint();
                localPaint.FilterBitmap = false;
                localPaint.AntiAlias = true;
                localPaint.SetXfermode(new PorterDuffXfermode(localMode));
                this.paint = localPaint;
            }

            int width = Width;
            int height = Height;
            /** 保存layer */
            int layer = canvas.SaveLayer(0.0F, 0.0F, width, height, null, SaveFlags.All);
            /** 设置drawable的大小 */
            localDrawable.SetBounds(0, 0, width, height);
            /** 将drawable绑定到bitmap(this.mask)上面（drawable只能通过bitmap显示出来） */
            localDrawable.Draw(canvas);
            if ((this.mask == null) || (this.mask.IsRecycled))
            {
                this.mask = CreateOvalBitmap(width, height);
            }
            /** 将bitmap画到canvas上面 */
            canvas.DrawBitmap(this.mask, 0.0F, 0.0F, this.paint);
            /** 将画布复制到layer上 */
            canvas.RestoreToCount(layer);
            DrawBorder(canvas, width, height);
        }


        private void DrawBorder(Canvas canvas, int width, int height)
        {
            if (mBorderWidth == 0)
                return;

            Paint mBorderPaint = new Paint();
            mBorderPaint.SetStyle(Paint.Style.Stroke);
            mBorderPaint.AntiAlias = true;
            mBorderPaint.Color = mBorderColor;
            mBorderPaint.StrokeWidth = mBorderWidth;
            /** 
             * 坐标x：view宽度的一般 坐标y：view高度的一般 半径r：因为是view的宽度-border的一半 
             */
            canvas.DrawCircle(width >> 1, height >> 1, (width - mBorderWidth) >> 1, mBorderPaint);
            canvas = null;
        }

        /// <summary>
        /// 获取一个bitmap，目的是用来承载drawable; 
        /// 将这个bitmap放在canvas上面承载，并在其上面画一个椭圆(其实也是一个圆，因为width=height)来固定显示区域 
        /// </summary>
        /// <param name="width"></param>
        /// <param name="height"></param>
        /// <returns></returns>
        public Bitmap CreateOvalBitmap(int width, int height)
        {
            Bitmap.Config localConfig = Bitmap.Config.Argb8888;
            Bitmap localBitmap = Bitmap.CreateBitmap(width, height, localConfig);
            Canvas localCanvas = new Canvas(localBitmap);
            Paint localPaint = new Paint();
            int padding = mBorderWidth - 3;
            /** 
             * 设置椭圆的大小(因为椭圆的最外边会和border的最外边重合的，如果图片最外边的颜色很深，有看出有棱边的效果，所以为了让体验更加好， 
             * 让其缩进padding px) 
             */
            RectF localRectF = new RectF(padding, padding, width - padding, height - padding);
            localCanvas.DrawOval(localRectF, localPaint);
            return localBitmap;
        }
    }
}

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:gravity="center_horizontal">
  <RoundImageViewDemo.RoundImageView
      android:src="@drawable/icon_boy"
      android:layout_height="80dp"
      android:layout_alignParentRight="true"
      android:id="@+id/iv_userphoto_mycenter_myprofile"
      android:layout_width="80dp"
      android:gravity="center_horizontal"/>
</LinearLayout>