public string AppVersion
{
    get
    {
         var appVersionString = NSBundle.MainBundle.ObjectForInfoDictionary("CFBundleShortVersionString").ToString();
         var appBuildNumber = NSBundle.MainBundle.ObjectForInfoDictionary("CFBundleVersion").ToString();

         return $"v{appVersionString} b{appBuildNumber}";
    }
}

using System;
using System.Linq;
using Android.Support.Design;
[assembly: Xamarin.Forms.Dependency(typeof(AppVersionProvider))]
namespace Shared.Services
{
    public class AppVersionProvider:IAppVersionProvider
    {
        public Version GetAppVersion()
        {
            var numbers = GetVersionString().Split(new[] {'.'}, 4).Select(s =>
            {
                int value;
                int.TryParse(s, out value);
                return value;
            }).ToArray();
            int major = numbers.ElementAtOrDefault(0);
            int minor = numbers.ElementAtOrDefault(1);
            int revision = numbers.ElementAtOrDefault(2);
            int build = numbers.ElementAtOrDefault(3);
            return new Version(major, minor, build, revision);

        }

        private string GetVersionString()
        {
#if __ANDROID__
            return BuildConfig.VersionName;
#endif
#if __IOS__
            //use the above logic to get the version string;
#endif
        }
    }
}