.NET Extensions IFileProvider for Android asset files.

AndroidAssetFileProvider.cs

using System;
using System.IO;
using System.Linq;
using Android.Content;
using Java.Lang;
using Microsoft.Extensions.FileProviders;
using Microsoft.Extensions.Primitives;

namespace XamarinNetExtensions.Droid
{
    internal class AndroidAssetFileProvider : IFileProvider
    {
        private readonly Context _context;

        public AndroidAssetFileProvider(Context context)
        {
            _context = context ?? throw new ArgumentNullException(nameof(context));
        }

        public IFileInfo GetFileInfo(string subpath)
        {
            var paths = subpath.Split(Path.PathSeparator);
            var parent = Path.Combine(paths.SkipLast(1).ToArray());

            if (_context.Assets.List(parent).All(path => path != subpath))
            {
                return new NotFoundFileInfo(subpath);
            }

            return new AndroidAssetFileInfo(_context, subpath);
        }

        public IDirectoryContents GetDirectoryContents(string subpath)
        {
            throw new UnsupportedOperationException("GetDirectoryContents is not supported.");
        }

        public IChangeToken Watch(string filter)
        {
            throw new UnsupportedOperationException("Watch is not supported.");
        }
    }

    internal class AndroidAssetFileInfo : IFileInfo
    {
        private readonly Context _context;
        private readonly string _filePath;

        private long _length = -1L;

        public AndroidAssetFileInfo(Context context, string filePath)
        {
            _context = context ?? throw new ArgumentNullException(nameof(context));
            _filePath = filePath ?? throw new ArgumentNullException(nameof(filePath));
            Name = filePath.Split(Path.PathSeparator).Last();
        }

        public Stream CreateReadStream()
        {
            return _context.Assets.Open(_filePath);
        }

        public bool Exists => true;

        public long Length
        {
            get
            {
                if (_length != -1)
                {
                    return _length;
                }

                using (var stream = CreateReadStream())
                {
                    return _length = stream.Length;
                }
            }
        }

        public string PhysicalPath => $"file:///android_assets/{_filePath}";
        public string Name { get; }
        public DateTimeOffset LastModified { get; } = DateTimeOffset.MinValue;
        public bool IsDirectory => false;
    }
}