A simple download manager to download files via http. Feature to show percentage and resume downloads.

gistfile1.cs

public interface IDownloadManager {
        event DownloadManager.PercentageChangedDelegate OnPercentageChanged;
        event DownloadManager.DownloadErrorDelegate OnDownloadError;
        event DownloadManager.DownloadCompleteDelegate OnDownloadComplete;
        Task DownloadFileAsync(string url, string localPath);
        void AbortDownload();
        bool IsTheSameFile(string expectedMD5, string filePath);
        double Percentage { get; }
    }
    public class DownloadManager : IDownloadManager {
        const int BlockSize = 512;
        HttpWebRequest request;
    
        public delegate void PercentageChangedDelegate(double percentage);
        public event PercentageChangedDelegate OnPercentageChanged = delegate {}; 
       
        public delegate void DownloadCompleteDelegate();
        public event DownloadCompleteDelegate OnDownloadComplete = delegate { }; 
        
        public delegate void DownloadErrorDelegate(string error);
        public event DownloadErrorDelegate OnDownloadError = delegate { };

        public double Percentage { get; private set; }

        public async Task DownloadFileAsync(string url, string localPath) {
            try {
                PrepareDownload(url, localPath);
                await StartDownload(localPath);
            }
            catch (Exception e) {
                OnDownloadError.Invoke(e.Message);
            }

        }
        void PrepareDownload(string url, string localPath) {
            request = (HttpWebRequest)HttpWebRequest.Create(url);
            ResumeDownload(localPath);
            Percentage = 0;
        }
        void ResumeDownload(string localPath) {
            if (!File.Exists(localPath)) return;
            using (var file = File.OpenRead(localPath))
                request.AddRange(file.Length);
        }
        async Task StartDownload(string localPath) {
           var response = await request.GetResponseAsync();
           
            using (var responseStream = response.GetResponseStream())
            using (var fileStream = new FileStream(localPath, FileMode.Append))
                DownloadAndStoreBlocks(responseStream, fileStream, response.ContentLength);
            OnDownloadComplete.Invoke();
        }
        
        void DownloadAndStoreBlocks(Stream origin, Stream destiny, long totalSize) {
            var bytes = new byte[BlockSize];
            while (true) {
                var readSize = origin.Read(bytes, 0, BlockSize);
                if (readSize <= 0) break;
                destiny.Write(bytes, 0, readSize);
                CalculateAndRaisePercentage(destiny.Length, totalSize);
            }
        }

        void CalculateAndRaisePercentage(long sizeDownloaded, long totalSize) {
            Percentage = Math.Round(((double)sizeDownloaded / (double)totalSize) * 100f,2);
            OnPercentageChanged.Invoke(Percentage);
        }

        public void AbortDownload() {
            request.Abort();
        }

        public bool IsTheSameFile(string expectedMD5, string filePath) {
            using (var hash = MD5.Create())
            using (var fs = File.OpenRead(filePath)) {
                var md5 = BitConverter.ToString(hash.ComputeHash(fs)).Replace("-", "").ToLower();
                return md5.Equals(expectedMD5);
            }
        }
    }