From .Net 4.5 onwards: Use IProgress<T>

Since .Net 4.5 you can handle asynchronous progress reporting with the IProgress<T> interface. You can write an extension method for downloading files using the HttpClient that can be called like this where progress is the implementation of IProgress<float> for your progress bar or other UI stuff:

// Seting up the http client used to download the data
using (var client = new HttpClient()) {
    client.Timeout = TimeSpan.FromMinutes(5);

    // Create a file stream to store the downloaded data.
    // This really can be any type of writeable stream.
    using (var file = new FileStream(filePath, FileMode.Create, FileAccess.Write, FileShare.None)) {

        // Use the custom extension method below to download the data.
        // The passed progress-instance will receive the download status updates.
        await client.DownloadAsync(DownloadUrl, file, progress, cancellationToken);
    }
}

Implementation

The code for this extension method looks like this. Note that this extension depends on another extension for handling asynchronous stream copying with progress reporting.

public static class HttpClientExtensions
{
    public static async Task DownloadAsync(this HttpClient client, string requestUri, Stream destination, IProgress<float> progress = null, CancellationToken cancellationToken = default) {
        // Get the http headers first to examine the content length
        using (var response = await client.GetAsync(requestUri, HttpCompletionOption.ResponseHeadersRead)) {
            var contentLength = response.Content.Headers.ContentLength;

            using (var download = await response.Content.ReadAsStreamAsync()) {

                // Ignore progress reporting when no progress reporter was 
                // passed or when the content length is unknown
                if (progress == null || !contentLength.HasValue) {
                    await download.CopyToAsync(destination);
                    return;
                }

                // Convert absolute progress (bytes downloaded) into relative progress (0% - 100%)
                var relativeProgress = new Progress<long>(totalBytes => progress.Report((float)totalBytes / contentLength.Value));
                // Use extension method to report progress while downloading
                await download.CopyToAsync(destination, 81920, relativeProgress, cancellationToken);
                progress.Report(1);
            }
        }
    }
}

With stream extension for the real progress reporting:

public static class StreamExtensions
{
    public static async Task CopyToAsync(this Stream source, Stream destination, int bufferSize, IProgress<long> progress = null, CancellationToken cancellationToken = default) {
        if (source == null)
            throw new ArgumentNullException(nameof(source));
        if (!source.CanRead)
            throw new ArgumentException("Has to be readable", nameof(source));
        if (destination == null)
            throw new ArgumentNullException(nameof(destination));
        if (!destination.CanWrite)
            throw new ArgumentException("Has to be writable", nameof(destination));
        if (bufferSize < 0)
            throw new ArgumentOutOfRangeException(nameof(bufferSize));

        var buffer = new byte[bufferSize];
        long totalBytesRead = 0;
        int bytesRead;
        while ((bytesRead = await source.ReadAsync(buffer, 0, buffer.Length, cancellationToken).ConfigureAwait(false)) != 0) {
            await destination.WriteAsync(buffer, 0, bytesRead, cancellationToken).ConfigureAwait(false);
            totalBytesRead += bytesRead;
            progress?.Report(totalBytesRead);
        }
    }
}

same as @René Sackers solution above, but added the ability to cancel the download

class HttpClientDownloadWithProgress : IDisposable
{
    private readonly string _downloadUrl;
    private readonly string _destinationFilePath;
    private readonly CancellationToken? _cancellationToken;

    private HttpClient _httpClient;

    public delegate void ProgressChangedHandler(long? totalFileSize, long totalBytesDownloaded, double? progressPercentage);

    public event ProgressChangedHandler ProgressChanged;

    public HttpClientDownloadWithProgress(string downloadUrl, string destinationFilePath, CancellationToken? cancellationToken = null)
    {
        _downloadUrl = downloadUrl;
        _destinationFilePath = destinationFilePath;
        _cancellationToken = cancellationToken;
    }

    public async Task StartDownload()
    {
        _httpClient = new HttpClient { Timeout = TimeSpan.FromDays(1) };

        using (var response = await _httpClient.GetAsync(_downloadUrl, HttpCompletionOption.ResponseHeadersRead))
            await DownloadFileFromHttpResponseMessage(response);
    }

    private async Task DownloadFileFromHttpResponseMessage(HttpResponseMessage response)
    {
        response.EnsureSuccessStatusCode();

        var totalBytes = response.Content.Headers.ContentLength;

        using (var contentStream = await response.Content.ReadAsStreamAsync())
            await ProcessContentStream(totalBytes, contentStream);
    }

    private async Task ProcessContentStream(long? totalDownloadSize, Stream contentStream)
    {
        var totalBytesRead = 0L;
        var readCount = 0L;
        var buffer = new byte[8192];
        var isMoreToRead = true;

        using (var fileStream = new FileStream(_destinationFilePath, FileMode.Create, FileAccess.Write, FileShare.None, 8192, true))
        {
            do
            {
                int bytesRead;
                if (_cancellationToken.HasValue)
                {
                    bytesRead = await contentStream.ReadAsync(buffer, 0, buffer.Length, _cancellationToken.Value);
                }
                else
                {
                    bytesRead = await contentStream.ReadAsync(buffer, 0, buffer.Length);
                }

                if (bytesRead == 0)
                {
                    isMoreToRead = false;
                    continue;
                }

                await fileStream.WriteAsync(buffer, 0, bytesRead);

                totalBytesRead += bytesRead;
                readCount += 1;

                if (readCount % 10 == 0)
                    TriggerProgressChanged(totalDownloadSize, totalBytesRead);
            }
            while (isMoreToRead);

        }

        //the last progress trigger should occur after the file handle has been released or you may get file locked error
        TriggerProgressChanged(totalDownloadSize, totalBytesRead);
    }

    private void TriggerProgressChanged(long? totalDownloadSize, long totalBytesRead)
    {
        if (ProgressChanged == null)
            return;

        double? progressPercentage = null;
        if (totalDownloadSize.HasValue)
            progressPercentage = Math.Round((double)totalBytesRead / totalDownloadSize.Value * 100, 2);

        ProgressChanged(totalDownloadSize, totalBytesRead, progressPercentage);
    }

    public void Dispose()
    {
        _httpClient?.Dispose();
    }
}

The simplest way to implement progress tracking for both uploading and downloading is to use ProgressMessageHandler from the Microsoft.AspNet.WebApi.Client nuget package.

Note: this library was originally named System.Net.Http.Formatting, and was renamed to Microsoft.AspNet.WebApi.Client. However, this library is not related to ASP.Net and can be used by any project looking for official Microsoft extensions to HttpClient. The source code is available here.

Example:

var handler = new HttpClientHandler() { AllowAutoRedirect = true };
var ph = new ProgressMessageHandler(handler);

ph.HttpSendProgress += (_, args) =>
{
    Console.WriteLine($"upload progress: {(double)args.BytesTransferred / args.TotalBytes}");
};

ph.HttpReceiveProgress += (_, args) =>
{
    Console.WriteLine($"download progress: {(double)args.BytesTransferred / args.TotalBytes}");
};

var client = new HttpClient(ph);
await client.SendAsync(...);

Note that this will not report progress if uploading a byte array. The request message content must be a stream.

René Sackers version is excellent but it could be better. Specifically, it has a subtle race condition caused by TriggerProgressChanged firing before the stream closes. The fix is to fire the event after the stream is explicitly disposed. The version below includes the above change, inherits from HttpClient and adds support for cancellation tokens.

public delegate void ProgressChangedHandler(long? totalFileSize, long totalBytesDownloaded, double? progressPercentage);

public class HttpClientWithProgress : HttpClient
{
    private readonly string _DownloadUrl;
    private readonly string _DestinationFilePath;

    public event ProgressChangedHandler ProgressChanged;

    public HttpClientWithProgress(string downloadUrl, string destinationFilePath)
    {
        _DownloadUrl = downloadUrl;
        _DestinationFilePath = destinationFilePath;
    }

    public async Task StartDownload()
    {
        using (var response = await GetAsync(_DownloadUrl, HttpCompletionOption.ResponseHeadersRead))
            await DownloadFileFromHttpResponseMessage(response);
    }

    public async Task StartDownload(CancellationToken cancellationToken)
    {
        using (var response = await GetAsync(_DownloadUrl, HttpCompletionOption.ResponseHeadersRead, cancellationToken))
            await DownloadFileFromHttpResponseMessage(response);
    }

    private async Task DownloadFileFromHttpResponseMessage(HttpResponseMessage response)
    {
        response.EnsureSuccessStatusCode();
        long? totalBytes = response.Content.Headers.ContentLength;
        using (var contentStream = await response.Content.ReadAsStreamAsync())
            await ProcessContentStream(totalBytes, contentStream);
    }

    private async Task ProcessContentStream(long? totalDownloadSize, Stream contentStream)
    {
        long totalBytesRead = 0L;
        long readCount = 0L;
        byte[] buffer = new byte[8192];
        bool isMoreToRead = true;

        using (FileStream fileStream = new FileStream(_DestinationFilePath, FileMode.Create, FileAccess.Write, FileShare.None, 8192, true))
        {
            do
            {
                int bytesRead = await contentStream.ReadAsync(buffer, 0, buffer.Length);
                if (bytesRead == 0)
                {
                    isMoreToRead = false;
                    continue;
                }

                await fileStream.WriteAsync(buffer, 0, bytesRead);

                totalBytesRead += bytesRead;
                readCount += 1;

                if (readCount % 10 == 0)
                    TriggerProgressChanged(totalDownloadSize, totalBytesRead);
            }
            while (isMoreToRead);
        }
        TriggerProgressChanged(totalDownloadSize, totalBytesRead);
    }

    private void TriggerProgressChanged(long? totalDownloadSize, long totalBytesRead)
    {
        if (ProgressChanged == null)
            return;

        double? progressPercentage = null;
        if (totalDownloadSize.HasValue)
            progressPercentage = Math.Round((double)totalBytesRead / totalDownloadSize.Value * 100, 2);

        ProgressChanged(totalDownloadSize, totalBytesRead, progressPercentage);
    }
}

This is my variation on the answer of René Sackers. Main differences:

• A more functional style.
• Only one method instead of a whole object.
• Can cancel the download

        public async static Task Download(
               string downloadUrl,
               string destinationFilePath,
               Func<long?, long, double?, bool> progressChanged)
        {
            using var httpClient = new HttpClient { Timeout = TimeSpan.FromDays(1) };
            using var response = await httpClient.GetAsync(downloadUrl, HttpCompletionOption.ResponseHeadersRead);

            response.EnsureSuccessStatusCode();
            var totalBytes = response.Content.Headers.ContentLength;

            using var contentStream = await response.Content.ReadAsStreamAsync();
            var totalBytesRead = 0L;
            var readCount = 0L;
            var buffer = new byte[8192];
            var isMoreToRead = true;

            static double? calculatePercentage(long? totalDownloadSize, long totalBytesRead) => totalDownloadSize.HasValue ? Math.Round((double)totalBytesRead / totalDownloadSize.Value * 100, 2) : null;

            using var fileStream = new FileStream(destinationFilePath, FileMode.Create, FileAccess.Write, FileShare.None, 8192, true);

            do
            {
                var bytesRead = await contentStream.ReadAsync(buffer);
                if (bytesRead == 0)
                {
                    isMoreToRead = false;

                    if (progressChanged(totalBytes, totalBytesRead, calculatePercentage(totalBytes, totalBytesRead)))
                    {
                        throw new OperationCanceledException();
                    }

                    continue;
                }

                await fileStream.WriteAsync(buffer.AsMemory(0, bytesRead));

                totalBytesRead += bytesRead;
                readCount++;

                if (readCount % 100 == 0)
                {
                    if (progressChanged(totalBytes, totalBytesRead, calculatePercentage(totalBytes, totalBytesRead)))
                    {
                        throw new OperationCanceledException();
                    }
                }
            }
            while (isMoreToRead);
        }

It can be called this way:

    // Change this variable to stop the download
    // You can use a global variable or some kind of state management
    var mustStop = false;

    var downloadProgress = (long? _, long __, double? progressPercentage) =>
    {
       if (progressPercentage.HasValue)
          progressBar.Value = progressPercentage.Value;

       // In this example only the variable is checked
       // You could write other code that evaluates other conditions
       return mustStop;
    };

    SomeClass.Download("https://example.com/bigfile.zip", "c:\downloads\file.zip", downloadProgress);

This is a modified version of René Sackers answer with the following functional changes:

• http client not disposed (because it should not be disposed)
• better progress handling
• callback to create httpRequest (custom header support)
• utilizes ArrayPool to reduce memory footprint
• automatic event subscribe+unsubscribe to prevent memory leaks by event handlers

Usage:

await DownloadWithProgress.ExecuteAsync(HttpClients.General, assetUrl, downloadFilePath, progressHandler, () =>
{
    var requestMessage = new HttpRequestMessage(HttpMethod.Get, assetUrl);
    requestMessage.Headers.Accept.TryParseAdd("application/octet-stream");
    return requestMessage;
});

I guess i am not the only one who needs custom headers so i figured i would share this rewrite

Implementation:

public delegate void DownloadProgressHandler(long? totalFileSize, long totalBytesDownloaded, double? progressPercentage);

public static class DownloadWithProgress
{
    public static async Task ExecuteAsync(HttpClient httpClient, string downloadPath, string destinationPath, DownloadProgressHandler progress, Func<HttpRequestMessage> requestMessageBuilder = null)
    {
        requestMessageBuilder ??= GetDefaultRequestBuilder(downloadPath);
        var download = new HttpClientDownloadWithProgress(httpClient, destinationPath, requestMessageBuilder);
        download.ProgressChanged += progress;
        await download.StartDownload();
        download.ProgressChanged -= progress;
    }

    private static Func<HttpRequestMessage> GetDefaultRequestBuilder(string downloadPath)
    {
        return () => new HttpRequestMessage(HttpMethod.Get, downloadPath);
    }
}

internal class HttpClientDownloadWithProgress
{
    private readonly HttpClient _httpClient;
    private readonly string _destinationFilePath;
    private readonly Func<HttpRequestMessage> _requestMessageBuilder;
    private int _bufferSize = 8192;

    public event DownloadProgressHandler ProgressChanged;

    public HttpClientDownloadWithProgress(HttpClient httpClient, string destinationFilePath, Func<HttpRequestMessage> requestMessageBuilder)
    {
        _httpClient = httpClient ?? throw new ArgumentNullException(nameof(httpClient));
        _destinationFilePath = destinationFilePath ?? throw new ArgumentNullException(nameof(destinationFilePath));
        _requestMessageBuilder = requestMessageBuilder ?? throw new ArgumentNullException(nameof(requestMessageBuilder));
    }

    public async Task StartDownload()
    {
        using var requestMessage = _requestMessageBuilder.Invoke();
        using var response = await _httpClient.SendAsync(requestMessage, HttpCompletionOption.ResponseHeadersRead);
        await DownloadAsync(response);
    }

    private async Task DownloadAsync(HttpResponseMessage response)
    {
        response.EnsureSuccessStatusCode();

        var totalBytes = response.Content.Headers.ContentLength;

        using (var contentStream = await response.Content.ReadAsStreamAsync())
            await ProcessContentStream(totalBytes, contentStream);
    }

    private async Task ProcessContentStream(long? totalDownloadSize, Stream contentStream)
    {
        var totalBytesRead = 0L;
        var readCount = 0L;
        var buffer = ArrayPool<byte>.Shared.Rent(_bufferSize);
        var isMoreToRead = true;

        using (var fileStream = new FileStream(_destinationFilePath, FileMode.Create, FileAccess.Write, FileShare.None, _bufferSize, true))
        {
            do
            {
                var bytesRead = await contentStream.ReadAsync(buffer, 0, buffer.Length);
                if (bytesRead == 0)
                {
                    isMoreToRead = false;
                    ReportProgress(totalDownloadSize, totalBytesRead);
                    continue;
                }

                await fileStream.WriteAsync(buffer, 0, bytesRead);

                totalBytesRead += bytesRead;
                readCount += 1;

                if (readCount % 100 == 0)
                    ReportProgress(totalDownloadSize, totalBytesRead);
            }
            while (isMoreToRead);
        }

        ArrayPool<byte>.Shared.Return(buffer);
    }

    private void ReportProgress(long? totalDownloadSize, long totalBytesRead)
    {
        double? progressPercentage = null;
        if (totalDownloadSize.HasValue)
            progressPercentage = Math.Round((double)totalBytesRead / totalDownloadSize.Value * 100, 2);

        ProgressChanged?.Invoke(totalDownloadSize, totalBytesRead, progressPercentage);
    }
}

Im not really sure how to measure how the completion logic, but for now this seems to do it.

public event ProgressChangedHandler ProgressChanged;
public event ProgressCompleteHandler DownloadComplete;

...

TriggerProgressChanged(totalDownloadSize, totalBytesRead);
TriggerDownloadComplete(totalBytesRead == totalDownloadSize);

private void TriggerDownloadComplete(bool status)
{
   DownloadComplete(status);
}

client.DownloadComplete += (status) =>
{
   if (status)
   {
       // success
   }
};