How to Implement a Progress Bar with HttpClient in .NET?

Use HttpClient to implement progress bar in .NET

The IProgress<T> interface introduced in .NET 4.5 allows handling of asynchronous progress reporting, thus implementing progress bar functionality for downloading files using HttpClient.

Extension methods using IProgress<T>

Creates an extension method for HttpClient's DownloadAsync operation that accepts a IProgress<float> implementation as a parameter. This implementation will update the progress bar or UI with the download status.

public static async Task DownloadAsync(this HttpClient client, string requestUri, Stream destination, IProgress<float> progress = null, CancellationToken cancellationToken = default)
{
    // 首先获取 http 头信息以检查内容长度
    using (var response = await client.GetAsync(requestUri, HttpCompletionOption.ResponseHeadersRead))
    {
        var contentLength = response.Content.Headers.ContentLength;

        using (var download = await response.Content.ReadAsStreamAsync(cancellationToken))
        {
            // 未传递进度报告器或内容长度未知时忽略进度报告
            if (progress == null || !contentLength.HasValue)
            {
                await download.CopyToAsync(destination);
                return;
            }

            // 将绝对进度（已下载字节数）转换为相对进度（0% - 100%）
            var relativeProgress = new Progress<long>(totalBytes => progress.Report((float)totalBytes / contentLength.Value));
            // 使用扩展方法在下载时报告进度
            await download.CopyToAsync(destination, 81920, relativeProgress, cancellationToken);
            progress.Report(1); // 报告100%完成
        }
    }
}

In this method, the progress.Report() method will pass the current download progress (in percent) to your IProgress<float> implementation, allowing it to update the progress bar or other UI elements accordingly.

Stream extension for progress reporting

To handle actual progress reporting as data is written to the target stream, consider creating an extension method for the Stream class.

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("必须可读", nameof(source));
    if (destination == null)
        throw new ArgumentNullException(nameof(destination));
    if (!destination.CanWrite)
        throw new ArgumentException("必须可写", 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);
    }
}

This extension method monitors the number of bytes written to the target stream and reports progress using the IProgress<long> implementation passed in.

By combining these extension methods, you can easily implement progress bar functionality for file download operations using HttpClient.

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