How do you resume a file download that was interrupted halfway?

Question

ICSM Computer · Answer

To resume a file download that was interrupted halfway, you need to support
HTTP Range requests — a mechanism that allows clients to request only a portion of a file. Here's how to implement it in both
client-side and server-side code using C#.
1. Client-Side: Resume Download with HttpClient
public async Task ResumeDownloadAsync(string url, string localPath)
{
    long existingLength = 0;
    if (File.Exists(localPath))
        existingLength = new FileInfo(localPath).Length;
    using var handler = new HttpClientHandler() { AllowAutoRedirect = true };
    using var client = new HttpClient(handler);
    // Set Range header
    client.DefaultRequestHeaders.Range = new System.Net.Http.Headers.RangeHeaderValue(existingLength, null);
    using var response = await client.GetAsync(url, HttpCompletionOption.ResponseHeadersRead);
    response.EnsureSuccessStatusCode();
    using var httpStream = await response.Content.ReadAsStreamAsync();
    using var fileStream = new FileStream(localPath, FileMode.Append, FileAccess.Write, FileShare.None);
    await httpStream.CopyToAsync(fileStream);
}
Behavior:
If localPath file exists, we calculate how many bytes have already been downloaded.Then we request only the remaining part of the file using Range: bytes=start-.
2. Server-Side: Support Range Requests (ASP.NET Core Example)
[HttpGet("download/{filename}")]
public IActionResult DownloadFile(string filename)
{
    var filePath = Path.Combine("files", filename);
    var file = new FileInfo(filePath);
    if (!file.Exists)
        return NotFound();
    var stream = new FileStream(filePath, FileMode.Open, FileAccess.Read);
    return File(stream, "application/octet-stream", file.Name, enableRangeProcessing: true);
}
Key Point:
enableRangeProcessing: true lets ASP.NET Core handle partial requests for you.The framework will respond with HTTP 206 (Partial Content) if a range is requested.
3. HTTP Header Behavior
When resuming, the server should:Accept Range: bytes=start-Respond with 206 Partial ContentInclude headers like Content-Range and Accept-Ranges: bytes
4. Things to Watch
Some servers do not support range requests. Test for Accept-Ranges in response headers.Ensure consistent file sizes and no corruption between resume attempts.Always handle the case where the server returns 200 OK instead of
206.
and

interview

How do you resume a file download that was interrupted halfway?

ICSM Computer

Can you answer this question?

1 Answers

1. Client-Side: Resume Download with `HttpClient`

2. Server-Side: Support Range Requests (ASP.NET Core Example)

3. HTTP Header Behavior

4. Things to Watch

Liked By