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Home Interviews How do you resume a file download that was interrupted halfway? – MindStick
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1 Answers

ICSM Computer
ICSM Computer 18 May 2025
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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:

  1. If localPath file exists, we calculate how many bytes have already been downloaded.
  2. 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:

  1. enableRangeProcessing: true lets ASP.NET Core handle partial requests for you.
  2. The framework will respond with HTTP 206 (Partial Content) if a range is requested.

3. HTTP Header Behavior

  1. When resuming, the server should:
  2. Accept Range: bytes=start-
  3. Respond with 206 Partial Content
  4. Include headers like Content-Range and Accept-Ranges: bytes

4. Things to Watch

  1. Some servers do not support range requests. Test for Accept-Ranges in response headers.
  2. Ensure consistent file sizes and no corruption between resume attempts.
  3. Always handle the case where the server returns 200 OK instead of 206.
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