ASP.NET Mvc : Download Large File Efficiently

When downloading large files efficiently in ASP.NET MVC, you need to optimize memory usage, prevent timeouts, and ensure a smooth user experience. Below are the best approaches:

1. Use FileStreamResult (Efficient for Large Files)

This streams the file without loading it entirely into memory.

public ActionResult DownloadLargeFile()
{
    string filePath = Server.MapPath("~/Files/LargeFile.zip"); // Adjust file path
    string fileName = "LargeFile.zip";

    if (!System.IO.File.Exists(filePath))
    {
        return HttpNotFound("File not found.");
    }

    var stream = new FileStream(filePath, FileMode.Open, FileAccess.Read);
    return File(stream, "application/octet-stream", fileName);
}

 ✅ Why?

• Streams the file without consuming memory.
• Works well for large files (e.g., >1GB).

2. Use Response.Write to Stream File in Chunks

This method writes file in chunks to prevent excessive memory usage.

public void StreamLargeFile()
{
    string filePath = Server.MapPath("~/Files/LargeFile.zip");

    if (!System.IO.File.Exists(filePath))
    {
        Response.StatusCode = 404;
        Response.End();
        return;
    }

    Response.Clear();
    Response.ContentType = "application/octet-stream";
    Response.AddHeader("Content-Disposition", "attachment; filename=LargeFile.zip");

    const int bufferSize = 1024 * 1024; // 1MB chunks
    byte[] buffer = new byte[bufferSize];

    using (FileStream fileStream = new FileStream(filePath, FileMode.Open, FileAccess.Read))
    {
        int bytesRead;
        while ((bytesRead = fileStream.Read(buffer, 0, buffer.Length)) > 0)
        {
            Response.OutputStream.Write(buffer, 0, bytesRead);
            Response.Flush(); // Sends the chunk immediately to the client
        }
    }

    Response.End();
}

 ✅ Why?

• Sends 1MB chunks, preventing high memory usage.
• Flushes data after each chunk to avoid server timeouts.

3. Asynchronous Streaming (Recommended for Web API)

For MVC + Web API applications, use an async stream for optimal performance.

public async Task<IActionResult> DownloadLargeFileAsync()
{
    string filePath = Path.Combine(Directory.GetCurrentDirectory(), "wwwroot", "Files", "LargeFile.zip");

    if (!System.IO.File.Exists(filePath))
    {
        return NotFound("File not found.");
    }

    var stream = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.Read, bufferSize: 4096, useAsync: true);
    return File(stream, "application/octet-stream", "LargeFile.zip");
}

✅ Why?

• Asynchronous file streaming prevents thread blocking.
• useAsync: true optimizes I/O operations.

4. Enable Large File Downloads in Web.config

Modify web.config to allow large downloads:

<system.webServer>
    <security>
        <requestFiltering>
            <requestLimits maxAllowedContentLength="2147483648" /> <!-- 2GB -->
        </requestFiltering>
    </security>
</system.webServer>

✅ Why?

• Increases max file size limit (default is 30MB in IIS).

5. Use Content-Disposition Header (For Better Browser Support)

If the filename contains special characters, encode it properly:

public ActionResult DownloadLargeFile()
{
    string filePath = Server.MapPath("~/Files/LargeFile.zip");
    string fileName = "Large File.zip";

    var stream = new FileStream(filePath, FileMode.Open, FileAccess.Read);
    var cd = new System.Net.Mime.ContentDisposition
    {
        FileName = fileName,
        Inline = false // Forces download
    };
    Response.Headers.Add("Content-Disposition", cd.ToString());

    return File(stream, "application/octet-stream");
}

✅ Why?

• Prevents filename issues (especially in Chrome & Edge).

6. Optimize IIS for Large Files

If you use IIS, increase timeout settings:

• appcmd set config /section:serverRuntime /uploadReadAheadSize:10485760
• appcmd set config /section:system.webServer/serverRuntime /maxRequestEntityAllowed:2147483648

✅ Why?

• Prevents timeouts for large file downloads.

Conclusion

Hope this will help to understand file download ways and setup.