---
title: "How do you implement file chunking for large file uploads?"  
description: "How do you implement file chunking for large file uploads?"  
author: "ICSM Computer"  
published: 2025-05-16  
updated: 2025-05-30  
canonical: https://www.mindstick.com/interview/34129/how-do-you-implement-file-chunking-for-large-file-uploads  
category: "c#"  
tags: ["c#"]  
reading_time: 4 minutes  

---

# How do you implement file chunking for large file uploads?

Implementing **file chunking for large file uploads** involves breaking the file into smaller chunks on the client side and sending them to the server, where the chunks are reassembled. Here's how to do this in C# for the **server-side** logic:

## Concept Overview

### Client:

1. Splits large file into chunks (e.g., 5 MB each).
2. Sends each chunk via HTTP POST/PUT.
3. Includes metadata: `fileId`, `chunkIndex`, `totalChunks`, etc.

### Server:

1. Receives and stores each chunk temporarily.
2. Once all chunks are received, merges them into a final file.

## 1. API Endpoint to Receive a Chunk

```cs
[HttpPost("upload-chunk")]
public async Task<IActionResult> UploadChunk(
    [FromForm] IFormFile chunk,
    [FromForm] string fileId,
    [FromForm] int chunkIndex,
    [FromForm] int totalChunks)
{
    string tempDir = Path.Combine("TempUploads", fileId);
    Directory.CreateDirectory(tempDir);

    string chunkPath = Path.Combine(tempDir, $"{chunkIndex}.chunk");

    using var stream = new FileStream(chunkPath, FileMode.Create);
    await chunk.CopyToAsync(stream);

    return Ok(new { message = "Chunk uploaded", chunkIndex });
}
```

## 2. Endpoint to Merge All Chunks

```cs
[HttpPost("merge-chunks")]
public IActionResult MergeChunks([FromForm] string fileId, [FromForm] string finalFileName)
{
    string tempDir = Path.Combine("TempUploads", fileId);
    string outputPath = Path.Combine("FinalUploads", finalFileName);

    Directory.CreateDirectory("FinalUploads");

    var chunkFiles = Directory.GetFiles(tempDir)
                              .OrderBy(f => int.Parse(Path.GetFileNameWithoutExtension(f)))
                              .ToList();

    using var output = new FileStream(outputPath, FileMode.Create);

    foreach (var chunkFile in chunkFiles)
    {
        using var input = new FileStream(chunkFile, FileMode.Open);
        input.CopyTo(output);
    }

    // Optionally clean up
    Directory.Delete(tempDir, recursive: true);

    return Ok(new { message = "File merged", outputPath });
}
```

## 3. Example Front-End (JavaScript File Chunking)

```javascript
async function uploadFileInChunks(file) {
  const chunkSize = 5 * 1024 * 1024; // 5MB
  const totalChunks = Math.ceil(file.size / chunkSize);
  const fileId = crypto.randomUUID(); // unique ID

  for (let i = 0; i < totalChunks; i++) {
    const start = i * chunkSize;
    const end = Math.min(file.size, start + chunkSize);
    const chunk = file.slice(start, end);

    const formData = new FormData();
    formData.append("chunk", chunk);
    formData.append("fileId", fileId);
    formData.append("chunkIndex", i);
    formData.append("totalChunks", totalChunks);

    await fetch("/upload-chunk", {
      method: "POST",
      body: formData
    });
  }

  // Trigger merge
  await fetch("/merge-chunks", {
    method: "POST",
    body: new URLSearchParams({
      fileId,
      finalFileName: file.name
    })
  });
}
```

## Benefits

1. Handles unstable connections (retry per chunk).
2. Works well for large files (even GBs).
3. Enables upload resumption and parallelism (with tracking).

## Answers

### Answer by ICSM Computer

Implementing **file chunking for large file uploads** involves breaking the file into smaller chunks on the client side and sending them to the server, where the chunks are reassembled. Here's how to do this in C# for the **server-side** logic:

## Concept Overview

### Client:

1. Splits large file into chunks (e.g., 5 MB each).
2. Sends each chunk via HTTP POST/PUT.
3. Includes metadata: `fileId`, `chunkIndex`, `totalChunks`, etc.

### Server:

1. Receives and stores each chunk temporarily.
2. Once all chunks are received, merges them into a final file.

## 1. API Endpoint to Receive a Chunk

```cs
[HttpPost("upload-chunk")]
public async Task<IActionResult> UploadChunk(
    [FromForm] IFormFile chunk,
    [FromForm] string fileId,
    [FromForm] int chunkIndex,
    [FromForm] int totalChunks)
{
    string tempDir = Path.Combine("TempUploads", fileId);
    Directory.CreateDirectory(tempDir);

    string chunkPath = Path.Combine(tempDir, $"{chunkIndex}.chunk");

    using var stream = new FileStream(chunkPath, FileMode.Create);
    await chunk.CopyToAsync(stream);

    return Ok(new { message = "Chunk uploaded", chunkIndex });
}
```

## 2. Endpoint to Merge All Chunks

```cs
[HttpPost("merge-chunks")]
public IActionResult MergeChunks([FromForm] string fileId, [FromForm] string finalFileName)
{
    string tempDir = Path.Combine("TempUploads", fileId);
    string outputPath = Path.Combine("FinalUploads", finalFileName);

    Directory.CreateDirectory("FinalUploads");

    var chunkFiles = Directory.GetFiles(tempDir)
                              .OrderBy(f => int.Parse(Path.GetFileNameWithoutExtension(f)))
                              .ToList();

    using var output = new FileStream(outputPath, FileMode.Create);

    foreach (var chunkFile in chunkFiles)
    {
        using var input = new FileStream(chunkFile, FileMode.Open);
        input.CopyTo(output);
    }

    // Optionally clean up
    Directory.Delete(tempDir, recursive: true);

    return Ok(new { message = "File merged", outputPath });
}
```

## 3. Example Front-End (JavaScript File Chunking)

```javascript
async function uploadFileInChunks(file) {
  const chunkSize = 5 * 1024 * 1024; // 5MB
  const totalChunks = Math.ceil(file.size / chunkSize);
  const fileId = crypto.randomUUID(); // unique ID

  for (let i = 0; i < totalChunks; i++) {
    const start = i * chunkSize;
    const end = Math.min(file.size, start + chunkSize);
    const chunk = file.slice(start, end);

    const formData = new FormData();
    formData.append("chunk", chunk);
    formData.append("fileId", fileId);
    formData.append("chunkIndex", i);
    formData.append("totalChunks", totalChunks);

    await fetch("/upload-chunk", {
      method: "POST",
      body: formData
    });
  }

  // Trigger merge
  await fetch("/merge-chunks", {
    method: "POST",
    body: new URLSearchParams({
      fileId,
      finalFileName: file.name
    })
  });
}
```

## Benefits

1. Handles unstable connections (retry per chunk).
2. Works well for large files (even GBs).
3. Enables upload resumption and parallelism (with tracking).


---

Original Source: https://www.mindstick.com/interview/34129/how-do-you-implement-file-chunking-for-large-file-uploads

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