Remove Excel Attachments Programmatically Using C#

Introduction

Ever opened an Excel spreadsheet only to find it’s mysteriously bloated to several megabytes, even though it contains just a few rows of data? You’re not alone. Excel worksheets often accumulate attachments—embedded files, broken links to external documents, encrypted objects—that clutter your workbooks and slow everything down.

If you’re managing Excel files at scale (think: automated reporting systems, data pipelines, or document management platforms), manually hunting down these attachments isn’t just tedious—it’s impossible. That’s where programmatic cleanup comes in.

In this guide, I’ll show you how to remove excel attachments programmatically using GroupDocs.Watermark for .NET. We’ll cover everything from identifying different attachment types to building a robust cleanup solution that handles edge cases gracefully. By the end, you’ll have a working C# implementation you can drop into your projects today.

Why Clean Up Excel Attachments?

Before we dive into code, let’s talk about why this matters (because it’s not just about saving a few kilobytes):

File Size Reduction: Attachments—especially high-resolution images or PDFs embedded as OLE objects—can balloon your Excel files. A 200 KB spreadsheet can easily become 20 MB with attachments.

Security & Compliance: Encrypted attachments or files with broken links can pose security risks. If you can’t verify what’s attached, you can’t ensure compliance with data policies.

Performance Impact: Large files take longer to open, process, and transfer. When you’re dealing with automated workflows processing hundreds of spreadsheets daily, every second counts.

Collaboration Issues: Broken links (files that were on someone’s local machine but aren’t accessible to others) frustrate team members and break workflows.

What Counts as an “Attachment” in Excel?

Let’s clarify what we’re actually removing here, because Excel has several ways to attach external content:

  1. Embedded Objects: Files inserted via “Insert > Object” (PDFs, Word docs, other Excel files)
  2. OLE Objects: Legacy embedded content that may or may not still open
  3. Linked Files: References to external files (think C:\Users\John\Document.xlsx)
  4. Encrypted Attachments: Password-protected embedded files
  5. Images and Charts: While technically attachments, we’ll focus on file-based objects

Our solution specifically targets broken links and encrypted files—the most common culprits behind Excel file bloat and security headaches.

Prerequisites

Before implementing our solution, make sure you’ve got these basics covered:

Required Libraries and Versions

  1. GroupDocs.Watermark for .NET (version 24.x or later): This library handles more than just watermarks—it’s a powerful document manipulation toolkit
  2. .NET Framework 4.6.1+ or .NET Core 3.1+: The library supports both, so pick what fits your project

Environment Setup

  • Visual Studio 2019+ (or your preferred .NET IDE)
  • Administrator access for reading/writing files (especially important for network drives)

Knowledge Prerequisites

You should be comfortable with:

  • Basic C# syntax (if you can work with classes and loops, you’re good)
  • File system operations in .NET
  • The concept of using statements and disposable objects

Don’t worry if you’re not an Excel API expert—I’ll explain everything as we go.

Setting Up GroupDocs.Watermark for .NET

Getting the library installed is straightforward. Pick your preferred method:

.NET CLI (if you’re command-line oriented):

dotnet add package GroupDocs.Watermark

Package Manager (the PowerShell way):

Install-Package GroupDocs.Watermark

NuGet Package Manager UI (the clicky way):

  • Open your project in Visual Studio
  • Right-click on your project → Manage NuGet Packages
  • Search for “GroupDocs.Watermark”
  • Click Install

License Acquisition Steps

Here’s the deal with licensing (because nobody likes surprise limitations):

  1. Free Trial: Grab it from the downloads page. It’s fully functional but adds a trial watermark to outputs and limits processing.

  2. Temporary License: Need more time to evaluate? Get a temporary license that removes restrictions for 30 days.

  3. Full License: For production use, you’ll need to purchase a license. It’s a one-time purchase with support and updates.

Pro tip: Start with the free trial to build your solution, then upgrade when you’re ready to deploy.

Implementation Guide: Building Your Attachment Cleaner

Alright, let’s build this thing. I’ll break it down step-by-step so you understand not just what the code does, but why we’re doing it this way.

Step 1: Set Up Your File Paths

First things first—tell your program where to find files and where to save the cleaned versions:

string documentPath = Path.Combine(@"YOUR_DOCUMENT_DIRECTORY", "YourSpreadsheet.xlsx");
string outputFileName = Path.Combine(@"YOUR_OUTPUT_DIRECTORY", "OutputSpreadsheet.xlsx");

What this does: Path.Combine is your friend here—it handles path separators correctly across Windows and Linux. Replace YOUR_DOCUMENT_DIRECTORY with your actual input folder path (e.g., @"C:\Projects\ExcelFiles\Input").

Common gotcha: Make sure your output directory exists. If it doesn’t, create it first or the save operation will throw an exception.

Step 2: Configure Loading Options

var loadOptions = new SpreadsheetLoadOptions();

What this does: This tells GroupDocs how to interpret your file. For Excel files, the default options usually work fine, but you can customize them if you’re dealing with password-protected workbooks or need to preserve specific formatting.

When you’d customize this: If your Excel file is password-protected, you’d add: loadOptions.Password = "yourPassword";

Step 3: Initialize the Watermarker

using (Watermarker watermarker = new Watermarker(documentPath, loadOptions))
{
    // All the magic happens inside this block
}

What this does: The Watermarker class is your gateway to the document. The using statement ensures proper cleanup—when this block ends, the file handle is released automatically (no memory leaks or locked files).

Why “Watermarker” for attachments? I know, the name is confusing. GroupDocs.Watermark started as a watermarking library but evolved into a broader document manipulation tool. Think of Watermarker as “DocumentEditor” and it makes more sense.

Step 4: Access Spreadsheet Content

SpreadsheetContent content = watermarker.GetContent<SpreadsheetContent>();

What this does: This casts the generic document content to a spreadsheet-specific object, giving you access to worksheets, cells, and (importantly) attachments.

Type safety note: The generic parameter <SpreadsheetContent> ensures compile-time type checking. If you accidentally pass a Word document, this line will throw an exception rather than fail silently later.

Step 5: Loop Through Worksheets

foreach (SpreadsheetWorksheet worksheet in content.Worksheets)
{
    // We'll process attachments for each worksheet
}

What this does: Excel workbooks can have multiple worksheets (tabs at the bottom). This loop ensures we clean up attachments across all of them, not just the first one.

Performance consideration: If you know attachments only exist on specific worksheets, you can target those directly using content.Worksheets[0] instead of looping.

Step 6: Identify and Remove Problematic Attachments

Here’s where the real work happens:

for (int i = worksheet.Attachments.Count - 1; i >= 0; i--)
{
    SpreadsheetAttachment attachment = worksheet.Attachments[i];
    
    if ((attachment.IsLink && !File.Exists(attachment.SourceFullName)) ||
        attachment.GetDocumentInfo().IsEncrypted)
    {
        worksheet.Attachments.RemoveAt(i);
    }
}

What this does:

  • Backward iteration (i--): We’re removing items from a collection while looping, so going backward prevents index shifting issues
  • First condition: attachment.IsLink && !File.Exists(...) catches broken links (files that were moved or deleted)
  • Second condition: attachment.GetDocumentInfo().IsEncrypted identifies password-protected embedded files

Why remove encrypted attachments? They’re often security risks—you can’t verify their contents. If your use case requires keeping them, remove that condition.

Customization point: Want to remove all attachments over a certain size? Add: || attachment.Size > 5 * 1024 * 1024 (removes anything over 5 MB).

Step 7: Save Your Cleaned File

watermarker.Save(outputFileName);

What this does: Writes the modified workbook to disk. The original file remains untouched (which is good for testing—always keep backups!).

File format preservation: GroupDocs automatically detects and preserves the original format (.xlsx, .xls, etc.). You don’t need to specify it.

When to Use This Solution

This approach shines in specific scenarios. Here’s when you should reach for it:

Automated Document Pipelines: You’re processing hundreds of Excel files daily (reporting systems, data exports, ETL workflows) and need to keep file sizes manageable.

Pre-Archive Cleanup: Before moving files to long-term storage, strip out attachments to reduce storage costs. A 70% file size reduction isn’t uncommon.

Compliance Workflows: Your organization requires that all distributed Excel files be free of external links or unverified embedded content.

Collaboration Platform Integration: You’re building a SharePoint or OneDrive integration that needs to sanitize uploaded files automatically.

When NOT to use this: If you’re only dealing with a handful of files occasionally, Excel’s built-in tools (Edit Links dialog) might be faster than writing code.

Common Pitfalls to Avoid

I’ve debugged enough Excel automation to know where things go wrong. Here are the gotchas:

1. File Permission Issues

Problem: Your code throws “Access Denied” or “File is being used by another process.”

Solution:

  • Ensure the Excel file isn’t open in Excel when you run your code
  • Run your application with appropriate permissions
  • Use File.Copy() to work on a temporary copy if you can’t control file locks

2. Incorrect Path Handling

Problem: Works on your machine, fails in production.

Solution:

  • Always use Path.Combine() instead of string concatenation
  • Convert relative paths to absolute: Path.GetFullPath(relativePath)
  • For network paths, use UNC format: \\server\share\file.xlsx

3. Memory Issues with Large Files

Problem: Processing a 50 MB Excel file crashes with out-of-memory exceptions.

Solution:

  • Process files in batches rather than loading dozens simultaneously
  • Explicitly dispose of Watermarker objects (the using statement handles this)
  • Increase your application’s memory limit if needed (app.config settings)

4. Assuming All Attachments Are Removable

Problem: Some “attachments” are actually critical embedded charts or pivot table data.

Solution: Test your removal logic thoroughly on sample files. Add logging to see what’s being removed. Consider whitelisting certain attachment types.

Performance Considerations

Let’s talk speed and efficiency, because nobody wants a script that takes 10 minutes to process one file.

Memory Management

  • Dispose promptly: The using statement handles this, but if you’re managing multiple Watermarker instances, ensure each is disposed before creating the next
  • Avoid loading all files into memory: Process one file at a time in a loop
  • Monitor with Task Manager: Keep an eye on memory usage during development to catch leaks early

Processing Speed

  • Benchmark: On a typical developer machine, expect 2-5 seconds per file for small spreadsheets (<5 MB), 10-20 seconds for larger ones (>20 MB)
  • Parallelize carefully: If processing multiple files, Parallel.ForEach can help, but limit concurrency (use MaxDegreeOfParallelism = 4) to avoid memory issues
  • Skip unchanged files: Add a hash check—if the file hasn’t changed since last processing, skip it

Optimization Tips

// Only process worksheets with attachments
foreach (var worksheet in content.Worksheets.Where(w => w.Attachments.Count > 0))
{
    // Your processing logic
}

This simple filter skips empty worksheets, cutting processing time significantly on workbooks with many tabs.

Practical Applications

Here’s how real organizations use this in the wild:

1. Financial Reporting Automation

Scenario: A finance team generates monthly reports with embedded supporting documents. Before archiving, they strip all attachments to reduce storage costs.

Implementation twist: They keep a log of removed attachments for audit purposes:

foreach (var attachment in worksheet.Attachments)
{
    Logger.Log($"Removing: {attachment.Name}, Size: {attachment.Size} bytes");
}

2. Email Attachment Sanitization

Scenario: An HR system receives expense reports via email. Before processing, it removes potentially malicious encrypted attachments.

Implementation twist: They quarantine removed attachments rather than deleting them:

if (attachment.GetDocumentInfo().IsEncrypted)
{
    SaveToQuarantine(attachment);
    worksheet.Attachments.RemoveAt(i);
}

3. Document Management System Integration

Scenario: A SharePoint integration automatically cleans up files when they’re uploaded to a specific library.

Implementation twist: They use Azure Functions to process files asynchronously, scaling automatically with upload volume.

Troubleshooting Guide

When things go wrong (and they will), here’s your debugging checklist:

“File Not Found” Errors

Check:

  • Print the full path: Console.WriteLine(Path.GetFullPath(documentPath));
  • Verify the file exists: if (!File.Exists(documentPath)) throw new Exception("Missing!");

“Document is Corrupted” Exceptions

Likely causes:

  • The file isn’t actually an Excel file (check extension vs. content)
  • The file was saved improperly by another program
  • The file is password-protected but you didn’t provide a password

Quick test: Try opening the file in Excel manually. If Excel complains, your code will too.

Attachments Not Being Removed

Debug by:

  • Add logging inside the removal loop: Console.WriteLine($"Found attachment: {attachment.Name}, IsLink: {attachment.IsLink}");
  • Check your conditions—maybe the attachment doesn’t meet your removal criteria
  • Verify you’re saving the file afterward (watermarker.Save())

Performance Degradation Over Time

Common cause: Memory leaks from not disposing objects properly.

Fix: Ensure every Watermarker is wrapped in a using statement. Use a memory profiler (dotMemory, ANTS) to identify leaks.

Conclusion

You’ve now got a robust solution for removing excel attachments programmatically. This isn’t just about making files smaller—it’s about building reliable, automated workflows that handle Excel documents at scale without manual intervention.

Key takeaways:

  • Use GroupDocs.Watermark for .NET to access and manipulate Excel attachments
  • Target broken links and encrypted files for removal to maximize impact
  • Always process files safely with proper error handling and disposal patterns
  • Test thoroughly on sample files before deploying to production

Next steps:

  • Integrate this into your existing document processing pipeline
  • Add logging and monitoring so you can track how many attachments are being removed
  • Explore other GroupDocs.Watermark features—you can also remove watermarks, add metadata, and more

Try it out on a few test files today. You’ll be surprised how much bloat you’re carrying around in those “small” Excel files!

FAQ Section

Q1: Can I remove specific types of attachments (e.g., only PDFs)?

Yes! Check the attachment’s file extension or MIME type. Add a condition like:

if (Path.GetExtension(attachment.Name).Equals(".pdf", StringComparison.OrdinalIgnoreCase))
{
    worksheet.Attachments.RemoveAt(i);
}

Q2: How do I handle password-protected Excel files?

Add the password to your loading options:

var loadOptions = new SpreadsheetLoadOptions { Password = "yourPassword" };

If you don’t know the password, you can’t process the file—that’s by design for security.

Q3: Will this work with older .xls files (not .xlsx)?

Absolutely. GroupDocs.Watermark supports both binary Excel formats (.xls) and modern Office Open XML formats (.xlsx, .xlsm). No code changes needed.

Q4: How do I preserve the original file and create a new cleaned version?

You’re already doing this! The code saves to a new file (outputFileName). If you want to overwrite the original instead, use:

watermarker.Save(documentPath);

But I’d recommend always keeping the original as a backup.

Q5: Can I remove images or charts, not just file attachments?

Different beast. Images and charts require accessing the worksheet.Shapes collection. GroupDocs supports this, but the API is different. Check the documentation for SpreadsheetShape operations.

Q6: How do I integrate this with a web application (ASP.NET)?

Upload the file to a temporary location, process it, then serve the cleaned file back to the user. Make sure to:

  • Clean up temporary files after processing
  • Implement proper error handling (don’t expose stack traces to users)
  • Consider async processing for large files to avoid request timeouts

Q7: What’s the performance impact on very large Excel files (100+ MB)?

Processing time scales roughly linearly with file size. A 100 MB file might take 30-60 seconds. For files this large, consider:

  • Processing asynchronously with progress reporting
  • Using a dedicated worker service rather than processing during a web request
  • Adding a timeout mechanism

Q8: Can I undo attachment removal if I make a mistake?

Not automatically. That’s why keeping the original file (by saving to a new filename) is crucial. Always test your removal logic on copies before applying it to production files.

Resources

Documentation:

Downloads and Licensing:

Support: