---
title: "Detect and add PDF form fields in C# | Nutrient .NET SDK"
canonical_url: "https://www.nutrient.io/guides/dotnet/csharp/editor/detect-and-add-form-fields/"
md_url: "https://www.nutrient.io/guides/dotnet/csharp/editor/detect-and-add-form-fields.md"
last_updated: "2026-10-08T00:00:00.000Z"
description: "How to detect form fields in a PDF and add interactive form fields using Nutrient .NET SDK."
---

# Detecting and adding form fields to a PDF document

Scanned forms — tax filings, healthcare intake sheets, lease agreements, expense reports — usually arrive as image-based PDFs. The page shows boxes, checkboxes, and signature lines, but the PDF has no AcroForm structure behind them, so end users can't fill the form in a viewer and downstream tools can't read the values. Adding those fields by hand means measuring rectangles for every blank on every page.

This sample shows how to detect form-field regions on each page of a document and add matching AcroForm fields automatically using Nutrient.NET SDK. The input can be any document format the SDK supports. If the input isn't already a PDF, the SDK converts it to PDF when you create the editor.

[Download sample](https://www.nutrient.io/downloads/samples/csharp/detect-and-add-form-fields.zip)

## How Nutrient helps

Nutrient.NET SDK handles the full detection-and-creation pipeline behind a single method call. The SDK takes care of:

- Implicitly converting non-PDF inputs (images, multi-page TIFFs, Office documents) to PDF when the editor is created

- Rendering each PDF page to a bitmap at the resolution the detection model expects

- Running the form-field detection model and classifying each region as text, checkbox, or signature

- Mapping image coordinates back to PDF page coordinates (including the Y-axis flip)

- Adding a matching AcroForm widget for each detected region with a unique field name

The result is a standard PDF with interactive form fields that any viewer can fill or any downstream tool can read.

## Supported field types and limits

The current detection model classifies regions into three types: text, checkbox, and signature. The SDK maps them to `PdfTextField`, `PdfCheckBoxField`, and `PdfSignatureField` respectively. Radio buttons aren't yet emitted by the model and are skipped. Detection runs on the rendered page image, so accuracy depends on the visual quality of the original document — clean scans produce better results than heavily compressed or skewed pages.

## Preparing the project

Import the Nutrient namespace:

```csharp

using Nutrient;

```

## Detecting and adding form fields

The sample opens the source document with a [using statement](https://learn.microsoft.com/dotnet/csharp/language-reference/statements/using), creates a PDF editor, and calls `DetectAndAddFormFields()` to process every page in a single call. The `using` statements release the native handles automatically when the block ends, even if an error is thrown:

```csharp

try
{
    using Document document = Document.Open("input_forms_detection.pdf");
    using PdfEditor editor = PdfEditor.Edit(document);
    editor.DetectAndAddFormFields();

```

`PdfEditor.Edit(document)` attaches an editor to the open document. If the document isn't already a PDF, the SDK converts it to PDF at this step so the rest of the pipeline works on a uniform page representation. Calling `editor.DetectAndAddFormFields()` walks every page, runs detection, and adds an AcroForm widget for each detected region. Existing form fields on the document aren't removed — detection only adds new fields and synthesizes unique names so it doesn't collide with anything already present.

## Saving the result

Save the modified document to a new file. The catch block surfaces any licensing, model-loading, or I/O issue that the SDK reports through a `NutrientException`:

```csharp

    editor.SaveAs("output.pdf");
    Console.WriteLine("Successfully detected and added form fields");
}
catch (NutrientException e)
{
    Console.Error.WriteLine($"Error: {e.Message}");
    Environment.Exit(1);
}

```

## Conclusion

The workflow for auto-populating an image-based form is:

1. Open the source document.

2. Create a `PdfEditor` for the document.

3. Call `DetectAndAddFormFields()` to detect and add fields across every page.

4. Save the result — the `using` statements release the handles.

The output is a standard PDF with interactive form fields, so existing PDF viewers can fill the form and downstream tools can read the values without any extra configuration.
---

## Related pages

- [Nutrient .NET SDK editor guides](/guides/dotnet/csharp/editor.md)
- [Adding annotations to a PDF document](/guides/dotnet/csharp/editor/add-annotations-to-pdf.md)
- [Adding a custom page to a PDF document](/guides/dotnet/csharp/editor/add-custom-page-to-pdf.md)
- [Adding interactive form fields to a PDF document](/guides/dotnet/csharp/editor/add-form-fields-to-pdf.md)
- [Adding free text annotations to a PDF document](/guides/dotnet/csharp/editor/add-freetext-annotations-to-pdf.md)
- [Adding invisible digital signatures to a PDF document](/guides/dotnet/csharp/editor/add-invisible-signature-to-pdf.md)
- [Adding link annotations to a PDF document](/guides/dotnet/csharp/editor/add-link-annotations-to-pdf.md)
- [Adding redaction annotations to a PDF document](/guides/dotnet/csharp/editor/add-redaction-annotations-to-pdf.md)
- [Adding shape annotations to a PDF document](/guides/dotnet/csharp/editor/add-shape-annotations-to-pdf.md)
- [Adding stamp annotations to a PDF document](/guides/dotnet/csharp/editor/add-stamp-annotations-to-pdf.md)
- [Adding sticky note annotations to a PDF document](/guides/dotnet/csharp/editor/add-sticky-note-annotations-to-pdf.md)
- [Adding text markup annotations to a PDF document](/guides/dotnet/csharp/editor/add-text-markup-annotations-to-pdf.md)
- [Adding visible digital signatures to a PDF document](/guides/dotnet/csharp/editor/add-visible-signature-to-pdf.md)
- [Advanced digital signature workflows](/guides/dotnet/csharp/editor/advanced-digital-signatures.md)
- [Editing PDF form fields](/guides/dotnet/csharp/editor/editing-pdf-form-fields.md)
- [Editing PDF metadata with Nutrient .NET SDK](/guides/dotnet/csharp/editor/editing-pdf-metadata.md)
- [Filling PDF form fields](/guides/dotnet/csharp/editor/fill-pdf-form.md)
- [Managing PDF page order](/guides/dotnet/csharp/editor/manage-pdf-page-order.md)
- [Merging PDFs](/guides/dotnet/csharp/editor/merge-pdf-into-other-pdf.md)