RichEditor.NET

A modern, feature-rich replacement for the traditional RichTextBox control in Windows Forms applications. RichEditor.NET leverages the Windows Text Object Model 2 (TOM2) via the RichEdit 4.1 control (msftedit.dll) to provide advanced text and image formatting, built-in spell checking, a context-menu popup toolbar, and first-class HTML and Markdown serialization.

Features

Text Formatting

• Bold / Italic / Strikethrough
• Underline with multiple styles: Single, Double, Dotted, Dashed, and Wavy
• Font color and background (highlight) color
• Font name and font size (free-form or HTML heading sizes)
• Superscript and Subscript

Block & Paragraph Formatting

• Block styles: Paragraph, Heading 1-6, and Preformatted
• Paragraph alignment: Left, Center, Right, and Justify
• Indentation: Indent and Outdent with configurable point increments
• Lists with multiple styles:
  • Bullet (unordered)
  • Decimal, Lower Alpha, Upper Alpha, Lower Roman, Upper Roman (ordered)
• Nested list levels with increase/decrease support

Image Support

• Embedded images: Insert JPEG, PNG, or GIF images directly into the document
• Linked thumbnails: Insert a downsampled thumbnail that links to the original full-size image URL
• Alt text support for inserted images
• Configurable default image dimensions (DefaultImageWidth / DefaultImageHeight)

Serialization

• HTML: Read and write document content as HTML with full round-trip support for all formatting features. Optionally enable strict mode (EnableStrictHtml) to throw on unsupported tags.
• Markdown: Read and write CommonMark or GitHub Flavored Markdown (GFM) via the EnableCommonMarkdown and EnableGithubMarkdown properties.

Other Features

• Built-in spell checking via the Windows system spell checker
• Popup toolbar that appears on right-click with formatting controls
• Hyperlink insertion with custom display text
• Per-feature enable/disable properties for fine-grained control over available formatting options
• Events for hyperlink, image, and linked thumbnail insertion via the popup toolbar

Requirements

• Windows 8 or later (for spell checking and TOM2 support)
• .NET 8+ or .NET Framework 4.8

Installation

Install via the .NET CLI:

dotnet add package EllipticBit.RichEditorNET

Or via the Package Manager Console in Visual Studio:

Install-Package EllipticBit.RichEditorNET

Usage

Basic Setup

Drop a RichEditBox onto your form or create one in code:

using EllipticBit.RichEditorNET;

var editor = new RichEditBox
{
    Dock = DockStyle.Fill,
    EnableSpellCheck = true,
};
this.Controls.Add(editor);

Applying Text Formatting

// Toggle bold on the current selection
editor.ToggleBold();

// Toggle italic
editor.ToggleItalic();

// Apply a wavy underline
editor.ToggleUnderline(UnderlineStyle.Wavy);

// Set font color to red
editor.SetFontColor(Color.Red);

// Set background highlight to yellow
editor.SetBackgroundColor(Color.Yellow);

// Change font name and size
editor.SetFontName("Georgia");
editor.SetFontSize(14f);

Working with Block Styles and Lists

// Apply Heading 1 block style (requires EnableHtmlFontSizing = true)
editor.EnableHtmlFontSizing = true;
editor.SetBlockStyle(BlockStyle.Heading1);

// Toggle a bullet list
editor.ToggleList(ListStyle.Bullet);

// Increase nesting level
editor.IncreaseListLevel();

// Set paragraph alignment to center
editor.SetAlignment(ParagraphAlignment.Center);

Inserting Hyperlinks

editor.InsertHyperlink("https://github.com", "Visit GitHub");

Inserting Images

// Insert an embedded image from a file
using (var stream = File.OpenRead("photo.png"))
{
    editor.InsertImage(stream, "A sample photo");
}

// Insert a linked thumbnail that opens the full image URL
using (var stream = File.OpenRead("photo.png"))
{
    editor.InsertLinkedThumbnail(
        "https://example.com/photo-full.png",
        stream,
        "Thumbnail preview");
}

HTML Round-Trip

// Load HTML content
editor.Html = "<p>Hello <b>World</b></p>";

// Retrieve the document as HTML
string html = editor.Html;

Markdown Round-Trip

// Enable GitHub Flavored Markdown
editor.EnableGithubMarkdown = true;

// Load Markdown
editor.Markdown = "# Hello\n\nThis is **bold** and ~~strikethrough~~ text.";

// Retrieve the document as Markdown
string md = editor.Markdown;

Markdown Mode

When EnableCommonMarkdown or EnableGithubMarkdown is set to true, formatting options that are not representable in the target Markdown flavor are automatically disabled. This ensures the content stays portable.

API Reference

Properties

Property	Type	Default	Description
EnableSpellCheck	bool	true	Enables the system spell checker
EnableBold	bool	true	Enables bold formatting
EnableItalic	bool	true	Enables italic formatting
EnableUnderline	bool	true	Enables underline formatting
EnableStrikeThrough	bool	true	Enables strikethrough formatting
EnableFontColor	bool	true	Enables font color formatting
EnableBackgroundColor	bool	true	Enables background color formatting
EnableFontName	bool	true	Enables font name formatting
EnableFontSize	bool	true	Enables font size formatting
EnableSuperscript	bool	true	Enables superscript formatting
EnableSubscript	bool	true	Enables subscript formatting
EnableLists	bool	true	Enables list formatting
EnableAlignment	bool	true	Enables paragraph alignment
EnableIndent	bool	true	Enables paragraph indentation
EnableHtmlFontSizing	bool	false	Uses HTML heading sizes instead of free-form font size/name
EnableHyperlinks	bool	true	Enables hyperlink insertion
EnableImages	bool	true	Enables image insertion
EnableCommonMarkdown	bool	false	Enables CommonMark Markdown mode
EnableGithubMarkdown	bool	false	Enables GitHub Flavored Markdown mode
EnableStrictHtml	bool	false	Throws on unsupported HTML tags/attributes when loading HTML
DefaultImageWidth	int	320	Default display width in pixels for inserted images
DefaultImageHeight	int	240	Default display height in pixels for inserted images
Html	string	-	Gets or sets document content as HTML
Markdown	string	-	Gets or sets document content as Markdown

Events

Event	Description
InsertHyperlinkClicked	Raised when the Insert Hyperlink button is clicked on the popup toolbar
InsertImageClicked	Raised when the Insert Image button is clicked on the popup toolbar
InsertLinkedThumbnailClicked	Raised when the Insert Linked Thumbnail button is clicked on the popup toolbar

Contributing

Contributions are welcome! To get started:

1. Fork this repository and create a feature branch from master.
2. Make your changes and ensure the project builds successfully for both net8.0-windows and net48 targets.
3. Add or update tests as appropriate.
4. Submit a Pull Request with a clear description of what you changed and why.

Coding Standards

• Follow the existing code style and conventions in the repository.
• Use the latest C# language features where appropriate.
• Ensure all COM objects are properly released via Marshal.ReleaseComObject.

LLM-Generated Contributions

If any part of your contribution was generated with the assistance of a Large Language Model (LLM) such as ChatGPT, GitHub Copilot, Claude, or similar tools, you must include the full prompt(s) used to generate the contribution in the Pull Request description. This is required for transparency and review purposes.

Reporting Issues

Please use GitHub Issues to report bugs or request features. Include steps to reproduce the problem and, if possible, a minimal code sample.

License

This project is licensed under the Boost Software License 1.0 (BSL-1.0).

Copyright (c) EllipticBit LLC 2026