Parse and Process Markdown in C#

Parse Markdown to a Syntax Tree in C#

Aspose.HTML for .NET provides a dedicated Markdown parsing API in the Aspose.Html.Toolkit.Markdown.Syntax namespace. The MarkdownParser class converts a .md file into a strongly typed MarkdownSyntaxTree – a Markdown Abstract Syntax Tree (AST) that represents the full document structure.

Each Markdown element is mapped to a specific node type:

• AtxHeadingSyntaxNode – ATX headings (#, ##, etc.)
• TableSyntaxNode – GFM tables
• InlineLinkSyntaxNode – inline links
• FencedCodeBlockSyntaxNode – fenced code blocks
• and other syntax node classes

Unlike text-based Markdown converters, this API gives you direct programmatic access to the document structure before rendering. Instead of converting Markdown to HTML first, you can analyze, validate, filter, or modify the syntax tree at the structural level.

To build a syntax tree:

using Aspose.Html.Toolkit.Markdown.Syntax.Parser;

var parser = new MarkdownParser();
var syntaxTree = parser.ParseFile("document.md");

syntaxTree is the root node of the Markdown AST and provides access to all child nodes in the document. In the following sections, you will learn how to traverse the syntax tree, extract headings and tables, and filter nodes by type.

Parsing builds the full syntax tree in memory. For very large Markdown files, consider memory usage and processing time when designing batch-processing workflows.

Traverse the Markdown Syntax Tree in C#

Once the Markdown file is parsed into a MarkdownSyntaxTree, you can traverse its nodes to inspect or process document elements. Traversal begins at syntaxTree.FirstChild and continues via NextSibling, following the linked-node structure of the Markdown syntax tree.

Example: iterating through top-level nodes of the document:

var node = syntaxTree.FirstChild;

while (node != null)
{
    // Process node
    Console.WriteLine(node.GetType().Name);

    node = node.NextSibling;
}

This pattern ensures efficient and safe traversal of the Markdown AST without relying on list indexing.

For more advanced scenarios, such as visiting only specific node types (e.g., tables or headings), you can use CreateTreeWalker() together with a MarkdownSyntaxNodeFilter. This approach enables selective traversal of the syntax tree.

Extract Headings from a Markdown in C#

After parsing a Markdown file into a MarkdownSyntaxTree, you can programmatically extract specific node types. This example demonstrates how to locate all ATX-style headings (#, ##, ###), determine their nesting level (H1–H6), and reconstruct their hierarchy using the syntax tree.

using System.Text;
using Aspose.Html.Toolkit.Markdown.Syntax;
using Aspose.Html.Toolkit.Markdown.Syntax.Parser;
using System.IO;
...

    // Parse a Markdown file and extract ATX headings (# H1, ## H2, ### H3)
    // preserving their hierarchy and nesting level using Aspose.HTML for .NET.

    // Initialize the parser and build a syntax tree from the .md file
    var parser = new MarkdownParser();
    var syntaxTree = parser.ParseFile("document.md");

    var headings = new List<(int Level, string Text)>();

    // Walk top-level nodes using FirstChild -> NextSibling
    var topNode = syntaxTree.FirstChild;
    while (topNode != null)
    {
        if (topNode is AtxHeadingSyntaxNode heading)
        {
            // Determine heading level (1–6)
                        int level = heading.GetOpeningTag()
                                        .ToString()
                                        .Count(c => c == '#');

            // Extract heading text from child nodes
            var sb = new StringBuilder();
            var child = heading.FirstChild;
            while (child != null)
            {
                if (child is TextSyntaxNode || child is WhitespaceSyntaxNode)
                    sb.Append(child.ToString());
                child = child.NextSibling;
            }

            headings.Add((level, sb.ToString().Trim()));
        }

        topNode = topNode.NextSibling;
    }

    // Output heading hierarchy with indentation reflecting nesting depth
    foreach (var (level, text) in headings)
    {
        string indent = new string(' ', (level - 1) * 2);
        string marker = new string('#', level);
        Console.WriteLine($"{indent}{marker} {text}");
    }

How ATX Heading Extraction Works

1. MarkdownParser.ParseFile() builds a MarkdownSyntaxTree, which represents the complete Markdown Abstract Syntax Tree (AST).
2. Traversal begins at syntaxTree.FirstChild and continues through NextSibling.
3. AtxHeadingSyntaxNode represents headings defined with leading # characters. (Setext-style headings are represented by SetextHeadingSyntaxNode and are not processed in this example.)
4. The heading level (H1–H6) is determined from the opening tag using GetOpeningTag(). The number of # characters in the opening token defines the nesting depth.
5. The heading text is reconstructed from TextSyntaxNode and WhitespaceSyntaxNode children. Structural nodes such as SoftBreakSyntaxNode are intentionally ignored.

Extract GFM Tables from the Markdown in C#

After parsing Markdown into a MarkdownSyntaxTree, you can selectively traverse the document tree to locate specific node types. This example demonstrates how to extract GitHub Flavored Markdown (GFM) tables using a TreeWalker together with a custom MarkdownSyntaxNodeFilter. This example extracts serialized Markdown content of each cell.

Unlike manual top-level traversal, this approach walks the entire syntax tree and returns only nodes that match the specified type.

using System.Text;
using Aspose.Html.Toolkit.Markdown.Syntax;
using Aspose.Html.Toolkit.Markdown.Syntax.Parser;
using System.IO;
...

    // Parse Markdown and extract GFM tables using TreeWalker

    // Initialize the parser and build a syntax tree from the .md file
    var parser = new MarkdownParser();
    var syntaxTree = parser.ParseFile("document.md");

    // Create a TreeWalker that visits only TableSyntaxNode nodes
    using var tableWalker = syntaxTree.CreateTreeWalker(new TypeFilter<TableSyntaxNode>());

    MarkdownSyntaxNode current;
    while ((current = tableWalker.NextNode()) != null)
    {
        if (current is not TableSyntaxNode table) continue;

        // Iterate rows via FirstChild -> NextSibling
        var row = table.FirstChild;
        while (row != null)
        {
            if (row is TableRowSyntaxNode tableRow)
            {
                var sb = new StringBuilder();
                var cell = tableRow.FirstChild;
                while (cell != null)
                {
                    if (cell is TableCellSyntaxNode)
                        sb.Append($"| {cell.ToString().Trim()} ");
                    cell = cell.NextSibling;
                }
                if (sb.Length > 0)
                    Console.WriteLine(sb.Append('|').ToString());
            }
            row = row.NextSibling;
        }
    }

    // Custom filter that accepts only nodes of type T
    class TypeFilter<T> : MarkdownSyntaxNodeFilter where T : MarkdownSyntaxNode
    {
        public override short AcceptNode(MarkdownSyntaxNode node)
            => node is T ? (short)1 : (short)3;
    }

How Markdown Table Extraction with TreeWalker Works

1. MarkdownParser.ParseFile() builds a MarkdownSyntaxTree, representing the full Markdown AST.
2. CreateTreeWalker(MarkdownSyntaxNodeFilter) creates a filtered traversal mechanism that walks the entire tree but returns only nodes accepted by the filter.
3. The custom TypeFilter<TableSyntaxNode> inherits from MarkdownSyntaxNodeFilter and overrides AcceptNode().
   • 1 (FILTER_ACCEPT) includes the node
   • 3 (FILTER_SKIP) ignores it
4. NextNode() advances through the Markdown AST and yields only TableSyntaxNode instances.
5. Rows (TableRowSyntaxNode) and cells (TableCellSyntaxNode) are accessed using pointer-based traversal (FirstChild → NextSibling), since NodeList does not support index access.
6. cell.ToString().Trim() returns the serialized Markdown content of the cell, including inline formatting.

Modify Markdown Syntax Tree in C#

MarkdownSyntaxTree is mutable. You can traverse nodes, modify their content, replace children, and save the updated Markdown document.

The following code modifies Markdown emphasis nodes in a syntax tree using Aspose.HTML for .NET. It replaces all occurrences of a deprecated product name (Aspose.HTML) with a new one (Aspose.HTML for .NET) – directly via the syntax tree, without using regex or string replacement.

using Aspose.Html.Toolkit.Markdown.Syntax;
using Aspose.Html.Toolkit.Markdown.Syntax.Parser;
using System.IO;
...

    // Replace text inside Emphasis nodes using Markdown DOM in C#

    // Parse Markdown string into a full syntax tree
    string markdownContent =
        "# Release Notes\n\n" +
        "*Aspose.HTML* is a powerful HTML processing library.\n" +
        "Use *Aspose.HTML* to convert, parse, and manipulate HTML documents.\n" +
        "The latest version of *Aspose.HTML* includes Markdown support.";

    var parser = new MarkdownParser();
    var markdown = parser.Parse(markdownContent);

    // SyntaxFactory is used to create new Markdown nodes
    var factory = markdown.SyntaxFactory;

    int replacementCount = 0;

    // Recursive traversal of the Markdown syntax tree
    void TraverseAndReplace(MarkdownSyntaxNode node)
    {
        // Detect inline emphasis nodes (e.g., *text*)
        if (node is EmphasisSyntaxNode emphasisNode)
        {
            // Ensure emphasis contains exactly one text node
            if (emphasisNode.FirstChild is TextSyntaxNode textNode &&
                emphasisNode.FirstChild.NextSibling == null)
            {
                // Extract plain text from TextSyntaxNode
                string currentText = textNode.ToString().Trim();

                // Perform semantic comparison instead of raw string replacement
                if (currentText == "Aspose.HTML")
                {
                    // Remove old text node
                    emphasisNode.RemoveChild(textNode);

                    // Insert new text node using SyntaxFactory
                    emphasisNode.AppendChild(
                        factory.Text("Aspose.HTML for .NET")
                    );
                    replacementCount++;
                }
            }
        }

        // Pointer-based traversal (FirstChild / NextSibling pattern)
        var child = node.FirstChild;
        while (child != null)
        {
            var next = child.NextSibling;
            TraverseAndReplace(child);
            child = next;
        }
    }

    // Start AST traversal from the root Markdown document node
    TraverseAndReplace(markdown);

    // Output statistics for debugging or logging
    Output.WriteLine($"Replaced {replacementCount} occurrence(s).");

    // Save modified Markdown file
    markdown.Save(Path.Combine(OutputDir, "modified-release-notes.md"));

Quick Reference: Markdown Parsing in C#

How to parse a Markdown file into a syntax tree?

var syntaxTree = new MarkdownParser().ParseFile("document.md");

Builds a MarkdownSyntaxTree – the root of the sintax tree that represents the full document structure.

How to iterate top-level nodes in the Markdown AST?

var node = syntaxTree.FirstChild;
while (node != null)
{
    /* process node */
    node = node.NextSibling;
}

NodeList returned by ChildNodes() does not support index access. Use pointer-based traversal: FirstChild → NextSibling.

How to check whether a node is a heading?

if (node is AtxHeadingSyntaxNode heading)
{
    /* H1–H6 heading */
}

AtxHeadingSyntaxNode represents ATX-style headings (#, ##, … ######) in the Markdown syntax tree.

How to traverse only specific node types using a filter?

using var walker = syntaxTree.CreateTreeWalker(new TypeFilter<TableSyntaxNode>());

while (walker.NextNode() != null)
{
    /* only TableSyntaxNode nodes */
}

TypeFilter<T> is a custom MarkdownSyntaxNodeFilter that accepts only nodes of type T, enabling selective traversal of the document tree.

FAQ: Markdown Parsing with Aspose.HTML

Q: What is the difference between AtxHeadingSyntaxNode and SetextHeadingSyntaxNode?

AtxHeadingSyntaxNode represents headings defined with leading # characters (# H1, ## H2). SetextHeadingSyntaxNode represents headings underlined with = or - on the next line. Both are valid Markdown heading styles, but they map to different node classes in the syntax tree.

Q: Why does NodeList not support index access like children[0]?

NodeList in the Aspose.HTML Markdown API does not implement IList<T> and has no indexer. The correct traversal pattern is pointer-based: start from FirstChild and advance through siblings using NextSibling until the value is null.

Q: Does TreeWalker traverse the entire document or only direct children?

CreateTreeWalker(syntaxTree.FirstChild) starts traversal from the first child node and visits only its descendants. To traverse the entire document, pass a MarkdownSyntaxNodeFilter directly: CreateTreeWalker(filter) – this starts from the tree root and visits all nodes.

In this guide, you learned how to:

• Parse Markdown
• Traverse the AST
• Extract structured elements
• Modify content programmatically

Related Articles

• Markdown Syntax – Basic Tutorial – Overview of Markdown elements and syntax rules.
• Markdown Converter – C# – Convert Markdown to HTML and other formats using Aspose.HTML for .NET.
• Create Markdown Documents in C# – Create Markdown documents programmatically using Aspose.HTML for .NET: build headings, paragraphs, lists, and save Markdown files from scratch.
• Edit Markdown Files in C# – Edit Markdown files in C# with Aspose.HTML for .NET: change headings, update paragraphs, remove list items. Working code examples + AST manipulation guide.