Treat a frontmatter block with no YAML content as unreadable instead of throwing

Frontmatter.Test/EmptyYamlBlockTests.cs

@@ -0,0 +1,71 @@
+// Copyright (c) 2023-2026 ktsu-dev contributors
+
+namespace ktsu.Frontmatter.Test;
+
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+/// <summary>
+/// Tests that a frontmatter block whose YAML has no content (a comment only, <c>~</c> or <c>null</c>)
+/// is treated as an empty header rather than throwing.
+/// </summary>
+[TestClass]
+public class EmptyYamlBlockTests
+{
+	private static readonly string Nl = Environment.NewLine;
+
+	private static string Document(string yaml) => $"---{Nl}{yaml}{Nl}---{Nl}Body text{Nl}";
+
+	[TestMethod]
+	[DataRow("# nothing here yet")]
+	[DataRow("~")]
+	[DataRow("null")]
+	public void ExtractBody_EmptyYamlBlock_ReturnsBody(string yaml) =>
+		Assert.AreEqual("Body text", Frontmatter.ExtractBody(Document(yaml)));
+
+	[TestMethod]
+	[DataRow("# draft")]
+	[DataRow("~")]
+	[DataRow("null")]
+	public void ExtractFrontmatter_EmptyYamlBlock_ReturnsNull(string yaml) =>
+		Assert.IsNull(Frontmatter.ExtractFrontmatter(Document(yaml)));
+
+	[TestMethod]
+	[DataRow("# draft")]
+	[DataRow("~")]
+	[DataRow("null")]
+	public void RemoveFrontmatter_EmptyYamlBlock_ReturnsBody(string yaml) =>
+		Assert.AreEqual($"Body text{Nl}", Frontmatter.RemoveFrontmatter(Document(yaml)));
+
+	[TestMethod]
+	[DataRow("# draft")]
+	[DataRow("~")]
+	[DataRow("null")]
+	public void CombineFrontmatter_EmptyYamlBlock_DoesNotThrow(string yaml)
+	{
+		string result = Frontmatter.CombineFrontmatter(Document(yaml));
+
+		Assert.Contains("Body text", result);
+	}
+
+	[TestMethod]
+	[DataRow("# draft")]
+	[DataRow("~")]
+	[DataRow("null")]
+	public void AddFrontmatter_EmptyYamlBlock_LeavesTheDocumentUnchanged(string yaml)
+	{
+		// A block with text in it that yields no properties is left alone, as for any block that cannot be read
+		string input = Document(yaml);
+
+		Assert.AreEqual(input, Frontmatter.AddFrontmatter(input, new Dictionary<string, object> { ["title"] = "A" }));
+	}
+
+	[TestMethod]
+	[DataRow("# draft")]
+	[DataRow("~")]
+	[DataRow("null")]
+	public void TryParseYamlObject_EmptyYamlDocument_ReturnsFalse(string yaml)
+	{
+		Assert.IsFalse(YamlSerializer.TryParseYamlObject(yaml, out Dictionary<string, object>? result));
+		Assert.IsNull(result);
+	}
+}

Frontmatter/YamlSerializer.cs

@@ -43,7 +43,7 @@
 	/// <param name="input">The YAML string to parse.</param>
 	/// <param name="result">When this method returns, contains the deserialized dictionary if parsing succeeded, or null if parsing failed.</param>
 	/// <returns>true if the YAML was successfully parsed; otherwise, false.</returns>
 	public static bool TryParseYamlObject(string input, [NotNullWhen(true)] out Dictionary<string, object>? result)
 	{
 		result = null;
 
@@ -68,7 +68,13 @@
 		try
 		{
 			// Simple approach with direct deserializer
-			Dictionary<object, object> rawData = Deserializer.Deserialize<Dictionary<object, object>>(input);
+			// A document with no content (a comment only, ~ or null) deserializes to null
+			Dictionary<object, object>? rawData = Deserializer.Deserialize<Dictionary<object, object>?>(input);
+			if (rawData is null)
+			{
+				return false;
+			}
+
 			result = [];
 
 			// Convert dictionary keys to strings and preserve the first occurrence of duplicate keys