Escape a lone surrogate in a quoted string literal [patch]

CodeBlocker.Test/TemplateTests.cs

@@ -2,6 +2,7 @@
 
 namespace CodeBlocker.Tests;
 
+using System.Text;
 using ktsu.CodeBlocker;
 using ktsu.CodeBlocker.Templates;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
@@ -48,6 +49,31 @@ public void ParameterEscapesAQuotedDefaultValue(string value, string expectedLit
 			$"string name = {expectedLiteral}",
 			Render(new ParameterTemplate { Type = "string", Name = "name", DefaultValue = value, DefaultValueIsQuoted = true }));
 
+	[TestMethod]
+	public void ParameterEscapesALoneSurrogateInAQuotedDefaultValue()
+	{
+		// These are built in code rather than passed through DataRow: an attribute argument is
+		// stored as UTF-8, which cannot carry a lone surrogate either.
+		(string Value, string ExpectedLiteral)[] cases =
+		[
+			("a\uD800b", @"""a\ud800b"""),
+			("a\uDC00b", @"""a\udc00b"""),
+			("end\uD83D", @"""end\ud83d"""),
+			("\uDE00start", @"""\ude00start"""),
+			("\uDE00\uD83D", @"""\ude00\ud83d"""),
+			("pair \uD83D\uDE00", "\"pair \uD83D\uDE00\""),
+		];
+
+		UTF8Encoding strictUtf8 = new(encoderShouldEmitUTF8Identifier: false, throwOnInvalidBytes: true);
+		foreach ((string value, string expectedLiteral) in cases)
+		{
+			string rendered = Render(new ParameterTemplate { Type = "string", Name = "name", DefaultValue = value, DefaultValueIsQuoted = true });
+
+			Assert.AreEqual($"string name = {expectedLiteral}", rendered);
+			_ = strictUtf8.GetBytes(rendered);
+		}
+	}
+
 	[TestMethod]
 	public void ParameterKeepsItsAttributesOnTheDeclarationLine()
 	{

CodeBlocker/Templates/TemplateRendering.cs

@@ -125,15 +125,18 @@ internal static void SpliceFragment(CodeBlocker parent, string fragment)
 	/// <returns>The quoted literal.</returns>
 	/// <remarks>
 	/// Backslashes, quotes and the common control characters take their short escapes; any other
-	/// control character, and the line and paragraph separators the compiler also treats as line
-	/// breaks, take a <c>\uXXXX</c> escape.
+	/// control character, the line and paragraph separators the compiler also treats as line
+	/// breaks, and any surrogate that is not half of a valid pair take a <c>\uXXXX</c> escape. A
+	/// lone surrogate has no UTF-8 encoding, so written raw it would make the generated file
+	/// impossible to save faithfully: a strict encoder throws and a replacing one changes the value.
 	/// </remarks>
 	internal static string QuoteStringLiteral(string value)
 	{
 		StringBuilder literal = new(value.Length + 2);
 		literal.Append('"');
-		foreach (char c in value)
+		for (int i = 0; i < value.Length; i++)
 		{
+			char c = value[i];
 			string? escape = c switch
 			{
 				'\\' => @"\\",
@@ -146,7 +149,7 @@ internal static string QuoteStringLiteral(string value)
 				'\r' => @"\r",
 				'\t' => @"\t",
 				'\v' => @"\v",
-				_ when char.IsControl(c) || c is '\u2028' or '\u2029' =>
+				_ when char.IsControl(c) || c is '\u2028' or '\u2029' || IsLoneSurrogate(value, i) =>
 					@"\u" + ((int)c).ToString("x4", CultureInfo.InvariantCulture),
 				_ => null,
 			};
@@ -165,6 +168,29 @@ _ when char.IsControl(c) || c is '\u2028' or '\u2029' =>
 		return literal.ToString();
 	}
 
+	/// <summary>
+	/// Reports whether the character at <paramref name="index"/> is a surrogate that is not half of
+	/// a high-low pair.
+	/// </summary>
+	/// <param name="value">The text.</param>
+	/// <param name="index">The position of the character to test.</param>
+	/// <returns><see langword="true"/> for an unpaired surrogate; otherwise <see langword="false"/>.</returns>
+	private static bool IsLoneSurrogate(string value, int index)
+	{
+		char c = value[index];
+		if (char.IsHighSurrogate(c))
+		{
+			return index + 1 >= value.Length || !char.IsLowSurrogate(value[index + 1]);
+		}
+
+		if (char.IsLowSurrogate(c))
+		{
+			return index == 0 || !char.IsHighSurrogate(value[index - 1]);
+		}
+
+		return false;
+	}
+
 	/// <summary>
 	/// Writes a parenthesised, comma-separated parameter list.
 	/// </summary>