Strip invalid char data from strings on save

src/SIL.LCModel.Core/Text/TsStringSerializer.cs

@@ -11,6 +11,7 @@
 using System;
 using System.Linq;
 using System.Text;
+using System.Text.RegularExpressions;
 using System.Xml;
 using System.Xml.Linq;
 using System.Xml.Schema;
@@ -173,7 +174,7 @@ public static string SerializeTsStringToXml(ITsString tss, ILgWritingSystemFacto
 						if (runText != string.Empty && runText.All(char.IsWhiteSpace))
 							writer.WriteAttributeString("xml", "space", "", "preserve");
 						// TODO: should we escape quotation marks? this is not necessary but different than the behavior of the C++ implementation
-						writer.WriteString(Normalizer.Normalize(runText, Normalizer.UNormalizationMode.UNORM_NFC));
+						writer.WriteString(StripInvalidXmlChars(Normalizer.Normalize(runText, Normalizer.UNormalizationMode.UNORM_NFC)));
 					}
 
 					writer.WriteEndElement();
@@ -187,6 +188,14 @@ public static string SerializeTsStringToXml(ITsString tss, ILgWritingSystemFacto
 			return xml.ToString();
 		}
 
+		private static readonly Regex InvalidXmlRegex = new Regex(@"[\x00-\x08\x0B\x0C\x0E-\x1F\uFFFE\uFFFF]", RegexOptions.Compiled);
+		public static string StripInvalidXmlChars(string text)
+		{
+			// Remove characters not allowed in XML
+			// Documented here: https://en.wikipedia.org/wiki/Valid_characters_in_XML
+			return InvalidXmlRegex.Replace(text, string.Empty);
+		}
+
 		#endregion
 
 		#region Serialization Helper Methods

src/SIL.LCModel/DomainImpl/Strings.cs

@@ -900,7 +900,7 @@ protected override void ToXml(XmlWriter writer, ILgWritingSystemFactory wsf, int
 
 			writer.WriteStartElement("AUni");
 			writer.WriteAttributeString("ws", m_object.Services.WritingSystemManager.Get(ws).Id);
-			text = Normalizer.Normalize(text, Normalizer.UNormalizationMode.UNORM_NFC);
+			text = TsStringSerializer.StripInvalidXmlChars(Normalizer.Normalize(text, Normalizer.UNormalizationMode.UNORM_NFC));
 			writer.WriteString(text);
 			writer.WriteEndElement();
 		}

tests/SIL.LCModel.Core.Tests/Text/TsStringSerializerTests.cs

@@ -42,6 +42,29 @@ public void SerializeTsStringToXml_Simple()
 			Assert.That(StripNewLines(xml), Is.EqualTo("<Str><Run ws=\"en\">This is a test!</Run></Str>"));
 		}
 
+		[Test]
+		public void SerializeTsStringToXml_StripsInvalidControlCharacter()
+		{
+			ITsString tss = TsStringUtils.MakeString("This is a te\u0002st!", EnWS);
+			string xml = TsStringSerializer.SerializeTsStringToXml(tss, WritingSystemManager);
+			Assert.That(StripNewLines(xml), Is.EqualTo("<Str><Run ws=\"en\">This is a test!</Run></Str>"));
+		}
+
+		[Test]
+		[TestCase("This is a test!")]
+		[TestCase(" 𐰉 (dǒng)")]//Nushu script
+		[TestCase("𠔤野 (Nishino)")]//Japanese Kanji
+		[TestCase("𠮷野家 (Yóu yě jiā)")]//Historic Chinese
+		[TestCase("🦊")]//emoji
+		[TestCase("\u200B\u200D\u200E\uDA00\uDC01")]
+		public void SerializeTsStringToXml_DoesNotStripValidCharacters(string word)
+		{
+			ITsString tss = TsStringUtils.MakeString(word, EnWS);
+			string xml = TsStringSerializer.SerializeTsStringToXml(tss, WritingSystemManager);
+			Assert.That(StripNewLines(xml),
+				Is.EqualTo($"<Str><Run ws=\"en\">{word}</Run></Str>"));
+		}
+
 		///--------------------------------------------------------------------------------------
 		/// <summary>
 		/// Tests the method SerializeTsStringToXml with a MultiString. This should

tests/SIL.LCModel.Tests/DomainImpl/StringsTests.cs

@@ -3,7 +3,9 @@
 // (http://www.gnu.org/licenses/lgpl-2.1.html)
 
 using System;
+using System.IO;
 using System.Linq;
+using System.Text;
 using NUnit.Framework;
 using SIL.LCModel.Core.KernelInterfaces;
 using SIL.LCModel.Core.Text;
@@ -283,6 +285,34 @@ public void AppendAlternativesTest()
 			Assert.AreEqual("Saltillo Mexico", Cache.LangProject.FieldWorkLocation.get_String(english.Handle).Text);
 			Assert.AreEqual("Saltillo Mejico", Cache.LangProject.FieldWorkLocation.get_String(spanish.Handle).Text);
 		}
+
+		[Test]
+		public void ToXml_WorksAsExpected()
+		{
+			var english = Cache.LangProject.CurrentAnalysisWritingSystems.First();
+			Cache.LangProject.MainCountry.set_String(english.Handle, TsStringUtils.MakeString("Mexico", english.Handle));
+			var xml = ToXml(Cache.LangProject.MainCountry);
+			Assert.AreEqual("<AUni ws=\"en\">Mexico</AUni>", xml, "XML does not contain expected string element.");
+		}
+
+		[Test]
+		public void ToXml_WithControlCharacterWorks()
+		{
+			var english = Cache.LangProject.CurrentAnalysisWritingSystems.First();
+			var tsString = TsStringUtils.MakeString("te\u0002st", english.Handle);
+			Cache.LangProject.MainCountry.set_String(english.Handle, tsString);
+			var xml = ToXml(Cache.LangProject.MainCountry);
+			Assert.AreEqual("<AUni ws=\"en\">test</AUni>", xml, "XML does not contain expected string element.");
+		}
+
+		private string ToXml(ITsMultiString multiString)
+		{
+			using var ms = new MemoryStream();
+			using var xmlWriter = XmlServices.CreateWriter(ms);
+			((MultiAccessor)multiString).ToXMLString(xmlWriter);
+			xmlWriter.Flush();
+			return Encoding.UTF8.GetString(ms.ToArray());
+		}
 	}
 
 	/// ----------------------------------------------------------------------------------------