Fix incorrect "illegal XML comment chars" check & add regression tests

src/libraries/System.Private.Xml/src/System/Xml/Core/XmlTextWriter.cs

@@ -760,7 +760,7 @@ public override void WriteComment(string? text)
         {
             try
             {
-                if (null != text && (text.Contains("--") || text.StartsWith('-')))
+                if (null != text && (text.Contains("--") || text.EndsWith('-')))
                 {
                     throw new ArgumentException(SR.Xml_InvalidCommentChars);
                 }

src/libraries/System.Private.Xml/tests/XmlWriter/System.Xml.RW.XmlWriter.Tests.csproj

@@ -7,5 +7,6 @@
     <Compile Include="WriteWithEncoding.cs" />
     <Compile Include="WriteWithEncodingWithFallback.cs" />
     <Compile Include="WriteWithInvalidSurrogate.cs" />
+    <Compile Include="XmlTextWriterTests.cs" />
   </ItemGroup>
 </Project>

src/libraries/System.Private.Xml/tests/XmlWriter/XmlTextWriterTests.cs

@@ -0,0 +1,92 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Collections.Generic;
+using System.IO;
+using Xunit;
+
+namespace System.Xml.Tests
+{
+    public class XmlTextWriterTests
+    {
+        public static IEnumerable<object[]> PositiveTestCases
+        {
+            get
+            {
+                yield return new string[] { null }; // will be normalized to empty string
+                yield return new string[] { "" };
+                yield return new string[] { "This is some data." };
+                yield return new string[] { " << brackets and whitespace >> " }; // brackets & surrounding whitespace are ok
+                yield return new string[] { "&amp;" }; // entities are ok (treated opaquely)
+                yield return new string[] { "Hello\r\nthere." }; // newlines are ok
+                yield return new string[] { "\U0001F643 Upside-down smiley \U0001F643" }; // correctly paired surrogates are ok
+                yield return new string[] { "\uFFFD\uFFFE\uFFFF" }; // replacement char & private use are ok
+            }
+        }
+
+        public static IEnumerable<object[]> BadSurrogateTestCases
+        {
+            get
+            {
+                yield return new string[] { "\uD800 Unpaired high surrogate." };
+                yield return new string[] { "\uDFFF Unpaired low surrogate." };
+                yield return new string[] { "Unpaired high surrogate at end. \uD800" };
+                yield return new string[] { "Unpaired low surrogate at end. \uDFFF" };
+                yield return new string[] { "Unpaired surrogates \uDFFF\uD800 in middle." };
+            }
+        }
+
+        [Theory]
+        [MemberData(nameof(PositiveTestCases))]
+        [InlineData("]]")] // ]] without trailing > is ok
+        [InlineData("-->")] // end of comment marker ok (meaningless to cdata tag)
+        public void WriteCData_SuccessCases(string cdataText)
+        {
+            StringWriter sw = new StringWriter();
+            XmlTextWriter xw = new XmlTextWriter(sw);
+
+            xw.WriteCData(cdataText);
+
+            Assert.Equal($"<![CDATA[{cdataText}]]>", sw.ToString());
+        }
+
+        [Theory]
+        [MemberData(nameof(BadSurrogateTestCases), DisableDiscoveryEnumeration = true)] // disable enumeration to avoid test harness misinterpreting unpaired surrogates
+        [InlineData("]]>")] // end of cdata marker forbidden (ambiguous close tag)
+        public void WriteCData_FailureCases(string cdataText)
+        {
+            StringWriter sw = new StringWriter();
+            XmlTextWriter xw = new XmlTextWriter(sw);
+
+            Assert.Throws<ArgumentException>(() => xw.WriteCData(cdataText));
+        }
+
+        [Theory]
+        [MemberData(nameof(PositiveTestCases))]
+        [InlineData("-12345")] // hyphen at beginning is ok
+        [InlineData("123- -45")] // single hyphens are ok in middle
+        [InlineData("]]>")] // end of cdata marker ok (meaningless to comment tag)
+        public void WriteComment_SuccessCases(string commentText)
+        {
+            StringWriter sw = new StringWriter();
+            XmlTextWriter xw = new XmlTextWriter(sw);
+
+            xw.WriteComment(commentText);
+
+            Assert.Equal($"<!--{commentText}-->", sw.ToString());
+        }
+
+        [Theory]
+        [MemberData(nameof(BadSurrogateTestCases), DisableDiscoveryEnumeration = true)] // disable enumeration to avoid test harness misinterpreting unpaired surrogates
+        [InlineData("123--45")] // double-hyphen in middle is forbidden (ambiguous comment close tag)
+        [InlineData("12345-")] // hyphen at end is forbidden (ambiguous comment close tag)
+        [InlineData("-->")] // end of comment marker forbidden (ambiguous close tag)
+        public void WriteComment_FailureCases(string commentText)
+        {
+            StringWriter sw = new StringWriter();
+            XmlTextWriter xw = new XmlTextWriter(sw);
+
+            Assert.Throws<ArgumentException>(() => xw.WriteComment(commentText));
+        }
+    }
+}