Use XmlWriter.Create instead of XmlTextWriter

src/libraries/System.Private.Xml/src/System/Xml/Serialization/XmlSerializer.cs

@@ -317,9 +317,10 @@ public void Serialize(TextWriter textWriter, object? o)
         [RequiresUnreferencedCode(TrimSerializationWarning)]
         public void Serialize(TextWriter textWriter, object? o, XmlSerializerNamespaces? namespaces)
         {
-            XmlTextWriter xmlWriter = new XmlTextWriter(textWriter);
-            xmlWriter.Formatting = Formatting.Indented;
-            xmlWriter.Indentation = 2;
+            //XmlTextWriter xmlWriter = new XmlTextWriter(textWriter);
+            //xmlWriter.Formatting = Formatting.Indented;
+            //xmlWriter.Indentation = 2;
+            XmlWriter xmlWriter = XmlWriter.Create(textWriter, new XmlWriterSettings() { CheckCharacters = true, IndentChars = "  ", Indent = true });
             Serialize(xmlWriter, o, namespaces);
         }
 
@@ -332,9 +333,10 @@ public void Serialize(Stream stream, object? o)
         [RequiresUnreferencedCode(TrimSerializationWarning)]
         public void Serialize(Stream stream, object? o, XmlSerializerNamespaces? namespaces)
         {
-            XmlTextWriter xmlWriter = new XmlTextWriter(stream, null);
-            xmlWriter.Formatting = Formatting.Indented;
-            xmlWriter.Indentation = 2;
+            //XmlTextWriter xmlWriter = new XmlTextWriter(stream, null);
+            //xmlWriter.Formatting = Formatting.Indented;
+            //xmlWriter.Indentation = 2;
+            XmlWriter xmlWriter = XmlWriter.Create(stream, new XmlWriterSettings() { CheckCharacters = true, IndentChars = "  ", Indent = true });
             Serialize(xmlWriter, o, namespaces);
         }
 
@@ -421,10 +423,11 @@ private XmlMapping GetMapping()
         [RequiresUnreferencedCode(TrimDeserializationWarning)]
         public object? Deserialize(Stream stream)
         {
-            XmlTextReader xmlReader = new XmlTextReader(stream);
-            xmlReader.WhitespaceHandling = WhitespaceHandling.Significant;
-            xmlReader.Normalization = true;
-            xmlReader.XmlResolver = null;
+            //XmlTextReader xmlReader = new XmlTextReader(stream);
+            //xmlReader.WhitespaceHandling = WhitespaceHandling.Significant;
+            //xmlReader.Normalization = true;
+            //xmlReader.XmlResolver = null;
+            XmlReader xmlReader = XmlReader.Create(stream, new XmlReaderSettings() { IgnoreWhitespace = true });
             return Deserialize(xmlReader, null);
         }
 

src/libraries/System.Private.Xml/tests/XmlSerializer/XmlSerializerTests.RuntimeOnly.cs

@@ -262,6 +262,13 @@ public static void Xml_StringAsRoot()
 "<?xml version=\"1.0\"?><string>Hello World! \u6F22 \u00F1</string>"));
     }
 
+    [Fact]
+    public static void Xml_StringWithNullChar()
+    {
+        Assert.Throws<InvalidOperationException>(() => SerializeWithDefaultValue<string>("Sample\0String", null));
+        Assert.Throws<InvalidOperationException>(() => DeserializeFromXmlString<string>("<?xml version=\"1.0\"?><string>Sample&#x0;String</string>"));
+    }
+
     [Fact]
     public static void Xml_UintAsRoot()
     {

src/libraries/System.Private.Xml/tests/XmlSerializer/XmlSerializerTests.cs

@@ -1726,6 +1726,17 @@ private static bool SerializeWithDefaultValue<T>(T value, string baseline)
         }
     }
 
+    private static T DeserializeFromXmlString<T>(string xmlString)
+    {
+        XmlSerializer serializer = new XmlSerializer(typeof(T));
+        using (Stream ms = GenerateStreamFromString(xmlString))
+        {
+            T value = (T)serializer.Deserialize(ms);
+            return value;
+        }
+
+    }
+
     [Fact]
     public static void Xml_TypeWithMismatchBetweenAttributeAndPropertyType()
     {