Fix XML serialization of xs:list elements to respect white-space separation

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

@@ -696,13 +696,24 @@ private bool WriteMemberText(Member anyText)
                 {
                     if (anyTextMapping.TypeDesc!.IsArrayLike)
                     {
+                        string textValue;
                         if (text.Mapping!.TypeDesc!.CollapseWhitespace)
                         {
-                            value = CollapseWhitespace(Reader.ReadString());
+                            textValue = CollapseWhitespace(Reader.ReadString());
                         }
                         else
                         {
-                            value = Reader.ReadString();
+                            textValue = Reader.ReadString();
+                        }
+                        // For string arrays with XmlText, split on whitespace to support XML Schema list types
+                        if (anyTextMapping.TypeDesc.Type == typeof(string[]))
+                        {
+                            string[] vals = textValue.Split((char[]?)null, StringSplitOptions.RemoveEmptyEntries);
+                            value = vals;
+                        }
+                        else
+                        {
+                            value = textValue;
                         }
                     }
                     else

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

@@ -360,7 +360,12 @@ private void WriteText(object o, TextAccessor text)
                 }
                 else
                 {
-                    if (!WritePrimitiveValue(primitiveMapping.TypeDesc!, o, out stringValue))
+                    // Handle string arrays as space-separated lists for XML Schema list types
+                    if (o is string[] stringArray)
+                    {
+                        stringValue = string.Join(" ", stringArray);
+                    }
+                    else if (!WritePrimitiveValue(primitiveMapping.TypeDesc!, o, out stringValue))
                     {
                         Debug.Assert(o is byte[]);
                     }

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

@@ -758,10 +758,11 @@ public static void XML_TypeWithXmlTextAttributeOnArray()
 
         var actual = SerializeAndDeserialize<TypeWithXmlTextAttributeOnArray>(original,
 @"<?xml version=""1.0""?>
-<TypeWithXmlTextAttributeOnArray xmlns:xsi=""http://www.w3.org/2001/XMLSchema-instance"" xmlns:xsd=""http://www.w3.org/2001/XMLSchema"" xmlns=""http://schemas.xmlsoap.org/ws/2005/04/discovery"">val1val2</TypeWithXmlTextAttributeOnArray>");
+<TypeWithXmlTextAttributeOnArray xmlns:xsi=""http://www.w3.org/2001/XMLSchema-instance"" xmlns:xsd=""http://www.w3.org/2001/XMLSchema"" xmlns=""http://schemas.xmlsoap.org/ws/2005/04/discovery"">val1 val2</TypeWithXmlTextAttributeOnArray>");
         Assert.NotNull(actual.Text);
-        Assert.StrictEqual(1, actual.Text.Length);
-        Assert.Equal("val1val2", actual.Text[0]);
+        Assert.StrictEqual(2, actual.Text.Length);
+        Assert.Equal("val1", actual.Text[0]);
+        Assert.Equal("val2", actual.Text[1]);
     }
 
     [Fact]