Fixes dotnet#1933 - CreateElement calls now parametrized UnknownOpenXmlElement constructor (dotnet#1936)

Fixes dotnet#1933

Author: mkaszewiak

src/DocumentFormat.OpenXml.Framework/OpenXmlPartReader.cs

@@ -765,7 +765,7 @@ private OpenXmlElement CreateElement(in OpenXmlQualifiedName qname)
             }
 
             // return unknown element instead of throw exception.
-            return new OpenXmlUnknownElement();
+            return new OpenXmlUnknownElement(_xmlReader.Prefix, _xmlReader.LocalName, _xmlReader.NamespaceURI);
         }
 
         private void LoadAttributes()

test/DocumentFormat.OpenXml.Packaging.Tests/OpenXmlPartReaderTests.cs

@@ -47,4 +47,53 @@ public void ExtractsInfoFromStream(string xml, string? encoding, bool? standalon
         // Ensure the part reader did not dispose the stream when it is disposed
         Assert.True(stream.CanRead);
     }
+
+    [Fact]
+    public void CreateElement()
+    {
+        // Arrange
+        var stream = new MemoryStream();
+        ExtendedPart extPart;
+        const string xmlContent = """
+            <?xml version='1.0' encoding='utf-8' standalone='yes'?>
+            <a:root xmlns:a="http://www.tests.com/">
+                <a:child>Test</a:child>
+                <a:child>Test2</a:child>
+            </a:root>
+            """;
+        const string nsUri = "http://www.tests.com/";
+        const string prefix = "a";
+        const string localName = "root";
+        {
+            using (var package = SpreadsheetDocument.Create(stream, SpreadsheetDocumentType.Workbook))
+            {
+                var workbook = package.AddWorkbookPart();
+                extPart = workbook.AddExtendedPart(nsUri, "application/xml", ".xml");
+
+                // Write some custom elements which are unknown to the OpenXmlPartReader
+                using (var writer = new StreamWriter(extPart.GetStream(FileMode.Create)))
+                {
+                    writer.Write(xmlContent);
+                }
+
+                OpenXmlReader reader = OpenXmlReader.Create(extPart);
+
+                // Assert
+                while (reader.Read())
+                {
+                    Assert.Equal(prefix, reader.Prefix);
+                    Assert.Equal(nsUri, reader.NamespaceUri);
+
+                    if (!reader.LocalName.Equals(localName))
+                    {
+                        Assert.Equal("child", reader.LocalName);
+                    }
+                    else
+                    {
+                        Assert.Equal(localName, reader.LocalName);
+                    }
+                }
+            }
+        }
+    }
 }