Log4JXmlNamespaceResolver hack

src/Log4JXmlNamespaceResolver.cs

@@ -0,0 +1,26 @@
+using System.Collections.Generic;
+using System.Xml;
+
+namespace Serilog.Formatting.Log4Net;
+
+internal sealed class Log4JXmlNamespaceResolver : IXmlNamespaceResolver
+{
+    private const string Prefix = "log4j";
+    private const string NamespaceName = "http://jakarta.apache.org/log4j/";
+
+    public static readonly Log4JXmlNamespaceResolver Instance = new();
+
+    /// <summary>
+    /// The XML namespace used for Log4j events.
+    /// </summary>
+    /// <remarks>https://github.com/apache/log4j/blob/v1_2_17/src/main/java/org/apache/log4j/xml/XMLLayout.java#L137</remarks>
+    public static readonly XmlQualifiedName Log4JXmlNamespace = new(Prefix, NamespaceName);
+
+    private static readonly Dictionary<string, string> Namespaces = new() { [Prefix] = NamespaceName };
+
+    IDictionary<string, string> IXmlNamespaceResolver.GetNamespacesInScope(XmlNamespaceScope scope) => Namespaces;
+
+    string? IXmlNamespaceResolver.LookupNamespace(string prefix) => prefix == Prefix ? NamespaceName : null;
+
+    string? IXmlNamespaceResolver.LookupPrefix(string namespaceName) => namespaceName == NamespaceName ? Prefix : null;
+}

src/Log4NetTextFormatter.cs

@@ -3,6 +3,7 @@
 using System.Diagnostics.CodeAnalysis;
 using System.IO;
 using System.Linq;
+using System.Reflection;
 using System.Text.RegularExpressions;
 using System.Xml;
 using Serilog.Core;
@@ -84,7 +85,7 @@ public Log4NetTextFormatter(Action<Log4NetTextFormatterOptionsBuilder>? configur
         var optionsBuilder = new Log4NetTextFormatterOptionsBuilder();
         configureOptions?.Invoke(optionsBuilder);
         _options = optionsBuilder.Build();
-        _usesLog4JCompatibility = ReferenceEquals(Log4NetTextFormatterOptionsBuilder.Log4JXmlNamespace, _options.XmlNamespace);
+        _usesLog4JCompatibility = ReferenceEquals(Log4JXmlNamespaceResolver.Log4JXmlNamespace, _options.XmlNamespace);
     }
 
     /// <summary>
@@ -104,29 +105,17 @@ public void Format(LogEvent logEvent, TextWriter output)
         {
             throw new ArgumentNullException(nameof(output));
         }
-        var xmlWriterOutput = _usesLog4JCompatibility ? new StringWriter() : output;
-        using var writer = XmlWriter.Create(xmlWriterOutput, _options.XmlWriterSettings);
-        WriteEvent(logEvent, writer);
-        writer.Flush();
+        using var writer = XmlWriter.Create(output, _options.XmlWriterSettings);
         if (_usesLog4JCompatibility)
         {
-            // log4j writes the XML "manually", see https://github.com/apache/log4j/blob/v1_2_17/src/main/java/org/apache/log4j/xml/XMLLayout.java#L137-L145
-            // The resulting XML is impossible to write with a standard compliant XML writer such as XmlWriter.
-            // That's why we write the event in a StringWriter then massage the output to remove the xmlns:log4j attribute to match log4j output.
-            // The XML fragment becomes valid when surrounded by an external entity, see https://github.com/apache/log4j/blob/v1_2_17/src/main/java/org/apache/log4j/xml/XMLLayout.java#L31-L49
-            const string log4JNamespaceAttribute = """
-                                                    xmlns:log4j="http://jakarta.apache.org/log4j/"
-                                                   """;
-            var xmlString = ((StringWriter)xmlWriterOutput).ToString();
-            var i = xmlString.IndexOf(log4JNamespaceAttribute, StringComparison.Ordinal);
-#if NETSTANDARD2_0
-            output.Write(xmlString.Substring(0, i));
-            output.Write(xmlString.Substring(i + log4JNamespaceAttribute.Length));
-#else
-            output.Write(xmlString.AsSpan(0, i));
-            output.Write(xmlString.AsSpan(i + log4JNamespaceAttribute.Length));
-#endif
+            var predefinedNamespaces = writer.GetType().GetField("_predefinedNamespaces", BindingFlags.Instance | BindingFlags.NonPublic);
+            if (predefinedNamespaces != null && typeof(IXmlNamespaceResolver).IsAssignableFrom(predefinedNamespaces.FieldType))
+            {
+                predefinedNamespaces.SetValue(writer, Log4JXmlNamespaceResolver.Instance);
+            }
         }
+        WriteEvent(logEvent, writer);
+        writer.Flush();
         output.Write(_options.XmlWriterSettings.NewLineChars);
     }
 

src/Log4NetTextFormatterOptionsBuilder.cs

@@ -15,12 +15,6 @@ public class Log4NetTextFormatterOptionsBuilder
     /// <remarks>https://github.com/apache/logging-log4net/blob/rel/2.0.8/src/Layout/XmlLayout.cs#L49</remarks>
     private static readonly XmlQualifiedName Log4NetXmlNamespace = new("log4net", "http://logging.apache.org/log4net/schemas/log4net-events-1.2/");
 
-    /// <summary>
-    /// The XML namespace used for Log4j events.
-    /// </summary>
-    /// <remarks>https://github.com/apache/log4j/blob/v1_2_17/src/main/java/org/apache/log4j/xml/XMLLayout.java#L137</remarks>
-    internal static readonly XmlQualifiedName Log4JXmlNamespace = new("log4j", "http://jakarta.apache.org/log4j/");
-
     /// <summary>
     /// Initialize a new instance of the <see cref="Log4NetTextFormatterOptionsBuilder"/> class.
     /// </summary>
@@ -170,7 +164,7 @@ public void UseLog4JCompatibility()
         _lineEnding = LineEnding.CarriageReturn | LineEnding.LineFeed;
 
         // https://github.com/apache/log4j/blob/v1_2_17/src/main/java/org/apache/log4j/xml/XMLLayout.java#L137
-        _xmlNamespace = Log4JXmlNamespace;
+        _xmlNamespace = Log4JXmlNamespaceResolver.Log4JXmlNamespace;
 
         // https://github.com/apache/log4j/blob/v1_2_17/src/main/java/org/apache/log4j/xml/XMLLayout.java#L147
         _cDataMode = CDataMode.Always;

tests/Log4JXmlNamespaceResolverTest.cs

@@ -0,0 +1,60 @@
+using System;
+using System.Collections.Generic;
+using System.Reflection;
+using System.Xml;
+using FluentAssertions;
+using Xunit;
+
+namespace Serilog.Formatting.Log4Net.Tests;
+
+public class Log4JXmlNamespaceResolverTest
+{
+    private readonly IXmlNamespaceResolver _resolver;
+
+    public Log4JXmlNamespaceResolverTest()
+    {
+        var log4JXmlNamespaceResolver = typeof(Log4NetTextFormatter).Assembly.GetType("Serilog.Formatting.Log4Net.Log4JXmlNamespaceResolver")
+            ?? throw new MissingMemberException("Serilog.Formatting.Log4Net.Log4JXmlNamespaceResolver");
+        var instance = log4JXmlNamespaceResolver.GetField("Instance", BindingFlags.Public | BindingFlags.Static)
+            ?? throw new MissingFieldException("Serilog.Formatting.Log4Net.Log4JXmlNamespaceResolver", "Instance");
+        _resolver = (IXmlNamespaceResolver)instance.GetValue(log4JXmlNamespaceResolver)!;
+    }
+
+    [Theory]
+    [InlineData(XmlNamespaceScope.All)]
+    [InlineData(XmlNamespaceScope.ExcludeXml)]
+    [InlineData(XmlNamespaceScope.Local)]
+    public void GetNamespacesInScope(XmlNamespaceScope scope)
+    {
+        var expected = new Dictionary<string, string> { ["log4j"] = "http://jakarta.apache.org/log4j/" };
+        _resolver.GetNamespacesInScope(scope).Should().BeEquivalentTo(expected);
+    }
+
+    [Fact]
+    public void LookupLog4JNamespace()
+    {
+        _resolver.LookupNamespace("log4j").Should().Be("http://jakarta.apache.org/log4j/");
+    }
+
+    [Fact]
+    public void LookupLog4JPrefix()
+    {
+        _resolver.LookupPrefix("http://jakarta.apache.org/log4j/").Should().Be("log4j");
+    }
+
+    [Theory]
+    [InlineData("xmlns")]
+    [InlineData("xml")]
+    public void LookupStandardXmlNamespace(string prefix)
+    {
+        _resolver.LookupNamespace(prefix).Should().BeNull();
+    }
+
+    [Theory]
+    [InlineData("http://www.w3.org/2000/xmlns/")]
+    [InlineData("http://www.w3.org/XML/1998/namespace")]
+    public void LookupStandardXmlPrefix(string namespaceName)
+    {
+        _resolver.LookupPrefix(namespaceName).Should().BeNull();
+    }
+}