Always write the value XML attributes in properties, even when the value is null

0xced · 0xced · commit 42b6770789b1 · 2025-08-26T12:43:16.000+02:00
Also make the null text configurable through a new `UseNullText()` method on the options builder. The default null text value is `(null)` in order to match the log4net behavior. Fixes #287
diff --git a/src/Log4NetTextFormatter.cs b/src/Log4NetTextFormatter.cs
@@ -406,11 +406,14 @@ private void WritePropertyElement(XmlWriter writer, string name, LogEventPropert
     {
         WriteStartElement(writer, "data");
         writer.WriteAttributeString("name", name);
-        var isNullValue = value is ScalarValue { Value: null };
-        if (!isNullValue)
+        if (value is not ScalarValue { Value: null })
         {
             writer.WriteAttributeString("value", RenderValue(value));
         }
+        else if (_options.NullText != null)
+        {
+            writer.WriteAttributeString("value", _options.NullText);
+        }
         writer.WriteEndElement();
     }
 
diff --git a/src/Log4NetTextFormatterOptions.cs b/src/Log4NetTextFormatterOptions.cs
@@ -8,10 +8,11 @@ namespace Serilog.Formatting.Log4Net;
 /// </summary>
 internal sealed class Log4NetTextFormatterOptions
 {
-    internal Log4NetTextFormatterOptions(IFormatProvider? formatProvider, CDataMode cDataMode, XmlQualifiedName? xmlNamespace, XmlWriterSettings xmlWriterSettings, PropertyFilter filterProperty, MessageFormatter formatMessage, ExceptionFormatter formatException)
+    internal Log4NetTextFormatterOptions(IFormatProvider? formatProvider, CDataMode cDataMode, string? nullText, XmlQualifiedName? xmlNamespace, XmlWriterSettings xmlWriterSettings, PropertyFilter filterProperty, MessageFormatter formatMessage, ExceptionFormatter formatException)
     {
         FormatProvider = formatProvider;
         CDataMode = cDataMode;
+        NullText = nullText;
         XmlNamespace = xmlNamespace;
         XmlWriterSettings = xmlWriterSettings;
         FilterProperty = filterProperty;
@@ -25,6 +26,9 @@ internal Log4NetTextFormatterOptions(IFormatProvider? formatProvider, CDataMode
     /// <summary>See <see cref="Log4NetTextFormatterOptionsBuilder.UseCDataMode"/></summary>
     internal CDataMode CDataMode { get; }
 
+    /// <summary>See <see cref="Log4NetTextFormatterOptionsBuilder.UseNullText"/></summary>
+    internal string? NullText { get; }
+
     /// <summary>See <see cref="Log4NetTextFormatterOptionsBuilder.UseNoXmlNamespace"/></summary>
     internal XmlQualifiedName? XmlNamespace { get; }
 
diff --git a/src/Log4NetTextFormatterOptionsBuilder.cs b/src/Log4NetTextFormatterOptionsBuilder.cs
@@ -31,9 +31,12 @@ internal Log4NetTextFormatterOptionsBuilder()
     /// <summary>See <see cref="UseFormatProvider"/></summary>
     private IFormatProvider? _formatProvider;
 
-    /// <summary> See <see cref="UseCDataMode"/></summary>
+    /// <summary>See <see cref="UseCDataMode"/></summary>
     private CDataMode _cDataMode = CDataMode.Always;
 
+    /// <summary>See <see cref="UseNullText"/></summary>
+    private string? _nullText = "(null)";
+
     /// <summary>See <see cref="UseNoXmlNamespace"/></summary>
     private XmlQualifiedName? _xmlNamespace = Log4NetXmlNamespace;
 
@@ -78,6 +81,23 @@ public Log4NetTextFormatterOptionsBuilder UseCDataMode(CDataMode cDataMode)
         return this;
     }
 
+    /// <summary>
+    /// Sets how <see langword="null"/> properties are rendered as textual representation inside XML attributes.
+    /// <para/>
+    /// The default value is <c>(null)</c>.
+    /// </summary>
+    /// <param name="nullText">The text to use to represent <see langword="null"/> properties.</param>
+    /// <returns>The builder in order to fluently chain all options.</returns>
+    /// <remarks>
+    /// Pass <see langword="null"/> for <paramref name="nullText"/> to omit the <c>value</c> attribute, i.e. <code>&lt;log4net:data name="MiddleName" /&gt;</code>
+    /// Note that this is not strictly compatible with log4net/log4j XML formats and should be used to keep compatibility with Serilog.Formatting.Log4Net 1.3.1 and earlier if needed.
+    /// </remarks>
+    public Log4NetTextFormatterOptionsBuilder UseNullText(string? nullText)
+    {
+        _nullText = nullText;
+        return this;
+    }
+
     /// <summary>
     /// Do not use any XML namespace for log4net events.
     /// <para/>
@@ -197,7 +217,7 @@ public void UseLog4JCompatibility()
     }
 
     internal Log4NetTextFormatterOptions Build()
-        => new(_formatProvider, _cDataMode, _xmlNamespace, CreateXmlWriterSettings(_lineEnding, _indentationSettings), _filterProperty, _formatMessage, _formatException);
+        => new(_formatProvider, _cDataMode, _nullText, _xmlNamespace, CreateXmlWriterSettings(_lineEnding, _indentationSettings), _filterProperty, _formatMessage, _formatException);
 
     private static XmlWriterSettings CreateXmlWriterSettings(LineEnding lineEnding, IndentationSettings? indentationSettings)
     {
diff --git a/tests/Log4NetTextFormatterTest.DefaultMessageFormatter_eventId=1_eventIdName=EventName.verified.xml b/tests/Log4NetTextFormatterTest.DefaultMessageFormatter_eventId=1_eventIdName=EventName.verified.xml
@@ -3,7 +3,7 @@
     <log4net:data name="Name" value="World" />
     <log4net:data name="EventId.Id" value="1" />
     <log4net:data name="EventId.Name" value="EventName" />
-    <log4net:data name="EventId.More" />
+    <log4net:data name="EventId.More" value="(null)" />
   </log4net:properties>
   <log4net:message><![CDATA[Hello World!]]></log4net:message>
 </log4net:event>
diff --git a/tests/Log4NetTextFormatterTest.NullProperty_nullText=-null-.verified.xml b/tests/Log4NetTextFormatterTest.NullProperty_nullText=-null-.verified.xml
@@ -0,0 +1,6 @@
+﻿<log4net:event timestamp="2003-01-04T15:09:26.535+01:00" level="INFO" xmlns:log4net="http://logging.apache.org/log4net/schemas/log4net-events-1.2/">
+  <log4net:properties>
+    <log4net:data name="n/a" value="&lt;null&gt;" />
+  </log4net:properties>
+  <log4net:message><![CDATA[Hello from Serilog]]></log4net:message>
+</log4net:event>
diff --git a/tests/Log4NetTextFormatterTest.NullProperty_nullText=.verified.xml b/tests/Log4NetTextFormatterTest.NullProperty_nullText=.verified.xml
@@ -0,0 +1,6 @@
+﻿<log4net:event timestamp="2003-01-04T15:09:26.535+01:00" level="INFO" xmlns:log4net="http://logging.apache.org/log4net/schemas/log4net-events-1.2/">
+  <log4net:properties>
+    <log4net:data name="n/a" value="" />
+  </log4net:properties>
+  <log4net:message><![CDATA[Hello from Serilog]]></log4net:message>
+</log4net:event>
diff --git a/tests/Log4NetTextFormatterTest.NullProperty_nullText=_DEFAULT_.verified.xml b/tests/Log4NetTextFormatterTest.NullProperty_nullText=_DEFAULT_.verified.xml
@@ -0,0 +1,6 @@
+﻿<log4net:event timestamp="2003-01-04T15:09:26.535+01:00" level="INFO" xmlns:log4net="http://logging.apache.org/log4net/schemas/log4net-events-1.2/">
+  <log4net:properties>
+    <log4net:data name="n/a" value="(null)" />
+  </log4net:properties>
+  <log4net:message><![CDATA[Hello from Serilog]]></log4net:message>
+</log4net:event>
diff --git a/tests/Log4NetTextFormatterTest.NullProperty_nullText=null.verified.xml b/tests/Log4NetTextFormatterTest.NullProperty_nullText=null.verified.xml
@@ -1,4 +1,4 @@
-<log4net:event timestamp="2003-01-04T15:09:26.535+01:00" level="INFO" xmlns:log4net="http://logging.apache.org/log4net/schemas/log4net-events-1.2/">
+﻿<log4net:event timestamp="2003-01-04T15:09:26.535+01:00" level="INFO" xmlns:log4net="http://logging.apache.org/log4net/schemas/log4net-events-1.2/">
   <log4net:properties>
     <log4net:data name="n/a" />
   </log4net:properties>
diff --git a/tests/Log4NetTextFormatterTest.NullProperty_nullText=🌀.verified.xml b/tests/Log4NetTextFormatterTest.NullProperty_nullText=🌀.verified.xml
@@ -0,0 +1,6 @@
+﻿<log4net:event timestamp="2003-01-04T15:09:26.535+01:00" level="INFO" xmlns:log4net="http://logging.apache.org/log4net/schemas/log4net-events-1.2/">
+  <log4net:properties>
+    <log4net:data name="n/a" value="🌀" />
+  </log4net:properties>
+  <log4net:message><![CDATA[Hello from Serilog]]></log4net:message>
+</log4net:event>
diff --git a/tests/Log4NetTextFormatterTest.TwoPropertiesOneNull.verified.xml b/tests/Log4NetTextFormatterTest.TwoPropertiesOneNull.verified.xml
@@ -1,6 +1,6 @@
 ﻿<log4net:event timestamp="2003-01-04T15:09:26.535+01:00" level="INFO" xmlns:log4net="http://logging.apache.org/log4net/schemas/log4net-events-1.2/">
   <log4net:properties>
-    <log4net:data name="n/a" />
+    <log4net:data name="n/a" value="(null)" />
     <log4net:data name="one" value="1" />
   </log4net:properties>
   <log4net:message><![CDATA[Hello from Serilog]]></log4net:message>
diff --git a/tests/Log4NetTextFormatterTest.cs b/tests/Log4NetTextFormatterTest.cs
@@ -243,19 +243,24 @@ public Task NoNamespace()
         return Verify(output);
     }
 
-    [Fact]
-    public Task NullProperty()
+    [Theory]
+    [InlineData("_DEFAULT_")]
+    [InlineData(null)]
+    [InlineData("")]
+    [InlineData("<null>")]
+    [InlineData("🌀")]
+    public Task NullProperty(string? nullText)
     {
         // Arrange
         using var output = new StringWriter();
         var logEvent = CreateLogEvent(properties: new LogEventProperty("n/a", new ScalarValue(null)));
-        var formatter = new Log4NetTextFormatter();
+        var formatter = new Log4NetTextFormatter(options => { if (nullText != "_DEFAULT_") { options.UseNullText(nullText); } });
 
         // Act
         formatter.Format(logEvent, output);
 
         // Assert
-        return Verify(output);
+        return Verify(output).UseParameters(nullText);
     }
 
     [Fact]
diff --git a/tests/PublicApi.net6.0.verified.cs b/tests/PublicApi.net6.0.verified.cs
@@ -45,6 +45,7 @@ public void UseLog4JCompatibility() { }
         public Serilog.Formatting.Log4Net.Log4NetTextFormatterOptionsBuilder UseMessageFormatter(Serilog.Formatting.Log4Net.MessageFormatter formatMessage) { }
         public Serilog.Formatting.Log4Net.Log4NetTextFormatterOptionsBuilder UseNoIndentation() { }
         public Serilog.Formatting.Log4Net.Log4NetTextFormatterOptionsBuilder UseNoXmlNamespace() { }
+        public Serilog.Formatting.Log4Net.Log4NetTextFormatterOptionsBuilder UseNullText(string? nullText) { }
         public Serilog.Formatting.Log4Net.Log4NetTextFormatterOptionsBuilder UsePropertyFilter(Serilog.Formatting.Log4Net.PropertyFilter filterProperty) { }
     }
     public delegate string MessageFormatter(Serilog.Events.LogEvent logEvent, System.IFormatProvider? formatProvider);
diff --git a/tests/PublicApi.net8.0.verified.cs b/tests/PublicApi.net8.0.verified.cs
@@ -46,6 +46,7 @@ public void UseLog4JCompatibility() { }
         public Serilog.Formatting.Log4Net.Log4NetTextFormatterOptionsBuilder UseMessageFormatter(Serilog.Formatting.Log4Net.MessageFormatter formatMessage) { }
         public Serilog.Formatting.Log4Net.Log4NetTextFormatterOptionsBuilder UseNoIndentation() { }
         public Serilog.Formatting.Log4Net.Log4NetTextFormatterOptionsBuilder UseNoXmlNamespace() { }
+        public Serilog.Formatting.Log4Net.Log4NetTextFormatterOptionsBuilder UseNullText(string? nullText) { }
         public Serilog.Formatting.Log4Net.Log4NetTextFormatterOptionsBuilder UsePropertyFilter(Serilog.Formatting.Log4Net.PropertyFilter filterProperty) { }
     }
     public delegate string MessageFormatter(Serilog.Events.LogEvent logEvent, System.IFormatProvider? formatProvider);
diff --git a/tests/PublicApi.netstandard2.0.verified.cs b/tests/PublicApi.netstandard2.0.verified.cs
@@ -45,6 +45,7 @@ public void UseLog4JCompatibility() { }
         public Serilog.Formatting.Log4Net.Log4NetTextFormatterOptionsBuilder UseMessageFormatter(Serilog.Formatting.Log4Net.MessageFormatter formatMessage) { }
         public Serilog.Formatting.Log4Net.Log4NetTextFormatterOptionsBuilder UseNoIndentation() { }
         public Serilog.Formatting.Log4Net.Log4NetTextFormatterOptionsBuilder UseNoXmlNamespace() { }
+        public Serilog.Formatting.Log4Net.Log4NetTextFormatterOptionsBuilder UseNullText(string? nullText) { }
         public Serilog.Formatting.Log4Net.Log4NetTextFormatterOptionsBuilder UsePropertyFilter(Serilog.Formatting.Log4Net.PropertyFilter filterProperty) { }
     }
     public delegate string MessageFormatter(Serilog.Events.LogEvent logEvent, System.IFormatProvider? formatProvider);

Original file line number	Diff line number	Diff line change
`@@ -406,11 +406,14 @@ private void WritePropertyElement(XmlWriter writer, string name, LogEventPropert`
`406`	`406`	`{`
`407`	`407`	`WriteStartElement(writer, "data");`
`408`	`408`	`writer.WriteAttributeString("name", name);`
`409`		`- var isNullValue = value is ScalarValue { Value: null };`
`410`		`- if (!isNullValue)`
	`409`	`+ if (value is not ScalarValue { Value: null })`
`411`	`410`	`{`
`412`	`411`	`writer.WriteAttributeString("value", RenderValue(value));`
`413`	`412`	`}`
	`413`	`+ else if (_options.NullText != null)`
	`414`	`+ {`
	`415`	`+ writer.WriteAttributeString("value", _options.NullText);`
	`416`	`+ }`
`414`	`417`	`writer.WriteEndElement();`
`415`	`418`	`}`
`416`	`419`
Original file line number	Diff line number	Diff line change
`@@ -1,4 +1,4 @@`
`1`		`-<log4net:event timestamp="2003-01-04T15:09:26.535+01:00" level="INFO" xmlns:log4net="http://logging.apache.org/log4net/schemas/log4net-events-1.2/">`
	`1`	`+<log4net:event timestamp="2003-01-04T15:09:26.535+01:00" level="INFO" xmlns:log4net="http://logging.apache.org/log4net/schemas/log4net-events-1.2/">`
`2`	`2`	`<log4net:properties>`
`3`	`3`	`<log4net:data name="n/a" />`
`4`	`4`	`</log4net:properties>`