Enable code quality analysis and fix reported issues

Note: the documentation on https://docs.microsoft.com/en-us/dotnet/core/project-sdk/msbuild-props#analysislevel says
> The latest code analyzers that have been released are used. This is the default.

This is true only if you are targeting .NET 5.0 or later. For .NET Standard 2.0, the `AnalysisLevel` must be explicitly set to `latest`.

Author: 0xced

src/Log4NetTextFormatter.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using System.IO;
 using System.Linq;
 using System.Xml;
@@ -72,6 +73,8 @@ public Log4NetTextFormatter(Action<Log4NetTextFormatterOptionsBuilder>? configur
         /// <param name="output">The output.</param>
         public void Format(LogEvent logEvent, TextWriter output)
         {
+            if (logEvent == null) throw new ArgumentNullException(nameof(logEvent));
+            if (output == null) throw new ArgumentNullException(nameof(output));
             using var writer = XmlWriter.Create(output, _options.XmlWriterSettings);
             WriteEvent(logEvent, writer);
             writer.Flush();
@@ -170,6 +173,7 @@ private static string LogLevel(LogEventLevel level)
         /// <param name="properties">The collection of properties to write.</param>
         /// <param name="machineNameProperty">The machine name property to write or <see langref="null"/> if doesn't exist.</param>
         /// <remarks>https://github.com/apache/logging-log4net/blob/rel/2.0.8/src/Layout/XmlLayout.cs#L262-L286</remarks>
+        [SuppressMessage("Microsoft.Design", "CA1031", Justification = "Protecting from user-provided code which might throw anything")]
         private void WriteProperties(LogEvent logEvent, XmlWriter writer, IEnumerable<KeyValuePair<string, LogEventPropertyValue>> properties, LogEventPropertyValue? machineNameProperty)
         {
             WriteStartElement(writer, "properties");
@@ -205,7 +209,7 @@ private void WriteProperties(LogEvent logEvent, XmlWriter writer, IEnumerable<Ke
         /// <returns>A string representation of the <paramref name="value"/>.</returns>
         private string RenderValue(LogEventPropertyValue value)
         {
-            var valueWriter = new StringWriter();
+            using var valueWriter = new StringWriter();
             // The "l" format specifier switches off quoting of strings, see https://github.com/serilog/serilog/wiki/Formatting-Output#formatting-plain-text
             value.Render(valueWriter, value is ScalarValue scalarValue && scalarValue.Value is string ? "l" : null, _options.FormatProvider);
             return valueWriter.ToString();
@@ -328,6 +332,7 @@ private void WriteMessage(LogEvent logEvent, XmlWriter writer)
         /// <param name="logEvent">The log event.</param>
         /// <param name="writer">The XML writer.</param>
         /// <remarks>https://github.com/apache/logging-log4net/blob/rel/2.0.8/src/Layout/XmlLayout.cs#L288-L295</remarks>
+        [SuppressMessage("Microsoft.Design", "CA1031", Justification = "Protecting from user-provided code which might throw anything")]
         private void WriteException(LogEvent logEvent, XmlWriter writer)
         {
             var exception = logEvent.Exception;

src/Properties/AssemblyInfo.cs

@@ -0,0 +1,4 @@
+using System;
+
+// Can't use AssemblyAttribute MSBuild item yet, see https://github.com/dotnet/msbuild/issues/2281
+[assembly: CLSCompliant(true)]

src/Serilog.Formatting.Log4Net.csproj

@@ -7,6 +7,8 @@
   <PropertyGroup Label="Compiling">
     <LangVersion>8.0</LangVersion>
     <Nullable>enable</Nullable>
+    <AnalysisLevel>latest</AnalysisLevel>
+    <AnalysisMode>AllEnabledByDefault</AnalysisMode>
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
   </PropertyGroup>
 

tests/Log4NetTextFormatterTest.cs

@@ -52,6 +52,33 @@ private static LogEvent CreateLogEvent(LogEventLevel level = Events.LogEventLeve
             );
         }
 
+        [Fact]
+        public void NullLogEventThrowsArgumentNullException()
+        {
+            // Arrange
+            var formatter = new Log4NetTextFormatter();
+
+            // Act
+            Action action = () => formatter.Format(null!, TextWriter.Null);
+
+            // Assert
+            action.Should().ThrowExactly<ArgumentNullException>().Which.ParamName.Should().Be("logEvent");
+        }
+
+        [Fact]
+        public void NullOutputThrowsArgumentNullException()
+        {
+            // Arrange
+            var logEvent = CreateLogEvent();
+            var formatter = new Log4NetTextFormatter();
+
+            // Act
+            Action action = () => formatter.Format(logEvent, null!);
+
+            // Assert
+            action.Should().ThrowExactly<ArgumentNullException>().Which.ParamName.Should().Be("output");
+        }
+
         [Theory]
         [InlineData(Events.LogEventLevel.Verbose)]
         [InlineData(Events.LogEventLevel.Debug)]