Add net6.0 target framework

It will be beneficial for [string interpolation][1] which is used extensively through the project. See [Are there benefits in producing a .NET 6.0 version of a .NET Standard 2.0 library?][1] on Stack Overflow.

Also, run mutation tests _after_ creating the NuGet package to ensure that Serilog.Formatting.Log4Net.dll is not altered by Styker.NET.

[1]: https://devblogs.microsoft.com/dotnet/performance-improvements-in-net-6/#arrays-strings-spans
[2]: https://stackoverflow.com/questions/70778797/are-there-benefits-in-producing-a-net-6-0-version-of-a-net-standard-2-0-librar/72266562#72266562

Author: 0xced

.github/workflows/continuous-integration.yml

@@ -78,12 +78,6 @@ jobs:
         with:
           project-token: ${{ secrets.CODACY_PROJECT_TOKEN }}
           coverage-reports: ${{ steps.dotnet-test.outputs.coverage-reports }}
-      - name: Run mutation tests and upload report to Stryker dashboard
-        if: matrix.os == 'ubuntu-latest' && env.STRYKER_DASHBOARD_API_KEY != ''
-        run: |
-          cd tests
-          dotnet tool restore
-          dotnet tool run dotnet-stryker --reporter dashboard --version ${GITHUB_REF##*/} --dashboard-api-key ${{ secrets.STRYKER_DASHBOARD_API_KEY }}
       - name: Create NuGet package
         run: dotnet pack --no-build --output .
         id: dotnet-pack
@@ -93,6 +87,12 @@ jobs:
         with:
           name: ${{ steps.dotnet-pack.outputs.nupkg-filename }}
           path: ${{ steps.dotnet-pack.outputs.nupkg-filename }}
+      - name: Run mutation tests and upload report to Stryker dashboard
+        if: matrix.os == 'ubuntu-latest' && env.STRYKER_DASHBOARD_API_KEY != ''
+        run: |
+          cd tests
+          dotnet tool restore
+          dotnet tool run dotnet-stryker --reporter dashboard --version ${GITHUB_REF##*/} --dashboard-api-key ${{ secrets.STRYKER_DASHBOARD_API_KEY }}
       - name: Retrieve tag message
         if: matrix.os == 'ubuntu-latest'
         run: |

CHANGELOG.md

@@ -3,6 +3,10 @@ All notable changes to this project will be documented in this file.
 
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/), and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [Unreleased][unreleased]
+
+- Add support for .NET 6. It will be beneficial for [string interpolation](https://devblogs.microsoft.com/dotnet/performance-improvements-in-net-6/#arrays-strings-spans) which is used extensively through the project. See [Are there benefits in producing a .NET 6.0 version of a .NET Standard 2.0 library?](https://stackoverflow.com/questions/70778797/are-there-benefits-in-producing-a-net-6-0-version-of-a-net-standard-2-0-librar/72266562#72266562) on Stack Overflow.
+
 ## [1.0.0][1.0.0] - 2022-03-08
 
 - First final version (i.e. non pre-release) which is identical to 1.0.0-rc.4
@@ -67,6 +71,7 @@ Still trying to figure out how to make everything fit together with [MinVer](htt
 
 - Implement log4j compatibility mode.
 
+[unreleased]: https://github.com/serilog-contrib/serilog-formatting-log4net/compare/1.0.0...HEAD
 [1.0.0]: https://github.com/serilog-contrib/serilog-formatting-log4net/compare/1.0.0-rc.4...1.0.0
 [1.0.0-rc.4]: https://github.com/serilog-contrib/serilog-formatting-log4net/compare/1.0.0-rc.3...1.0.0-rc.4
 [1.0.0-rc.3]: https://github.com/serilog-contrib/serilog-formatting-log4net/compare/1.0.0-rc.2...1.0.0-rc.3

src/Log4NetTextFormatter.cs

@@ -95,10 +95,15 @@ public void Format(LogEvent logEvent, TextWriter output)
             // 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 = xmlWriterOutput.ToString();
+            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
         }
         output.Write(_options.XmlWriterSettings.NewLineChars);
     }
@@ -401,7 +406,7 @@ private void WriteException(LogEvent logEvent, XmlWriter writer)
     {
         try
         {
-            return _options.FormatException(exception);
+            return (string?)_options.FormatException(exception);
         }
         catch (Exception formattingException)
         {

src/Serilog.Formatting.Log4Net.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>netstandard2.0</TargetFramework>
+    <TargetFrameworks>netstandard2.0;net6.0</TargetFrameworks>
   </PropertyGroup>
 
   <PropertyGroup Label="Compiling">