AV2210: Removed check on warning level, because since .NET 5, the default warning level depends on the target framework and is driven by the AnalysisLevel MSBuild property (see https://devblogs.microsoft.com/dotnet/automatically-find-latent-bugs-in-your-code-with-net-5/ and https://docs.microsoft.com/en-us/dotnet/core/project-sdk/msbuild-props#analysislevel), which analyzers do not have access to by default (it would require user intervention as described at https://stackoverflow.com/questions/69215975/roslyn-codefix-msbuild-properties-metadata-and-unit-testing). So to get a good out-of-the-box experience, we'd need to duplicate the logic from Microsoft.NET.Sdk.Analyzers.targets, which only applies to SDK-style projects and is subject to change in each .NET release.

Setting the warning level to 9999 as recommended in https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/compiler-options/errors-warnings#warninglevel does not work anymore since .NET 5, because it gets overwritten at build time by Microsoft.NET.Sdk.Analyzers.targets (dotnet/sdk#21599).

Author: Bart Koelman

docs/Overview.md

@@ -162,9 +162,7 @@ This analyzer reports when:
 - a comparison with a nullable value type contains a redundant null check.
 
 ### [AV2210](https://github.com/dennisdoomen/CSharpGuidelines/blob/7a66f7468da6ce1477753a02e416e04bc9a44e45/_pages/2200_FrameworkGuidelines.md#av2210): Build with the highest warning level ![](/images/warn.png "severity: warning")
-This analyzer reports when:
-- the compiler warning level is below [9999](https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/compiler-options/errors-warnings#warninglevel) in the build settings
-- warnings are not treated as errors in the build settings.
+This analyzer reports when warnings are not treated as errors in the build settings.
 
 Note: this analyzer requires [Full Solution Analysis](/docs/Full%20Solution%20Analysis.md) enabled.
 

src/CSharpGuidelinesAnalyzer/CSharpGuidelinesAnalyzer.Test/Specs/Framework/BuildWithTheHighestWarningLevelSpecs.cs

@@ -10,43 +10,27 @@ public sealed class BuildWithTheHighestWarningLevelSpecs : CSharpGuidelinesAnaly
         protected override string DiagnosticId => BuildWithTheHighestWarningLevelAnalyzer.DiagnosticId;
 
         [Fact]
-        internal void When_warning_level_is_9999_with_warnings_as_errors_it_must_be_skipped()
+        internal void When_compiling_with_warnings_as_errors_it_must_be_skipped()
         {
             // Arrange
             ParsedSourceCode source = new TypeSourceCodeBuilder()
-                .CompileAtWarningLevel(9999)
                 .CompileWithWarningAsError()
                 .Build();
 
             // Act and assert
             VerifyGuidelineDiagnostic(source);
         }
 
-        [Fact]
-        internal void When_warning_level_is_below_9999_it_must_be_reported()
-        {
-            // Arrange
-            ParsedSourceCode source = new TypeSourceCodeBuilder()
-                .CompileAtWarningLevel(5)
-                .CompileWithWarningAsError()
-                .Build();
-
-            // Act and assert
-            VerifyGuidelineDiagnostic(source,
-                "Build with warning level 9999");
-        }
-
         [Fact]
         internal void When_compiling_with_warnings_not_as_errors_it_must_be_reported()
         {
             // Arrange
             ParsedSourceCode source = new TypeSourceCodeBuilder()
-                .CompileAtWarningLevel(9999)
                 .Build();
 
             // Act and assert
             VerifyGuidelineDiagnostic(source,
-                "Build with -warnaserror");
+                "Pass -warnaserror to the compiler or add <TreatWarningsAsErrors>True</TreatWarningsAsErrors> to your project file");
         }
 
         protected override DiagnosticAnalyzer CreateAnalyzer()

src/CSharpGuidelinesAnalyzer/CSharpGuidelinesAnalyzer.Test/TestDataBuilders/SourceCodeBuilderExtensions.cs

@@ -83,17 +83,6 @@ public static TBuilder WithOutputKind<TBuilder>([NotNull] this TBuilder source,
             return source;
         }
 
-        [NotNull]
-        public static TBuilder CompileAtWarningLevel<TBuilder>([NotNull] this TBuilder source, int warningLevel)
-            where TBuilder : SourceCodeBuilder
-        {
-            Guard.NotNull(source, nameof(source));
-
-            source.Editor.UpdateTestContext(context => context.CompileAtWarningLevel(warningLevel));
-
-            return source;
-        }
-
         [NotNull]
         public static TBuilder CompileWithWarningAsError<TBuilder>([NotNull] this TBuilder source)
             where TBuilder : SourceCodeBuilder

src/CSharpGuidelinesAnalyzer/CSharpGuidelinesAnalyzer/Rules/Framework/BuildWithTheHighestWarningLevelAnalyzer.cs

@@ -9,33 +9,27 @@ namespace CSharpGuidelinesAnalyzer.Rules.Framework
     [DiagnosticAnalyzer(LanguageNames.CSharp)]
     public sealed class BuildWithTheHighestWarningLevelAnalyzer : DiagnosticAnalyzer
     {
-        private const string Title = "Compiler warnings are not treated as errors or warning level is too low";
-        private const string WarningLevelMessageFormat = "Build with warning level 9999";
-        private const string WarningAsErrorMessageFormat = "Build with -warnaserror";
-        private const string Description = "Build with the highest warning level.";
+        private const string Title = "Compiler warnings are not treated as errors";
 
-        // The highest warning level used to be 4, until C# 9 added warning level 5. Microsoft now recommends to use 9999, which should include all
-        // future warning levels. See https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/compiler-options/errors-warnings#warninglevel.
-        private const int MaxWarningLevel = 9999;
+        private const string MessageFormat =
+            "Pass -warnaserror to the compiler or add <TreatWarningsAsErrors>True</TreatWarningsAsErrors> to your project file";
+
+        private const string Description = "Build with the highest warning level";
 
         public const string DiagnosticId = "AV2210";
 
         [NotNull]
         private static readonly AnalyzerCategory Category = AnalyzerCategory.Framework;
 
         [NotNull]
-        private static readonly DiagnosticDescriptor WarningLevelRule = new DiagnosticDescriptor(DiagnosticId, Title, WarningLevelMessageFormat,
-            Category.DisplayName, DiagnosticSeverity.Warning, true, Description, Category.GetHelpLinkUri(DiagnosticId));
-
-        [NotNull]
-        private static readonly DiagnosticDescriptor WarningAsErrorRule = new DiagnosticDescriptor(DiagnosticId, Title, WarningAsErrorMessageFormat,
-            Category.DisplayName, DiagnosticSeverity.Warning, true, Description, Category.GetHelpLinkUri(DiagnosticId));
+        private static readonly DiagnosticDescriptor Rule = new DiagnosticDescriptor(DiagnosticId, Title, MessageFormat, Category.DisplayName,
+            DiagnosticSeverity.Warning, true, Description, Category.GetHelpLinkUri(DiagnosticId));
 
         [NotNull]
         private static readonly Action<CompilationAnalysisContext> AnalyzeCompilationOptionsAction = AnalyzeCompilationOptions;
 
         [ItemNotNull]
-        public override ImmutableArray<DiagnosticDescriptor> SupportedDiagnostics => ImmutableArray.Create(WarningLevelRule, WarningAsErrorRule);
+        public override ImmutableArray<DiagnosticDescriptor> SupportedDiagnostics => ImmutableArray.Create(Rule);
 
         public override void Initialize([NotNull] AnalysisContext context)
         {
@@ -47,16 +41,9 @@ public override void Initialize([NotNull] AnalysisContext context)
 
         private static void AnalyzeCompilationOptions(CompilationAnalysisContext context)
         {
-            CompilationOptions options = context.Compilation.Options;
-
-            if (options.GeneralDiagnosticOption != ReportDiagnostic.Error)
-            {
-                context.ReportDiagnostic(Diagnostic.Create(WarningAsErrorRule, Location.None));
-            }
-
-            if (options.WarningLevel < MaxWarningLevel)
+            if (context.Compilation.Options.GeneralDiagnosticOption != ReportDiagnostic.Error)
             {
-                context.ReportDiagnostic(Diagnostic.Create(WarningLevelRule, Location.None));
+                context.ReportDiagnostic(Diagnostic.Create(Rule, Location.None));
             }
         }
     }