Added a suppressor when CA1812 fires on NUnit Test classes.

documentation/NUnit3001.md

@@ -98,6 +98,7 @@ To disable the rule for a project, you need to add a
   <Rules AnalyzerId="DiagnosticSuppressors" RuleNamespace="NUnit.NUnitAnalyzers">
     <Rule Id="NUnit3001" Action="Info" /> <!-- Possible Null Reference -->
     <Rule Id="NUnit3002" Action="Info" /> <!-- NonNullableField/Property is Uninitialized -->
+    <Rule Id="NUnit3003" Action="Info" /> <!-- Avoid Uninstantiated Internal Classes -->
   </Rules>
 </RuleSet>
 ```

documentation/NUnit3002.md

@@ -68,6 +68,7 @@ To disable the rule for a project, you need to add a
   <Rules AnalyzerId="DiagnosticSuppressors" RuleNamespace="NUnit.NUnitAnalyzers">
     <Rule Id="NUnit3001" Action="Info" /> <!-- Possible Null Reference -->
     <Rule Id="NUnit3002" Action="Info" /> <!-- NonNullableField/Property is Uninitialized -->
+    <Rule Id="NUnit3003" Action="Info" /> <!-- Avoid Uninstantiated Internal Classes -->
   </Rules>
 </RuleSet>
 ```

documentation/NUnit3003.md

@@ -0,0 +1,63 @@
+# NUnit3003
+
+## Class is a NUnit TestFixture and called by reflection
+
+| Topic    | Value
+| :--      | :--
+| Id       | NUnit3003
+| Severity | Info
+| Enabled  | True
+| Category | Suppressor
+| Code     | [AvoidUninstantiatedInternalClassesSuppressor](https://github.com/nunit/nunit.analyzers/blob/master/src/nunit.analyzers/DiagnosticSuppressors/AvoidUninstantiatedInternalClassesSuppressor.cs)
+
+## Description
+
+Class is a NUnit TestFixture and called by reflection
+
+## Motivation
+
+The default roslyn analyzer has rule [CA1812](https://learn.microsoft.com/en-us/dotnet/fundamentals/code-analysis/quality-rules/ca1812)
+which warns about internal classes not being used.
+That analyzer doesn't know about NUnit test classes.
+This suppressor catches the error, verifies the class is an NUNit TestFixture and if so suppresses the error.
+
+<!-- start generated config severity -->
+## Configure severity
+
+The rule has no severity, but can be disabled.
+
+### Via ruleset file
+
+To disable the rule for a project, you need to add a
+[ruleset file](https://github.com/nunit/nunit.analyzers/blob/master/src/nunit.analyzers/DiagnosticSuppressors/NUnit.Analyzers.Suppressions.ruleset)
+
+```xml
+<?xml version="1.0" encoding="utf-8"?>
+<RuleSet Name="NUnit.Analyzer Suppressions" Description="DiagnosticSuppression Rules" ToolsVersion="12.0">
+  <Rules AnalyzerId="DiagnosticSuppressors" RuleNamespace="NUnit.NUnitAnalyzers">
+    <Rule Id="NUnit3001" Action="Info" /> <!-- Possible Null Reference -->
+    <Rule Id="NUnit3002" Action="Info" /> <!-- NonNullableField/Property is Uninitialized -->
+    <Rule Id="NUnit3003" Action="Info" /> <!-- Avoid Uninstantiated Internal Classes -->
+  </Rules>
+</RuleSet>
+```
+
+and add it to the project like:
+
+```xml
+<PropertyGroup>
+  <CodeAnalysisRuleSet>NUnit.Analyzers.Suppressions.ruleset</CodeAnalysisRuleSet>
+</PropertyGroup>
+```
+
+For more info about rulesets see [MSDN](https://learn.microsoft.com/en-us/visualstudio/code-quality/using-rule-sets-to-group-code-analysis-rules?view=vs-2022).
+
+### Via .editorconfig file
+
+This is currently not working. Waiting for [Roslyn](https://github.com/dotnet/roslyn/issues/49727)
+
+```ini
+# NUnit3003: Class is a NUnit TestFixture and called by reflection
+dotnet_diagnostic.NUnit3003.severity = none
+```
+<!-- end generated config severity -->

documentation/index.md

@@ -112,3 +112,4 @@ Rules which suppress compiler errors based on context. Note that these rules are
 | :--      | :--         | :--:  | :--:   | :--:   |
 | [NUnit3001](https://github.com/nunit/nunit.analyzers/tree/master/documentation/NUnit3001.md) | Expression was checked in an Assert.NotNull, Assert.IsNotNull or Assert.That call | :white_check_mark: | :information_source: | :x: |
 | [NUnit3002](https://github.com/nunit/nunit.analyzers/tree/master/documentation/NUnit3002.md) | Field/Property is initialized in SetUp or OneTimeSetUp method | :white_check_mark: | :information_source: | :x: |
+| [NUnit3003](https://github.com/nunit/nunit.analyzers/tree/master/documentation/NUnit3003.md) | Class is a NUnit TestFixture and called by reflection | :white_check_mark: | :information_source: | :x: |

src/nunit.analyzers.sln

@@ -91,6 +91,7 @@ Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "documentation", "documentat
 		..\documentation\NUnit2047.md = ..\documentation\NUnit2047.md
 		..\documentation\NUnit3001.md = ..\documentation\NUnit3001.md
 		..\documentation\NUnit3002.md = ..\documentation\NUnit3002.md
+		..\documentation\NUnit3003.md = ..\documentation\NUnit3003.md
 		..\README.md = ..\README.md
 	EndProjectSection
 EndProject

src/nunit.analyzers.tests/DefaultEnabledAnalyzer.cs

@@ -0,0 +1,49 @@
+using System;
+using System.Collections.Immutable;
+using System.Linq;
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.Diagnostics;
+
+namespace NUnit.Analyzers.Tests
+{
+    [DiagnosticAnalyzer(LanguageNames.CSharp)]
+#pragma warning disable RS1036 // Specify analyzer banned API enforcement setting
+    internal sealed class DefaultEnabledAnalyzer : DiagnosticAnalyzer
+#pragma warning restore RS1036 // Specify analyzer banned API enforcement setting
+    {
+        private readonly DiagnosticAnalyzer inner;
+
+        internal DefaultEnabledAnalyzer(DiagnosticAnalyzer inner)
+        {
+            this.inner = inner;
+            this.SupportedDiagnostics = EnabledDiagnostics(inner.SupportedDiagnostics);
+
+            static ImmutableArray<DiagnosticDescriptor> EnabledDiagnostics(ImmutableArray<DiagnosticDescriptor> source)
+            {
+                var builder = ImmutableArray.CreateBuilder<DiagnosticDescriptor>(source.Length);
+                foreach (var diagnostic in source)
+                {
+                    builder.Add(
+                        new DiagnosticDescriptor(
+                            diagnostic.Id,
+                            diagnostic.Title,
+                            diagnostic.MessageFormat,
+                            diagnostic.Category,
+                            diagnostic.DefaultSeverity,
+                            isEnabledByDefault: true,
+                            diagnostic.Description,
+                            diagnostic.HelpLinkUri,
+                            diagnostic.CustomTags?.ToArray() ?? Array.Empty<string>()));
+                }
+
+                return builder.MoveToImmutable();
+            }
+        }
+
+        public override ImmutableArray<DiagnosticDescriptor> SupportedDiagnostics { get; }
+
+#pragma warning disable RS1025, RS1026
+        public override void Initialize(AnalysisContext context) => this.inner.Initialize(context);
+#pragma warning restore RS1025, RS1026
+    }
+}

src/nunit.analyzers.tests/DiagnosticSuppressors/AvoidUninstantiatedInternalClassesSuppressorTests.cs

@@ -0,0 +1,89 @@
+using System;
+using System.IO;
+using System.Reflection;
+using System.Threading.Tasks;
+using Microsoft.CodeAnalysis.Diagnostics;
+using NUnit.Analyzers.DiagnosticSuppressors;
+using NUnit.Framework;
+
+namespace NUnit.Analyzers.Tests.DiagnosticSuppressors
+{
+    internal class AvoidUninstantiatedInternalClassesSuppressorTests
+    {
+        private const string CalculatorClass = @"
+            internal sealed class Calculator
+            {
+                private readonly Calculation _calculation;
+
+                public Calculator(Calculation calculation) => _calculation = calculation;
+
+                public int Calculate(int op1, int op2)
+                {
+                    return _calculation == Calculation.Add ? op1 + op2 : op1 - op2;
+                }
+
+                public enum Calculation
+                {
+                    Add,
+                    Subtract,
+                }
+            }
+        ";
+
+        private const string CalculatorTest = @"
+            internal sealed class CalculatorTests
+            {
+                [Test]
+                public void TestAdd()
+                {
+                    var instance = new Calculator(Calculator.Calculation.Add);
+                    var result = instance.Calculate(3, 4);
+                    Assert.That(result, Is.EqualTo(7));
+                }
+
+                [Test]
+                public void TestSubtract()
+                {
+                    var instance = new Calculator(Calculator.Calculation.Subtract);
+                    var result = instance.Calculate(3, 4);
+                    Assert.That(result, Is.EqualTo(-1));
+                }
+            }
+        ";
+
+        private static readonly DiagnosticSuppressor suppressor = new AvoidUninstantiatedInternalClassesSuppressor();
+        private DiagnosticAnalyzer analyzer;
+
+        [OneTimeSetUp]
+        public void OneTimeSetUp()
+        {
+            // Find the NetAnalyzers assembly (note version should match the one referenced)
+            string netAnalyzersPath = Path.Combine(PathHelper.GetNuGetPackageDirectory(),
+                "microsoft.codeanalysis.netanalyzers/7.0.3/analyzers/dotnet/cs/Microsoft.CodeAnalysis.CSharp.NetAnalyzers.dll");
+            Assembly netAnalyzerAssembly = Assembly.LoadFrom(netAnalyzersPath);
+            Type analyzerType = netAnalyzerAssembly.GetType("Microsoft.CodeQuality.CSharp.Analyzers.Maintainability.CSharpAvoidUninstantiatedInternalClasses", true)!;
+            this.analyzer = (DiagnosticAnalyzer)Activator.CreateInstance(analyzerType!)!;
+
+            this.analyzer = new DefaultEnabledAnalyzer(this.analyzer);
+        }
+
+        [Test]
+        public async Task NonTestClass()
+        {
+            var testCode = TestUtility.WrapClassInNamespaceAndAddUsing(CalculatorClass);
+
+            await TestHelpers.NotSuppressed(this.analyzer, suppressor, testCode).ConfigureAwait(true);
+        }
+
+        [Test]
+        public async Task TestClass()
+        {
+            var testCode = TestUtility.WrapClassInNamespaceAndAddUsing($@"
+                {CalculatorClass}
+                {CalculatorTest}
+            ");
+
+            await TestHelpers.Suppressed(this.analyzer, suppressor, testCode).ConfigureAwait(true);
+        }
+    }
+}

src/nunit.analyzers.tests/DocumentationTests.cs

@@ -513,6 +513,7 @@ private static string CreateStub(DiagnosticSuppressor suppressor, SuppressionDes
   <Rules AnalyzerId=""DiagnosticSuppressors"" RuleNamespace=""NUnit.NUnitAnalyzers"">
     <Rule Id=""NUnit3001"" Action=""Info"" /> <!-- Possible Null Reference -->
     <Rule Id=""NUnit3002"" Action=""Info"" /> <!-- NonNullableField/Property is Uninitialized -->
+    <Rule Id=""NUnit3003"" Action=""Info"" /> <!-- Avoid Uninstantiated Internal Classes -->
   </Rules>
 </RuleSet>
 ```

src/nunit.analyzers.tests/PathHelper.cs

@@ -0,0 +1,21 @@
+using System;
+using System.IO;
+
+namespace NUnit.Analyzers.Tests
+{
+    internal static class PathHelper
+    {
+        public static string GetNuGetPackageDirectory()
+        {
+            return Environment.GetEnvironmentVariable("NUGET_PACKAGES") ??
+                   Path.Combine(GetHomeDirectory(), ".nuget/packages");
+        }
+
+        public static string GetHomeDirectory()
+        {
+            return Environment.GetEnvironmentVariable("HOME") ?? // Linux
+                   Environment.GetEnvironmentVariable("USERPROFILE") ?? // Windows
+                   throw new NotSupportedException("Cannot determine home directory");
+        }
+    }
+}

src/nunit.analyzers.tests/TestHelpers.cs

@@ -1,27 +1,60 @@
 using System;
+using System.Collections.Immutable;
 using System.Threading.Tasks;
 using Gu.Roslyn.Asserts;
 using Microsoft.CodeAnalysis;
 using Microsoft.CodeAnalysis.CSharp;
+using Microsoft.CodeAnalysis.Diagnostics;
+using NUnit.Framework;
 
 namespace NUnit.Analyzers.Tests
 {
     internal static class TestHelpers
     {
-        internal static Compilation CreateCompilation(string? code = null)
+        internal static Compilation CreateCompilation(string? code = null, Settings? settings = null)
         {
             var syntaxTrees = code is null
                 ? null
                 : new[] { CSharpSyntaxTree.ParseText(code) };
 
+            settings ??= Settings.Default;
+
             return CSharpCompilation.Create(Guid.NewGuid().ToString("N"),
                 syntaxTrees,
-                references: Settings.Default.MetadataReferences,
-                options: new CSharpCompilationOptions(OutputKind.DynamicallyLinkedLibrary,
-                    nullableContextOptions: NullableContextOptions.Enable,
-                    reportSuppressedDiagnostics: true));
+                references: settings.MetadataReferences,
+                options: settings.CompilationOptions);
         }
 
+        internal static async Task SuppressedOrNot(DiagnosticAnalyzer analyzer, DiagnosticSuppressor suppressor, string code, bool isSuppressed, Settings? settings = null)
+        {
+            string id = analyzer.SupportedDiagnostics[0].Id;
+            Assert.That(suppressor.SupportedSuppressions[0].SuppressedDiagnosticId, Is.EqualTo(id));
+
+            settings ??= Settings.Default;
+            settings = settings.WithCompilationOptions(Settings.Default.CompilationOptions.WithWarningOrError(analyzer.SupportedDiagnostics));
+
+            Compilation compilation = CreateCompilation(code, settings);
+
+            CompilationWithAnalyzers compilationWithAnalyzer = compilation
+                .WithAnalyzers(ImmutableArray.Create(analyzer));
+            ImmutableArray<Diagnostic> diagnostics = await compilationWithAnalyzer.GetAllDiagnosticsAsync().ConfigureAwait(false);
+            Assert.That(diagnostics, Has.Length.EqualTo(1));
+            Assert.That(diagnostics[0].Id, Is.EqualTo(id));
+
+            CompilationWithAnalyzers compilationWithAnalyzerAndSuppressor = compilation
+                .WithAnalyzers(ImmutableArray.Create(analyzer, suppressor));
+            diagnostics = await compilationWithAnalyzerAndSuppressor.GetAllDiagnosticsAsync().ConfigureAwait(false);
+            Assert.That(diagnostics, Has.Length.EqualTo(1));
+            Assert.That(diagnostics[0].Id, Is.EqualTo(id));
+            Assert.That(diagnostics[0].IsSuppressed, Is.EqualTo(isSuppressed));
+        }
+
+        internal static Task NotSuppressed(DiagnosticAnalyzer analyzer, DiagnosticSuppressor suppressor, string code, Settings? settings = null)
+            => SuppressedOrNot(analyzer, suppressor, code, false, settings);
+
+        internal static Task Suppressed(DiagnosticAnalyzer analyzer, DiagnosticSuppressor suppressor, string code, Settings? settings = null)
+            => SuppressedOrNot(analyzer, suppressor, code, true, settings);
+
         internal static async Task<(SyntaxNode Node, SemanticModel Model)> GetRootAndModel(string code)
         {
             var tree = CSharpSyntaxTree.ParseText(code);

src/nunit.analyzers.tests/nunit.analyzers.tests.csproj

@@ -14,6 +14,7 @@
     <PackageReference Include="Gu.Roslyn.Asserts.Analyzers" Version="4.0.0" />
     <PackageReference Include="NUnit" Version="3.13.3" />
     <PackageReference Include="NUnit3TestAdapter" Version="4.5.0" />
+    <PackageReference Include="System.Collections.Immutable" Version="6.0.0" />
   </ItemGroup>
 
   <ItemGroup>

src/nunit.analyzers/Constants/AnalyzerIdentifiers.cs

@@ -93,6 +93,7 @@ internal static class AnalyzerIdentifiers
 
         internal const string DereferencePossibleNullReference = "NUnit3001";
         internal const string NonNullableFieldOrPropertyIsUninitialized = "NUnit3002";
+        internal const string AvoidUninstantiatedInternalClasses = "NUnit3003";
 
         #endregion
     }