diff --git a/csharp/extractor/Semmle.Extraction.CIL/Context.cs b/csharp/extractor/Semmle.Extraction.CIL/Context.cs
index 825a05b5ad1..95e59f1544a 100644
--- a/csharp/extractor/Semmle.Extraction.CIL/Context.cs
+++ b/csharp/extractor/Semmle.Extraction.CIL/Context.cs
@@ -12,7 +12,7 @@ namespace Semmle.Extraction.CIL
     /// Adds additional context that is specific for CIL extraction.
     /// One context = one DLL/EXE.
     /// </summary>
-    partial class Context : IDisposable
+    sealed partial class Context : IDisposable
     {
         readonly FileStream stream;
         Entities.Assembly? assemblyNull;
@@ -58,7 +58,7 @@ namespace Semmle.Extraction.CIL
             }
         }
 
-        void IDisposable.Dispose()
+        public void Dispose()
         {
             if (pdb != null)
                 pdb.Dispose();
diff --git a/csharp/extractor/Semmle.Extraction.CIL/Factories.cs b/csharp/extractor/Semmle.Extraction.CIL/Factories.cs
index c6fb673c940..841afb99a4b 100644
--- a/csharp/extractor/Semmle.Extraction.CIL/Factories.cs
+++ b/csharp/extractor/Semmle.Extraction.CIL/Factories.cs
@@ -9,7 +9,7 @@ namespace Semmle.Extraction.CIL
     /// <summary>
     /// Provides methods for creating and caching various entities.
     /// </summary>
-    public partial class Context
+    public sealed partial class Context
     {
         readonly Dictionary<object, Label> ids = new Dictionary<object, Label>();
 
diff --git a/csharp/extractor/Semmle.Extraction.CIL/PDB/MetadataPdbReader.cs b/csharp/extractor/Semmle.Extraction.CIL/PDB/MetadataPdbReader.cs
index ab929af0499..4901d9f54cb 100644
--- a/csharp/extractor/Semmle.Extraction.CIL/PDB/MetadataPdbReader.cs
+++ b/csharp/extractor/Semmle.Extraction.CIL/PDB/MetadataPdbReader.cs
@@ -13,7 +13,7 @@ namespace Semmle.Extraction.PDB
     ///
     /// PDB information can be in a separate PDB file, or embedded in the DLL.
     /// </summary>
-    class MetadataPdbReader : IPdb
+    sealed class MetadataPdbReader : IPdb
     {
         class SourceFile : ISourceFile
         {
diff --git a/csharp/extractor/Semmle.Extraction.CIL/PDB/NativePdbReader.cs b/csharp/extractor/Semmle.Extraction.CIL/PDB/NativePdbReader.cs
index 052b4f69c8d..f6cca049ed6 100644
--- a/csharp/extractor/Semmle.Extraction.CIL/PDB/NativePdbReader.cs
+++ b/csharp/extractor/Semmle.Extraction.CIL/PDB/NativePdbReader.cs
@@ -14,7 +14,7 @@ namespace Semmle.Extraction.PDB
     /// A PDB reader using Microsoft.DiaSymReader.Native.
     /// This is an unmanaged Windows DLL, which therefore only works on Windows.
     /// </summary>
-    class NativePdbReader : IPdb
+    sealed class NativePdbReader : IPdb
     {
         sealed class Document : ISourceFile
         {
diff --git a/csharp/extractor/Semmle.Extraction.CSharp.Standalone/BuildAnalysis.cs b/csharp/extractor/Semmle.Extraction.CSharp.Standalone/BuildAnalysis.cs
index 3f0a46f7969..dc0c4145dec 100644
--- a/csharp/extractor/Semmle.Extraction.CSharp.Standalone/BuildAnalysis.cs
+++ b/csharp/extractor/Semmle.Extraction.CSharp.Standalone/BuildAnalysis.cs
@@ -46,7 +46,7 @@ namespace Semmle.BuildAnalyser
     /// <summary>
     /// Main implementation of the build analysis.
     /// </summary>
-    class BuildAnalysis : IBuildAnalysis, IDisposable
+    sealed class BuildAnalysis : IBuildAnalysis, IDisposable
     {
         private readonly AssemblyCache assemblyCache;
         private readonly IProgressMonitor progressMonitor;
diff --git a/csharp/extractor/Semmle.Extraction.CSharp.Standalone/Program.cs b/csharp/extractor/Semmle.Extraction.CSharp.Standalone/Program.cs
index 0dc72f74237..ea03768f856 100644
--- a/csharp/extractor/Semmle.Extraction.CSharp.Standalone/Program.cs
+++ b/csharp/extractor/Semmle.Extraction.CSharp.Standalone/Program.cs
@@ -22,7 +22,7 @@ namespace Semmle.Extraction.CSharp.Standalone
     /// <summary>
     ///     Searches for source/references and creates separate extractions.
     /// </summary>
-    class Analysis : IDisposable
+    sealed class Analysis : IDisposable
     {
         public Analysis(ILogger logger, Options options)
         {
diff --git a/csharp/extractor/Semmle.Extraction.CSharp/Analyser.cs b/csharp/extractor/Semmle.Extraction.CSharp/Analyser.cs
index 6b5b2fac317..e9df51880ae 100644
--- a/csharp/extractor/Semmle.Extraction.CSharp/Analyser.cs
+++ b/csharp/extractor/Semmle.Extraction.CSharp/Analyser.cs
@@ -15,7 +15,7 @@ namespace Semmle.Extraction.CSharp
     /// <summary>
     /// Encapsulates a C# analysis task.
     /// </summary>
-    public class Analyser : IDisposable
+    public sealed class Analyser : IDisposable
     {
         IExtractor extractor;
 
diff --git a/csharp/extractor/Semmle.Extraction.Tests/Layout.cs b/csharp/extractor/Semmle.Extraction.Tests/Layout.cs
index e7f06b02307..26890460fdf 100644
--- a/csharp/extractor/Semmle.Extraction.Tests/Layout.cs
+++ b/csharp/extractor/Semmle.Extraction.Tests/Layout.cs
@@ -199,7 +199,7 @@ namespace Semmle.Extraction.Tests
                 new Semmle.Extraction.Layout(null, null, "layout.txt"));
         }
 
-        class LoggerMock : ILogger
+        sealed class LoggerMock : ILogger
         {
             public void Dispose() { }
 
diff --git a/csharp/extractor/Semmle.Extraction.Tests/TrapWriter.cs b/csharp/extractor/Semmle.Extraction.Tests/TrapWriter.cs
index a1d45c2bdc2..aff2d6a6b45 100644
--- a/csharp/extractor/Semmle.Extraction.Tests/TrapWriter.cs
+++ b/csharp/extractor/Semmle.Extraction.Tests/TrapWriter.cs
@@ -42,7 +42,7 @@ namespace Semmle.Extraction.Tests
             Assert.Equal(@"C:\Temp\source_archive\diskstation\share\source\def.cs", TrapWriter.NestPaths(logger, @"C:\Temp\source_archive", $@"{root3}{root3}diskstation\share\source\def.cs").Replace('/', '\\'));
         }
 
-        class LoggerMock : ILogger
+        sealed class LoggerMock : ILogger
         {
             public void Dispose() { }
 
diff --git a/csharp/extractor/Semmle.Extraction/Context.cs b/csharp/extractor/Semmle.Extraction/Context.cs
index 8e11b937c18..d075e18c93e 100644
--- a/csharp/extractor/Semmle.Extraction/Context.cs
+++ b/csharp/extractor/Semmle.Extraction/Context.cs
@@ -283,7 +283,7 @@ namespace Semmle.Extraction
 
         public IDisposable StackGuard => new ScopeGuard(this);
 
-        private class ScopeGuard : IDisposable
+        private sealed class ScopeGuard : IDisposable
         {
             readonly Context cx;
 
diff --git a/csharp/extractor/Semmle.Util.Tests/CanonicalPathCache.cs b/csharp/extractor/Semmle.Util.Tests/CanonicalPathCache.cs
index 1c1d1e9d627..97a6d885ae6 100644
--- a/csharp/extractor/Semmle.Util.Tests/CanonicalPathCache.cs
+++ b/csharp/extractor/Semmle.Util.Tests/CanonicalPathCache.cs
@@ -6,7 +6,7 @@ using System;
 
 namespace SemmleTests.Semmle.Util
 {
-    public class CanonicalPathCacheTest : IDisposable
+    public sealed class CanonicalPathCacheTest : IDisposable
     {
         readonly ILogger Logger = new LoggerMock();
         readonly string root;
@@ -24,9 +24,10 @@ namespace SemmleTests.Semmle.Util
             root = Win32.IsWindows() ? @"X:\" : "/";
         }
 
-        void IDisposable.Dispose()
+        public void Dispose()
         {
             File.Delete("abc");
+            Logger.Dispose();
         }
 
         [Fact]
@@ -172,7 +173,7 @@ namespace SemmleTests.Semmle.Util
             CanonicalPathDots();
         }
 
-        class LoggerMock : ILogger
+        sealed class LoggerMock : ILogger
         {
             public void Dispose() { }
 
diff --git a/csharp/extractor/Semmle.Util.Tests/LongPaths.cs b/csharp/extractor/Semmle.Util.Tests/LongPaths.cs
index 9a03fbc2013..d20d3a28b54 100644
--- a/csharp/extractor/Semmle.Util.Tests/LongPaths.cs
+++ b/csharp/extractor/Semmle.Util.Tests/LongPaths.cs
@@ -10,20 +10,20 @@ namespace SemmleTests.Semmle.Util
     /// Ensure that the Extractor works with long paths.
     /// These should be handled by .NET Core.
     /// </summary>
-    public class LongPaths : IDisposable
+    public sealed class LongPaths : IDisposable
     {
         static readonly string tmpDir = Path.GetTempPath();
         static readonly string shortPath = Path.Combine(tmpDir, "test.txt");
         static readonly string longPath = Path.Combine(tmpDir, "aaaaaaaaaaaaaaaaaaaaaaaaaaaa", "bbbbbbbbbbbbbbbbbbbbbbbbbbbbb",
             "ccccccccccccccccccccccccccccccc", "ddddddddddddddddddddddddddddddddddddd", "eeeeeeeeeeeeeeeeeeeeeeeeeeeeeee", "fffffffffffffffffffffffffffffffff",
-            "ggggggggggggggggggggggggggggggggggg","hhhhhhhhhhhhhhhhhhhhhhhhhhhhhh","iiiiiiiiiiiiiiii.txt");
+            "ggggggggggggggggggggggggggggggggggg", "hhhhhhhhhhhhhhhhhhhhhhhhhhhhhh", "iiiiiiiiiiiiiiii.txt");
 
         public LongPaths()
         {
             CleanUp();
         }
 
-        void IDisposable.Dispose()
+        public void Dispose()
         {
             CleanUp();
         }
@@ -34,14 +34,14 @@ namespace SemmleTests.Semmle.Util
             {
                 File.Delete(shortPath);
             }
-            catch(DirectoryNotFoundException)
+            catch (DirectoryNotFoundException)
             {
             }
             try
             {
                 File.Delete(longPath);
             }
-            catch(DirectoryNotFoundException)
+            catch (DirectoryNotFoundException)
             {
             }
         }
diff --git a/csharp/extractor/Semmle.Util/Logger.cs b/csharp/extractor/Semmle.Util/Logger.cs
index 13877448093..5ef5c8fd039 100644
--- a/csharp/extractor/Semmle.Util/Logger.cs
+++ b/csharp/extractor/Semmle.Util/Logger.cs
@@ -55,7 +55,7 @@ namespace Semmle.Util.Logging
     /// A logger that outputs to a <code>csharp.log</code>
     /// file.
     /// </summary>
-    public class FileLogger : ILogger
+    public sealed class FileLogger : ILogger
     {
         readonly StreamWriter writer;
         readonly Verbosity verbosity;
@@ -100,7 +100,7 @@ namespace Semmle.Util.Logging
     /// <summary>
     /// A logger that outputs to stdout/stderr.
     /// </summary>
-    public class ConsoleLogger : ILogger
+    public sealed class ConsoleLogger : ILogger
     {
         readonly Verbosity verbosity;
 
@@ -143,7 +143,7 @@ namespace Semmle.Util.Logging
     /// <summary>
     /// A combined logger.
     /// </summary>
-    public class CombinedLogger : ILogger
+    public sealed class CombinedLogger : ILogger
     {
         readonly ILogger logger1;
         readonly ILogger logger2;