diff --git a/cpp/autobuilder/Semmle.Autobuild.Cpp.Tests/BuildScripts.cs b/cpp/autobuilder/Semmle.Autobuild.Cpp.Tests/BuildScripts.cs
index 3855428a5ae9..f243bdb54013 100644
--- a/cpp/autobuilder/Semmle.Autobuild.Cpp.Tests/BuildScripts.cs
+++ b/cpp/autobuilder/Semmle.Autobuild.Cpp.Tests/BuildScripts.cs
@@ -203,6 +203,8 @@ internal class TestDiagnosticWriter : IDiagnosticsWriter
         public IList<DiagnosticMessage> Diagnostics { get; } = new List<DiagnosticMessage>();
 
         public void AddEntry(DiagnosticMessage message) => this.Diagnostics.Add(message);
+
+        public void Dispose() { }
     }
 
     /// <summary>
diff --git a/csharp/autobuilder/Semmle.Autobuild.CSharp.Tests/BuildScripts.cs b/csharp/autobuilder/Semmle.Autobuild.CSharp.Tests/BuildScripts.cs
index 2b00d9db742a..3288eb5e852e 100644
--- a/csharp/autobuilder/Semmle.Autobuild.CSharp.Tests/BuildScripts.cs
+++ b/csharp/autobuilder/Semmle.Autobuild.CSharp.Tests/BuildScripts.cs
@@ -218,6 +218,8 @@ internal class TestDiagnosticWriter : IDiagnosticsWriter
         public IList<DiagnosticMessage> Diagnostics { get; } = new List<DiagnosticMessage>();
 
         public void AddEntry(DiagnosticMessage message) => this.Diagnostics.Add(message);
+
+        public void Dispose() { }
     }
 
     /// <summary>
diff --git a/csharp/autobuilder/Semmle.Autobuild.Shared/Autobuilder.cs b/csharp/autobuilder/Semmle.Autobuild.Shared/Autobuilder.cs
index 6fe258abc0e3..2414791ad4ce 100644
--- a/csharp/autobuilder/Semmle.Autobuild.Shared/Autobuilder.cs
+++ b/csharp/autobuilder/Semmle.Autobuild.Shared/Autobuilder.cs
@@ -361,7 +361,7 @@ protected virtual void Dispose(bool disposing)
         {
             if (disposing)
             {
-                (diagnostics as IDisposable)?.Dispose();
+                diagnostics.Dispose();
             }
         }
 
diff --git a/csharp/extractor/Semmle.Util/ToolStatusPage.cs b/csharp/extractor/Semmle.Util/ToolStatusPage.cs
index 1be700654af2..a93cdec5fdf6 100644
--- a/csharp/extractor/Semmle.Util/ToolStatusPage.cs
+++ b/csharp/extractor/Semmle.Util/ToolStatusPage.cs
@@ -183,7 +183,7 @@ public DiagnosticMessage(
     /// <summary>
     /// Provides the ability to write diagnostic messages to some output.
     /// </summary>
-    public interface IDiagnosticsWriter
+    public interface IDiagnosticsWriter : IDisposable
     {
         /// <summary>
         /// Adds <paramref name="message" /> as a new diagnostics entry.