diff --git a/NUnit.Agent.Core.slnx b/NUnit.Agent.Core.slnx
index f692e6a..a4caaad 100644
--- a/NUnit.Agent.Core.slnx
+++ b/NUnit.Agent.Core.slnx
@@ -9,6 +9,9 @@
     <File Path="build.ps1" />
     <File Path="build.sh" />
     <File Path="NuGet.config" />
+    <File Path="src/Directory.Build.props" />
+    <File Path="src/Directory.Packages.props" />
+    <File Path="src/Nullable.props" />
   </Folder>
   <Folder Name="/Solution Items/.config/">
     <File Path=".config/dotnet-tools.json" />
diff --git a/src/nunit.agent.core.tests/Runners/TestAgentRunnerTests.cs b/src/nunit.agent.core.tests/Runners/TestAgentRunnerTests.cs
index b995197..57bf029 100644
--- a/src/nunit.agent.core.tests/Runners/TestAgentRunnerTests.cs
+++ b/src/nunit.agent.core.tests/Runners/TestAgentRunnerTests.cs
@@ -14,7 +14,7 @@ namespace NUnit.Engine.Runners
 #if NETFRAMEWORK
     [TestFixture(typeof(TestDomainRunner))]
 #endif
-    public class TestAgentRunnerTests<TRunner> : ITestEventListener
+    internal class TestAgentRunnerTests<TRunner> : ITestEventListener
         where TRunner : TestAgentRunner
     {
         protected TestPackage _package;
diff --git a/src/nunit.agent.core/CallbackHandler.cs b/src/nunit.agent.core/CallbackHandler.cs
index cec5743..f7c21d7 100644
--- a/src/nunit.agent.core/CallbackHandler.cs
+++ b/src/nunit.agent.core/CallbackHandler.cs
@@ -6,7 +6,7 @@
 
 namespace NUnit.Engine
 {
-    public class CallbackHandler : MarshalByRefObject, ICallbackEventHandler
+    internal class CallbackHandler : MarshalByRefObject, ICallbackEventHandler
     {
         public string? Result { get; private set; }
 
diff --git a/src/nunit.agent.core/Drivers/DriverService.cs b/src/nunit.agent.core/Drivers/DriverService.cs
index cea15c1..7233315 100644
--- a/src/nunit.agent.core/Drivers/DriverService.cs
+++ b/src/nunit.agent.core/Drivers/DriverService.cs
@@ -15,7 +15,7 @@ namespace NUnit.Engine.Drivers
     /// The DriverService provides drivers able to load and run tests
     /// using various frameworks.
     /// </summary>
-    public class DriverService : IDriverService
+    internal class DriverService : IDriverService
     {
 #if NETFRAMEWORK
         private const string TYPE_EXTENSION_PATH = "/NUnit/Engine/TypeExtensions/";
diff --git a/src/nunit.agent.core/Drivers/InvalidAssemblyFrameworkDriver.cs b/src/nunit.agent.core/Drivers/InvalidAssemblyFrameworkDriver.cs
index ae01a40..87a352f 100644
--- a/src/nunit.agent.core/Drivers/InvalidAssemblyFrameworkDriver.cs
+++ b/src/nunit.agent.core/Drivers/InvalidAssemblyFrameworkDriver.cs
@@ -8,7 +8,7 @@
 
 namespace NUnit.Engine.Drivers
 {
-    public sealed class InvalidAssemblyFrameworkDriver : IFrameworkDriver
+    internal sealed class InvalidAssemblyFrameworkDriver : IFrameworkDriver
     {
         private readonly string _name;
         private readonly string _fullname;
diff --git a/src/nunit.agent.core/Drivers/NUnit2DriverFactory.cs b/src/nunit.agent.core/Drivers/NUnit2DriverFactory.cs
index 3d730da..5123e42 100644
--- a/src/nunit.agent.core/Drivers/NUnit2DriverFactory.cs
+++ b/src/nunit.agent.core/Drivers/NUnit2DriverFactory.cs
@@ -8,7 +8,7 @@
 
 namespace NUnit.Engine.Drivers
 {
-    public class NUnit2DriverFactory : IDriverFactory
+    internal class NUnit2DriverFactory : IDriverFactory
     {
         private const string NUNIT_FRAMEWORK = "nunit.framework";
         private const string NUNITLITE_FRAMEWORK = "nunitlite";
diff --git a/src/nunit.agent.core/Drivers/NUnit3DriverFactory.cs b/src/nunit.agent.core/Drivers/NUnit3DriverFactory.cs
index 0ed280d..4fb3aad 100644
--- a/src/nunit.agent.core/Drivers/NUnit3DriverFactory.cs
+++ b/src/nunit.agent.core/Drivers/NUnit3DriverFactory.cs
@@ -6,7 +6,7 @@
 
 namespace NUnit.Engine.Drivers
 {
-    public class NUnit3DriverFactory : IDriverFactory
+    internal class NUnit3DriverFactory : IDriverFactory
     {
         internal const string NUNIT_FRAMEWORK = "nunit.framework";
         private static readonly Logger log = InternalTrace.GetLogger(typeof(NUnit3DriverFactory));
diff --git a/src/nunit.agent.core/Drivers/NUnitFrameworkApi.cs b/src/nunit.agent.core/Drivers/NUnitFrameworkApi.cs
index c4910e1..41a75da 100644
--- a/src/nunit.agent.core/Drivers/NUnitFrameworkApi.cs
+++ b/src/nunit.agent.core/Drivers/NUnitFrameworkApi.cs
@@ -5,7 +5,7 @@
 
 namespace NUnit.Engine.Drivers
 {
-    public interface NUnitFrameworkApi
+    internal interface NUnitFrameworkApi
     {
         /// <summary>
         /// Loads the tests in an assembly.
diff --git a/src/nunit.agent.core/Drivers/NUnitFrameworkApi2018.cs b/src/nunit.agent.core/Drivers/NUnitFrameworkApi2018.cs
index f366e5e..f0281b1 100644
--- a/src/nunit.agent.core/Drivers/NUnitFrameworkApi2018.cs
+++ b/src/nunit.agent.core/Drivers/NUnitFrameworkApi2018.cs
@@ -19,9 +19,9 @@ namespace NUnit.Engine.Drivers
     /// may be used for NUnit 3.10 or higher.
     /// </summary>
 #if NETFRAMEWORK
-    public class NUnitFrameworkApi2018 : MarshalByRefObject, NUnitFrameworkApi
+    internal class NUnitFrameworkApi2018 : MarshalByRefObject, NUnitFrameworkApi
 #else
-    public class NUnitFrameworkApi2018 : NUnitFrameworkApi
+    internal class NUnitFrameworkApi2018 : NUnitFrameworkApi
 #endif
     {
         private static readonly Logger log = InternalTrace.GetLogger(nameof(NUnitFrameworkApi2018));
diff --git a/src/nunit.agent.core/Drivers/NUnitFrameworkDriver.cs b/src/nunit.agent.core/Drivers/NUnitFrameworkDriver.cs
index a2ccf09..9d06acc 100644
--- a/src/nunit.agent.core/Drivers/NUnitFrameworkDriver.cs
+++ b/src/nunit.agent.core/Drivers/NUnitFrameworkDriver.cs
@@ -15,7 +15,7 @@ namespace NUnit.Engine.Drivers
     /// NUnitFrameworkDriver is used by the test-runner to load and run
     /// tests using the NUnit framework assembly, versions 3 and up.
     /// </summary>
-    public class NUnitFrameworkDriver : IFrameworkDriver
+    internal class NUnitFrameworkDriver : IFrameworkDriver
     {
         private static readonly Version MINIMUM_NUNIT_VERSION = new(3, 2, 0);
         private static readonly Logger log = InternalTrace.GetLogger(nameof(NUnitFrameworkDriver));
diff --git a/src/nunit.agent.core/Drivers/ProvidedPathsAssemblyResolver.cs b/src/nunit.agent.core/Drivers/ProvidedPathsAssemblyResolver.cs
index 42a16af..2a987c2 100644
--- a/src/nunit.agent.core/Drivers/ProvidedPathsAssemblyResolver.cs
+++ b/src/nunit.agent.core/Drivers/ProvidedPathsAssemblyResolver.cs
@@ -8,7 +8,7 @@
 
 namespace NUnit.Engine.Drivers
 {
-    public class ProvidedPathsAssemblyResolver
+    internal class ProvidedPathsAssemblyResolver
     {
         private static readonly Logger log = InternalTrace.GetLogger(typeof(ProvidedPathsAssemblyResolver));
 
diff --git a/src/nunit.agent.core/RunTestsCallbackHandler.cs b/src/nunit.agent.core/RunTestsCallbackHandler.cs
index a5c279d..beb6388 100644
--- a/src/nunit.agent.core/RunTestsCallbackHandler.cs
+++ b/src/nunit.agent.core/RunTestsCallbackHandler.cs
@@ -7,7 +7,7 @@
 
 namespace NUnit.Engine
 {
-    public class RunTestsCallbackHandler : MarshalByRefObject, ICallbackEventHandler
+    internal class RunTestsCallbackHandler : MarshalByRefObject, ICallbackEventHandler
     {
         private readonly ITestEventListener? _listener;
 
diff --git a/src/nunit.agent.core/Runners/DomainManager.cs b/src/nunit.agent.core/Runners/DomainManager.cs
index 592411e..6fee33c 100644
--- a/src/nunit.agent.core/Runners/DomainManager.cs
+++ b/src/nunit.agent.core/Runners/DomainManager.cs
@@ -19,7 +19,7 @@ namespace NUnit.Engine.Runners
     /// The DomainManager class handles the creation and unloading
     /// of domains as needed and keeps track of all existing domains.
     /// </summary>
-    public class DomainManager
+    internal class DomainManager
     {
         private static readonly Logger log = InternalTrace.GetLogger(typeof(DomainManager));
 
diff --git a/src/nunit.agent.core/Runners/LocalTestRunner.cs b/src/nunit.agent.core/Runners/LocalTestRunner.cs
index ded26d9..85637ee 100644
--- a/src/nunit.agent.core/Runners/LocalTestRunner.cs
+++ b/src/nunit.agent.core/Runners/LocalTestRunner.cs
@@ -7,7 +7,7 @@ namespace NUnit.Engine.Runners
     /// <summary>
     /// LocalTestRunner runs tests in the current application domain.
     /// </summary>
-    public class LocalTestRunner : TestAgentRunner
+    internal class LocalTestRunner : TestAgentRunner
     {
         public LocalTestRunner(TestPackage package) : base(package)
         {
diff --git a/src/nunit.agent.core/Runners/TestAgentRunner.cs b/src/nunit.agent.core/Runners/TestAgentRunner.cs
index 1494159..ed0d0d9 100644
--- a/src/nunit.agent.core/Runners/TestAgentRunner.cs
+++ b/src/nunit.agent.core/Runners/TestAgentRunner.cs
@@ -18,7 +18,7 @@ namespace NUnit.Engine.Runners
     /// deal directly with a framework driver. It loads and runs tests in a single
     /// assembly, creating an <see cref="IFrameworkDriver"/> to do so.
     /// </summary>
-    public abstract class TestAgentRunner : ITestEngineRunner
+    internal abstract class TestAgentRunner : ITestEngineRunner
     {
         private readonly Logger log = InternalTrace.GetLogger(typeof(TestAgentRunner));
 
diff --git a/src/nunit.agent.core/Runners/TestDomainRunner.cs b/src/nunit.agent.core/Runners/TestDomainRunner.cs
index b48a9ff..61f9865 100644
--- a/src/nunit.agent.core/Runners/TestDomainRunner.cs
+++ b/src/nunit.agent.core/Runners/TestDomainRunner.cs
@@ -8,7 +8,7 @@ namespace NUnit.Engine.Runners
     /// TestDomainRunner loads and runs tests in a separate
     /// domain whose lifetime it controls.
     /// </summary>
-    public class TestDomainRunner : TestAgentRunner
+    internal class TestDomainRunner : TestAgentRunner
     {
         private static readonly Logger log = InternalTrace.GetLogger(typeof(TestDomainRunner));
 
diff --git a/src/nunit.agent.core/TestAssemblyResolver.cs b/src/nunit.agent.core/TestAssemblyResolver.cs
index e983d41..df866d6 100644
--- a/src/nunit.agent.core/TestAssemblyResolver.cs
+++ b/src/nunit.agent.core/TestAssemblyResolver.cs
@@ -131,7 +131,7 @@ private static string GetAssemblyLocationInfo(Assembly assembly)
 
     #region ResolutionStrategy Classes
 
-    public abstract class ResolutionStrategy
+    internal abstract class ResolutionStrategy
     {
         public string Name => GetType().Name;
         public int Calls { get; set; }
@@ -141,7 +141,7 @@ public abstract bool TryToResolve(
             AssemblyLoadContext loadContext, AssemblyName assemblyName, [NotNullWhen(true)] out Assembly? loadedAssembly);
     }
 
-    public class TrustedPlatformAssembliesStrategy : ResolutionStrategy
+    internal class TrustedPlatformAssembliesStrategy : ResolutionStrategy
     {
         private static readonly Logger log = InternalTrace.GetLogger(typeof(TrustedPlatformAssembliesStrategy));
         public override bool TryToResolve(
@@ -181,7 +181,7 @@ bool FileMatchesAssembly(string fileName) =>
         }
     }
 
-    public class RuntimeLibrariesStrategy : ResolutionStrategy
+    internal class RuntimeLibrariesStrategy : ResolutionStrategy
     {
         private static readonly Logger log = InternalTrace.GetLogger(typeof(RuntimeLibrariesStrategy));
 
@@ -244,7 +244,7 @@ public override bool TryToResolve(
         }
     }
 
-    public class AdditionalRuntimesStrategy : ResolutionStrategy
+    internal class AdditionalRuntimesStrategy : ResolutionStrategy
     {
         private string _runtimeName;
         private bool _x86;
@@ -273,14 +273,14 @@ public override bool TryToResolve(AssemblyLoadContext loadContext, AssemblyName
         }
     }
 
-    public class WindowsDesktopStrategy : AdditionalRuntimesStrategy
+    internal class WindowsDesktopStrategy : AdditionalRuntimesStrategy
     {
         public WindowsDesktopStrategy(bool x86) : base("Microsoft.WindowsDesktop.App", x86)
         {
         }
     }
 
-    public class AspNetCoreStrategy : AdditionalRuntimesStrategy
+    internal class AspNetCoreStrategy : AdditionalRuntimesStrategy
     {
         public AspNetCoreStrategy(bool x86) : base("Microsoft.AspNetCore.App", x86)
         {