diff --git a/ICSharpCode.Decompiler.Tests/RoundtripAssembly.cs b/ICSharpCode.Decompiler.Tests/RoundtripAssembly.cs
index dc1808f10..6fc0feea9 100644
--- a/ICSharpCode.Decompiler.Tests/RoundtripAssembly.cs
+++ b/ICSharpCode.Decompiler.Tests/RoundtripAssembly.cs
@@ -290,8 +290,8 @@ namespace ICSharpCode.Decompiler.Roundtrip
 
 		class TestProjectDecompiler : WholeProjectDecompiler
 		{
-			public TestProjectDecompiler(Guid projectGuid, IAssemblyResolver resolver, AssemblyReferenceClassifier assemblyReferenceClassifier, DecompilerSettings settings)
-				: base(settings, projectGuid, resolver, null, assemblyReferenceClassifier, debugInfoProvider: null)
+			public TestProjectDecompiler(Guid projectGuid, IAssemblyResolver resolver, IAssemblyReferenceClassifier assemblyReferenceClassifier, DecompilerSettings settings)
+				: base(settings, projectGuid, resolver, IProjectFileWriter.FromSettings(settings), assemblyReferenceClassifier, debugInfoProvider: null)
 			{
 			}
 		}
diff --git a/ICSharpCode.Decompiler/CSharp/ProjectDecompiler/IProjectInfoProvider.cs b/ICSharpCode.Decompiler/CSharp/ProjectDecompiler/IProjectInfoProvider.cs
index cd29f1d80..cdc1ec2db 100644
--- a/ICSharpCode.Decompiler/CSharp/ProjectDecompiler/IProjectInfoProvider.cs
+++ b/ICSharpCode.Decompiler/CSharp/ProjectDecompiler/IProjectInfoProvider.cs
@@ -32,7 +32,7 @@ namespace ICSharpCode.Decompiler.CSharp.ProjectDecompiler
 		/// </summary>
 		IAssemblyResolver AssemblyResolver { get; }
 
-		AssemblyReferenceClassifier AssemblyReferenceClassifier { get; }
+		IAssemblyReferenceClassifier AssemblyReferenceClassifier { get; }
 
 		/// <summary>
 		/// Gets the C# language version of the project.
diff --git a/ICSharpCode.Decompiler/CSharp/ProjectDecompiler/WholeProjectDecompiler.cs b/ICSharpCode.Decompiler/CSharp/ProjectDecompiler/WholeProjectDecompiler.cs
index 067c03010..5ded93304 100644
--- a/ICSharpCode.Decompiler/CSharp/ProjectDecompiler/WholeProjectDecompiler.cs
+++ b/ICSharpCode.Decompiler/CSharp/ProjectDecompiler/WholeProjectDecompiler.cs
@@ -73,7 +73,7 @@ namespace ICSharpCode.Decompiler.CSharp.ProjectDecompiler
 
 		public IAssemblyResolver AssemblyResolver { get; }
 
-		public AssemblyReferenceClassifier AssemblyReferenceClassifier { get; }
+		public IAssemblyReferenceClassifier AssemblyReferenceClassifier { get; }
 
 		public IDebugInfoProvider DebugInfoProvider { get; }
 
@@ -111,9 +111,9 @@ namespace ICSharpCode.Decompiler.CSharp.ProjectDecompiler
 			DecompilerSettings settings,
 			IAssemblyResolver assemblyResolver,
 			IProjectFileWriter projectWriter,
-			AssemblyReferenceClassifier assemblyReferenceClassifier,
+			IAssemblyReferenceClassifier assemblyReferenceClassifier,
 			IDebugInfoProvider debugInfoProvider)
-			: this(settings, Guid.NewGuid(), assemblyResolver, projectWriter, assemblyReferenceClassifier, debugInfoProvider)
+			: this(settings, Guid.NewGuid(), assemblyResolver, projectWriter ?? IProjectFileWriter.FromSettings(settings), assemblyReferenceClassifier ?? new AssemblyReferenceClassifier(), debugInfoProvider)
 		{
 		}
 
@@ -122,15 +122,15 @@ namespace ICSharpCode.Decompiler.CSharp.ProjectDecompiler
 			Guid projectGuid,
 			IAssemblyResolver assemblyResolver,
 			IProjectFileWriter projectWriter,
-			AssemblyReferenceClassifier assemblyReferenceClassifier,
+			IAssemblyReferenceClassifier assemblyReferenceClassifier,
 			IDebugInfoProvider debugInfoProvider)
 		{
 			Settings = settings ?? throw new ArgumentNullException(nameof(settings));
 			ProjectGuid = projectGuid;
 			AssemblyResolver = assemblyResolver ?? throw new ArgumentNullException(nameof(assemblyResolver));
-			AssemblyReferenceClassifier = assemblyReferenceClassifier ?? new AssemblyReferenceClassifier();
+			AssemblyReferenceClassifier = assemblyReferenceClassifier ?? throw new ArgumentNullException(nameof(assemblyReferenceClassifier));
 			DebugInfoProvider = debugInfoProvider;
-			this.projectWriter = projectWriter ?? IProjectFileWriter.FromSettings(settings);
+			this.projectWriter = projectWriter ?? throw new ArgumentNullException(nameof(projectWriter));
 		}
 
 		// per-run members
diff --git a/ICSharpCode.Decompiler/Metadata/AssemblyReferences.cs b/ICSharpCode.Decompiler/Metadata/AssemblyReferences.cs
index d59ff406e..834334aa8 100644
--- a/ICSharpCode.Decompiler/Metadata/AssemblyReferences.cs
+++ b/ICSharpCode.Decompiler/Metadata/AssemblyReferences.cs
@@ -67,7 +67,13 @@ namespace ICSharpCode.Decompiler.Metadata
 #endif
 	}
 
-	public class AssemblyReferenceClassifier
+	public interface IAssemblyReferenceClassifier
+	{
+		bool IsGacAssembly(IAssemblyReference reference);
+		bool IsSharedAssembly(IAssemblyReference reference, [NotNullWhen(true)] out string? runtimePack);
+	}
+
+	public class AssemblyReferenceClassifier : IAssemblyReferenceClassifier
 	{
 		/// <summary>
 		/// For GAC assembly references, the WholeProjectDecompiler will omit the HintPath in the
diff --git a/ICSharpCode.ILSpyCmd/BamlAwareWholeProjectDecompiler.cs b/ICSharpCode.ILSpyCmd/BamlAwareWholeProjectDecompiler.cs
index 9fd07c685..c03334e09 100644
--- a/ICSharpCode.ILSpyCmd/BamlAwareWholeProjectDecompiler.cs
+++ b/ICSharpCode.ILSpyCmd/BamlAwareWholeProjectDecompiler.cs
@@ -37,7 +37,7 @@ namespace ICSharpCode.ILSpyCmd
 		public BamlAwareWholeProjectDecompiler(
 			DecompilerSettings settings,
 			IAssemblyResolver assemblyResolver,
-			AssemblyReferenceClassifier assemblyReferenceClassifier,
+			IAssemblyReferenceClassifier assemblyReferenceClassifier,
 			IDebugInfoProvider debugInfoProvider,
 			BamlDecompilerTypeSystem bamlTypeSystem,
 			BamlDecompilerSettings bamlSettings)
diff --git a/ILSpy/Languages/CSharpLanguage.cs b/ILSpy/Languages/CSharpLanguage.cs
index fc027541e..67edae098 100644
--- a/ILSpy/Languages/CSharpLanguage.cs
+++ b/ILSpy/Languages/CSharpLanguage.cs
@@ -581,7 +581,7 @@ namespace ICSharpCode.ILSpy.Languages
 				DecompilerSettings settings,
 				IAssemblyResolver resolver,
 				IProjectFileWriter? projectWriter,
-				AssemblyReferenceClassifier? assemblyReferenceClassifier,
+				IAssemblyReferenceClassifier? assemblyReferenceClassifier,
 				ICSharpCode.Decompiler.DebugInfo.IDebugInfoProvider? debugInfoProvider)
 				: base(settings, resolver, projectWriter!, assemblyReferenceClassifier!, debugInfoProvider!)
 			{