diff --git a/src/Microsoft.CodeAnalysis.Testing/Microsoft.CodeAnalysis.Analyzer.Testing/AnalyzerTest`1.cs b/src/Microsoft.CodeAnalysis.Testing/Microsoft.CodeAnalysis.Analyzer.Testing/AnalyzerTest`1.cs
index 2a8886732..a5b7ec1cf 100644
--- a/src/Microsoft.CodeAnalysis.Testing/Microsoft.CodeAnalysis.Analyzer.Testing/AnalyzerTest`1.cs
+++ b/src/Microsoft.CodeAnalysis.Testing/Microsoft.CodeAnalysis.Analyzer.Testing/AnalyzerTest`1.cs
@@ -930,7 +930,7 @@ private static bool IsInSourceFile(DiagnosticResult result, (string filename, So
 
             diagnostics.AddRange(additionalDiagnostics);
             var results = SortDistinctDiagnostics(diagnostics);
-            return results.ToImmutableArray();
+            return results;
         }
 
         protected virtual Task<Compilation> GetProjectCompilationAsync(Project project, IVerifier verifier, CancellationToken cancellationToken)
@@ -1317,14 +1317,14 @@ protected virtual Workspace CreateWorkspaceImpl()
         /// <param name="diagnostics">A collection of <see cref="Diagnostic"/>s to be sorted.</param>
         /// <returns>A collection containing the input <paramref name="diagnostics"/>, sorted by
         /// <see cref="Diagnostic.Location"/> and <see cref="Diagnostic.Id"/>.</returns>
-        private static (Project project, Diagnostic diagnostic)[] SortDistinctDiagnostics(IEnumerable<(Project project, Diagnostic diagnostic)> diagnostics)
+        protected virtual ImmutableArray<(Project project, Diagnostic diagnostic)> SortDistinctDiagnostics(IEnumerable<(Project project, Diagnostic diagnostic)> diagnostics)
         {
             return diagnostics
                 .OrderBy(d => d.diagnostic.Location.GetLineSpan().Path, StringComparer.Ordinal)
                 .ThenBy(d => d.diagnostic.Location.SourceSpan.Start)
                 .ThenBy(d => d.diagnostic.Location.SourceSpan.End)
                 .ThenBy(d => d.diagnostic.Id)
-                .ThenBy(d => GetArguments(d.diagnostic), LexicographicComparer.Instance).ToArray();
+                .ThenBy(d => GetArguments(d.diagnostic), LexicographicComparer.Instance).ToImmutableArray();
         }
 
         private static IReadOnlyList<object?> GetArguments(Diagnostic diagnostic)
diff --git a/src/Microsoft.CodeAnalysis.Testing/Microsoft.CodeAnalysis.Analyzer.Testing/PublicAPI.Unshipped.txt b/src/Microsoft.CodeAnalysis.Testing/Microsoft.CodeAnalysis.Analyzer.Testing/PublicAPI.Unshipped.txt
index 85a267c28..9a2b5ff44 100644
--- a/src/Microsoft.CodeAnalysis.Testing/Microsoft.CodeAnalysis.Analyzer.Testing/PublicAPI.Unshipped.txt
+++ b/src/Microsoft.CodeAnalysis.Testing/Microsoft.CodeAnalysis.Analyzer.Testing/PublicAPI.Unshipped.txt
@@ -348,6 +348,7 @@ virtual Microsoft.CodeAnalysis.Testing.AnalyzerTest<TVerifier>.GetDefaultDiagnos
 virtual Microsoft.CodeAnalysis.Testing.AnalyzerTest<TVerifier>.GetProjectCompilationAsync(Microsoft.CodeAnalysis.Project project, Microsoft.CodeAnalysis.Testing.IVerifier verifier, System.Threading.CancellationToken cancellationToken) -> System.Threading.Tasks.Task<Microsoft.CodeAnalysis.Compilation>
 virtual Microsoft.CodeAnalysis.Testing.AnalyzerTest<TVerifier>.IsCompilerDiagnosticIncluded(Microsoft.CodeAnalysis.Diagnostic diagnostic, Microsoft.CodeAnalysis.Testing.CompilerDiagnostics compilerDiagnostics) -> bool
 virtual Microsoft.CodeAnalysis.Testing.AnalyzerTest<TVerifier>.RunImplAsync(System.Threading.CancellationToken cancellationToken) -> System.Threading.Tasks.Task
+virtual Microsoft.CodeAnalysis.Testing.AnalyzerTest<TVerifier>.SortDistinctDiagnostics(System.Collections.Generic.IEnumerable<(Microsoft.CodeAnalysis.Project project, Microsoft.CodeAnalysis.Diagnostic diagnostic)> diagnostics) -> System.Collections.Immutable.ImmutableArray<(Microsoft.CodeAnalysis.Project project, Microsoft.CodeAnalysis.Diagnostic diagnostic)>
 virtual Microsoft.CodeAnalysis.Testing.CodeActionTest<TVerifier>.FilterCodeActions(System.Collections.Immutable.ImmutableArray<Microsoft.CodeAnalysis.CodeActions.CodeAction> actions) -> System.Collections.Immutable.ImmutableArray<Microsoft.CodeAnalysis.CodeActions.CodeAction>
 virtual Microsoft.CodeAnalysis.Testing.DefaultVerifier.CreateMessage(string message) -> string
 virtual Microsoft.CodeAnalysis.Testing.DefaultVerifier.Empty<T>(string collectionName, System.Collections.Generic.IEnumerable<T> collection) -> void