Update applications/evaluation/Evaluators/Faithfulness/FaithfulnessEvaluator.cs

Author: dluc

applications/evaluation/Evaluators/Faithfulness/FaithfulnessEvaluator.cs

@@ -1,5 +1,6 @@
 // Copyright (c) Microsoft. All rights reserved.
 
+using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Text.Json;