Reproduce S2583 FP: indexer does not throw exception (#9585)

analyzers/tests/SonarAnalyzer.Test/TestCases/SymbolicExecution/Roslyn/ConditionEvaluatesToConstant.cs

@@ -5,6 +5,7 @@
 using System.IO;
 using System.Linq;
 using System.Timers;
+using System.Collections.Specialized;
 
 namespace Tests.Diagnostics
 {
@@ -4310,3 +4311,20 @@ void Test(List<Repro9425> aList)
         }
     }
 }
+
+// https://github.com/SonarSource/sonar-dotnet/issues/9580
+public class Repro_9580
+{
+    public void IndexerReturnsNullInsteadOfThrowingException(NameValueCollection collection)
+    {
+        var element = collection["key"];
+        if (element != null)
+        {
+            Console.WriteLine(element.ToString());
+        }
+        if (collection.Count == 0)          // Noncompliant - FP: the indexer with string argument returns null if the key is not found rather than throwing an exception,
+        {                                   // so at this point we can't know for sure that the collection is not empty.
+            Console.WriteLine("Empty!");    // Secondary - FP
+        }
+    }
+}