DYN-7082: Adding a new node List.StringifyIndexOf (#15440)

Author: reddyashish

src/Libraries/CoreNodes/List.cs

@@ -161,16 +161,17 @@ public static IList SetUnion(IList<object> list1, IList<object> list2)
         }
 
         /// <summary>
-        ///     Returns the index of the element in the given list. Match between given list and target element must be a strict match (i.e. int to int, double to double, string to string, object to object etc.)
+        ///     Returns the index of the element in the given list. Match between given list and target element must be a strict match (i.e. int to int, double to double, string to string, object to object etc.).
         /// </summary>
         /// <param name="list">The list to find the element in.</param>
         /// <param name="element">The element whose index is to be returned.</param>
         /// <returns name="int">The index of the element in the list. Invalid index -1 will be returned if strict match not found.</returns>
         /// <search>index,indexof</search>
         [IsVisibleInDynamoLibrary(true)]
-        public static int IndexOf(IList list, object element)
+        public static int? IndexOf(IList list, object element)
         {
-            return list.IndexOf(element);
+            var index = list.IndexOf(element);
+            return index < 0 ? null : index;
         }
 
         /// <summary>

test/Libraries/CoreNodesTests/ListTests.cs

@@ -2,7 +2,6 @@
 using System.Collections;
 using System.Collections.Generic;
 using System.Linq;
-using Dynamo.Graph.Nodes;
 using NUnit.Framework;
 using List = DSCore.List;
 
@@ -180,7 +179,7 @@ public static void ListSetUnion()
         public static void ListIndexOf()
         {
             Assert.AreEqual(1, List.IndexOf(new ArrayList { "x", "y", 1 }, "y"));
-            Assert.AreEqual(-1, List.IndexOf(new ArrayList { 3, 4, 6, 8 }, 9));
+            Assert.AreEqual(null, List.IndexOf(new ArrayList { 3, 4, 6, 8 }, 9));
         }
 
         [Test]