DYN-6985 Get item from list with negative index (Wishlist #187) (#15205)

Co-authored-by: Aaron (Qilong) <[email protected]>

src/Libraries/CoreNodes/List.cs

@@ -625,6 +625,16 @@ public static IList ShiftIndices(IList list, int amount)
         [IsVisibleInDynamoLibrary(true)]
         public static object GetItemAtIndex(IList list, int index)
         {
+            if (index<0)
+            {
+                index = list.Count + index;
+            }
+
+            // When calculated index is more than list count or still negative, throw exception
+            if (index >= list.Count || index < 0)
+            {
+                throw new IndexOutOfRangeException();
+            }
             return list[index];
         }
 

test/Libraries/CoreNodesTests/ListTests.cs

@@ -2,6 +2,7 @@
 using System.Collections;
 using System.Collections.Generic;
 using System.Linq;
+using Dynamo.Graph.Nodes;
 using NUnit.Framework;
 using List = DSCore.List;
 
@@ -500,6 +501,35 @@ public static void GetFromList()
             Assert.AreEqual(2, List.GetItemAtIndex(new List<int> { 0, 1, 2, 3 }, 2));
         }
 
+        [Test]
+        [Category("UnitTests")]
+        public static void GetNegtiveIndexItemFromList()
+        {
+            Assert.AreEqual(3, List.GetItemAtIndex(new List<int> { 0, 1, 2, 3 }, -1));
+        }
+
+        [Test]
+        [Category("UnitTests")]
+        public static void GetNegtiveIndexItemFromListCouldThrow()
+        {
+            Assert.Throws<IndexOutOfRangeException>(() =>
+            {
+                // -7 as index argument will cause exception.
+                List.GetItemAtIndex(new List<int> { 0, 1, 2, 3 }, -7);
+            });
+        }
+
+        [Test]
+        [Category("UnitTests")]
+        public static void GetPositiveIndexItemFromListCouldThrow()
+        {
+            Assert.Throws<IndexOutOfRangeException>(() =>
+            {
+                // 7 as index argument will cause exception.
+                List.GetItemAtIndex(new List<int> { 0, 1, 2, 3 }, 7);
+            });
+        }
+
         [Test]
         [Category("UnitTests")]
         public static void TakeListSlice()