*

Masuit.Tools.Abstractions/Systems/LookupX.cs

@@ -61,9 +61,9 @@ public class Grouping<TKey, TElement> : IEnumerable<TElement>
 
     public TKey Key { get; }
 
-    public void Deconstruct(out TKey key, out IEnumerable<TElement> elements)
+    public void Deconstruct(out TKey key, out List<TElement> elements)
     {
         key = Key;
         elements = _list;
     }
-}
+}

Test/Masuit.Tools.Abstractions.Test/Extensions/BaseType/IDictionaryExtensionsTests.cs

@@ -202,6 +202,20 @@ public class IDictionaryExtensionsTests
         Assert.Single(lookup["b"]);
     }
 
+    [Fact]
+    public void ToLookupX_ShouldConvertToLookup2()
+    {
+        // Arrange
+        var list = new List<string> { "a", "b", "a" };
+
+        // Act
+        var lookup = list.ToLookupX(x => x, s => list.IndexOf(s));
+
+        // Assert
+        Assert.Equal(2, lookup["a"].Count);
+        Assert.Single(lookup["b"]);
+    }
+
     [Fact]
     public async Task ToLookupAsync_ShouldConvertToLookup()
     {