Don't allow setting Variant to Variant (#38592)

* Don't allow setting Variant to Variant

* pr fb

Author: annelo-msft

sdk/core/Azure.Core.Experimental/src/Variant/Variant.cs

@@ -22,8 +22,16 @@ public readonly partial struct Variant
         /// <param name="value"></param>
         public Variant(object? value)
         {
-            _object = value;
-            _union = default;
+            if (value is Variant variant)
+            {
+                _object = variant._object;
+                _union = variant._union;
+            }
+            else
+            {
+                _object = value;
+                _union = default;
+            }
         }
 
         /// <summary>

sdk/core/Azure.Core.Experimental/tests/Variant/VariantUsage.cs

@@ -39,6 +39,58 @@ public void CanGetAsString(Variant v, Variant s)
             Assert.AreEqual(value, v.ToString());
         }
 
+        [Test]
+        public void VariantDoesntStoreVariant()
+        {
+            Variant a = new("hi");
+            Variant b = new(a);
+
+            Assert.AreEqual(a, b);
+            Assert.AreEqual(typeof(string), b.Type);
+        }
+
+        [Test]
+        public void VariantAssignmentHasReferenceSemantics()
+        {
+            // Variant should use reference semantics with reference types
+            // so that it behaves like object in these cases.
+            //
+            // e.g. since:
+            //   List<string> list = new List<string> { "1" };
+            //   object oa = list;
+            //   object ob = oa;
+            //   list[0] = "2";
+            //
+            //   Assert.AreEqual("2", list[0]);
+            //   Assert.AreEqual("2", ((List<string>)oa)[0]);
+            //   Assert.AreEqual("2", ((List<string>)ob)[0]);
+            //
+            // Variant should do the same.
+            // The following test validates this functionality.
+
+            List<string> list = new List<string> { "1" };
+            Variant a = new(list);
+
+            Assert.AreEqual("1", list[0]);
+            Assert.AreEqual("1", a.As<List<string>>()[0]);
+
+            list[0] = "2";
+
+            Assert.AreEqual("2", list[0]);
+            Assert.AreEqual("2", a.As<List<string>>()[0]);
+
+            Variant b = new(a);
+
+            Assert.AreEqual(a, b);
+            Assert.AreEqual("2", b.As<List<string>>()[0]);
+
+            list[0] = "3";
+
+            Assert.AreEqual("3", list[0]);
+            Assert.AreEqual("3", a.As<List<string>>()[0]);
+            Assert.AreEqual("3", b.As<List<string>>()[0]);
+        }
+
         #region Helpers
         public static IEnumerable<Variant[]> VariantValues()
         {