Adjust comment and add tests.

src/libraries/Microsoft.Win32.Registry/src/Microsoft/Win32/RegistryKey.cs

@@ -334,22 +334,23 @@ public void DeleteSubKeyTree(string subkey, bool throwOnMissingSubKey)
 
             subkey = FixupName(subkey); // Fixup multiple slashes to a single slash
 
-            RegistryKey? key = InternalOpenSubKeyWithoutSecurityChecks(subkey, false);
+            // If the key has values, it must be opened with KEY_SET_VALUE,
+            // or RegDeleteTree will fail with ERROR_ACCESS_DENIED.
+
+            RegistryKey? key = InternalOpenSubKeyWithoutSecurityChecks(subkey, true);
             if (key != null)
             {
                 using (key)
                 {
-                    // The access requirement of RegDeleteTree is different with old implementation.
-                    // Open a new handle to the subkey to restore old behavior.
                     int ret = Interop.Advapi32.RegDeleteTree(key._hkey, string.Empty);
                     if (ret != 0)
                     {
                         Win32Error(ret, null);
                     }
 
-                    // RegDeleteTree only delete subkeys and values of the key
-                    // if subkey is null.
-                    // Also delete the key.
+                    // RegDeleteTree doesn't self-delete when lpSubKey is empty.
+                    // Manually delete the key to restore old behavior.
+
                     ret = Interop.Advapi32.RegDeleteKeyEx(key._hkey, string.Empty, (int)_regView, 0);
                     if (ret != 0)
                     {

src/libraries/Microsoft.Win32.Registry/tests/RegistryKey/RegistryKey_DeleteSubKeyTree_str.cs

@@ -3,7 +3,6 @@
 
 using System;
 using System.Linq;
-using System.Reflection;
 using Xunit;
 
 namespace Microsoft.Win32.RegistryTests
@@ -49,6 +48,38 @@ public void SelfDeleteTest()
 
             Assert.Null(TestRegistryKey.OpenSubKey(TestRegistryKeyName));
         }
+
+        [Fact]
+        public void SelfDeleteWithValuesTest()
+        {
+            using (var rk = TestRegistryKey.CreateSubKey(TestRegistryKeyName))
+            {
+                rk.SetValue("VAL", "Dummy", RegistryValueKind.String);
+                rk.SetDefaultValue("Default");
+                using RegistryKey created = rk.CreateSubKey(TestRegistryKeyName);
+                created.SetValue("Value", 42, RegistryValueKind.DWord);
+                rk.DeleteSubKeyTree("");
+            }
+
+            Assert.Null(TestRegistryKey.OpenSubKey(TestRegistryKeyName));
+        }
+
+        [Fact]
+        public void SelfDeleteWithValuesTest_AnotherHandlePresent()
+        {
+            using (var rk = TestRegistryKey.CreateSubKey(TestRegistryKeyName))
+            {
+                rk.SetValue("VAL", "Dummy", RegistryValueKind.String);
+                rk.SetDefaultValue("Default");
+                using RegistryKey created = rk.CreateSubKey(TestRegistryKeyName);
+                created.SetValue("Value", 42, RegistryValueKind.DWord);
+
+                using var rk2 = TestRegistryKey.OpenSubKey(TestRegistryKeyName);
+                rk.DeleteSubKeyTree("");
+            }
+
+            Assert.Null(TestRegistryKey.OpenSubKey(TestRegistryKeyName));
+        }
 
         [Fact]
         public void DeleteSubKeyTreeTest()
@@ -85,6 +116,37 @@ public void DeleteSubKeyTreeTest2()
             TestRegistryKey.DeleteSubKeyTree(TestRegistryKeyName);
             Assert.Null(TestRegistryKey.OpenSubKey(TestRegistryKeyName));
         }
+
+        [Fact]
+        public void DeleteSubKeyTreeTest3()
+        {
+            // [] Add in multiple subkeys and then delete the root key
+            string[] subKeyNames = Enumerable.Range(1, 9).Select(x => "BLAH_" + x.ToString()).ToArray();
+
+            using (RegistryKey rk = TestRegistryKey.CreateSubKey(TestRegistryKeyName))
+            {
+                foreach (var subKeyName in subKeyNames)
+                {
+                    using RegistryKey rk2 = rk.CreateSubKey(subKeyName);
+                    Assert.NotNull(rk2);
+
+                    using RegistryKey rk3 = rk2.CreateSubKey("Test");
+                    Assert.NotNull(rk3);
+                }
+
+                Assert.Equal(subKeyNames, rk.GetSubKeyNames());
+
+                // Add multiple values to the key being deleted
+                foreach (int i in Enumerable.Range(1, 9))
+                {
+                    rk.SetValue("STRVAL_" + i, i.ToString(), RegistryValueKind.String);
+                    rk.SetValue("INTVAL_" + i, i, RegistryValueKind.DWord);
+                }
+            }
+
+            TestRegistryKey.DeleteSubKeyTree(TestRegistryKeyName);
+            Assert.Null(TestRegistryKey.OpenSubKey(TestRegistryKeyName));
+        }
 
         [Theory]
         [MemberData(nameof(TestRegistrySubKeyNames))]