Fix break in DirectoryServices SearchResultCollection

src/libraries/System.DirectoryServices/src/System/DirectoryServices/SearchResultCollection.cs

@@ -47,15 +47,17 @@ private ArrayList InnerList
             {
                 if (_innerList == null)
                 {
-                    _innerList = new ArrayList();
+                    var eagerList = new ArrayList();
                     var enumerator = new ResultsEnumerator(
                         this,
                         _rootEntry.GetUsername(),
                         _rootEntry.GetPassword(),
                         _rootEntry.AuthenticationType);
 
                     while (enumerator.MoveNext())
-                        _innerList.Add(enumerator.Current);
+                        eagerList.Add(enumerator.Current);
+
+                    _innerList = eagerList;
                 }
 
                 return _innerList;
@@ -188,12 +190,10 @@ protected virtual void Dispose(bool disposing)
 
         public IEnumerator GetEnumerator()
         {
-            // Two ResultsEnumerators can't exist at the same time over the
-            // same object. Need to get a new handle, which means re-querying.
-            return new ResultsEnumerator(this,
-                                                       _rootEntry.GetUsername(),
-                                                       _rootEntry.GetPassword(),
-                                                       _rootEntry.AuthenticationType);
+            if (_innerList != null)
+                return new AlreadyReadResultsEnumerator(_innerList);
+            else
+                return new ResultsEnumerator(this, _rootEntry.GetUsername(), _rootEntry.GetPassword(), _rootEntry.AuthenticationType);
         }
 
         public bool Contains(SearchResult result) => InnerList.Contains(result);
@@ -214,6 +214,49 @@ void ICollection.CopyTo(Array array, int index)
             InnerList.CopyTo(array, index);
         }
 
+        /// <devdoc>
+        /// Supports a simple type-specific wrapper for the underlying cached list
+        /// </devdoc>
+        private sealed class AlreadyReadResultsEnumerator : IEnumerator
+        {
+            private readonly IEnumerator _innerEnumerator;
+
+            internal AlreadyReadResultsEnumerator(ArrayList innerList)
+            {
+                _innerEnumerator = innerList.GetEnumerator();
+            }
+
+            /// <devdoc>
+            /// Gets the current element in the collection.
+            /// </devdoc>
+            public SearchResult Current
+            {
+                get
+                {
+                    return (SearchResult)(_innerEnumerator.Current);
+                }
+            }
+
+            /// <devdoc>
+            /// Advances the enumerator to the next element of the collection
+            /// and returns a Boolean value indicating whether a valid element is available.
+            /// </devdoc>
+            public bool MoveNext()
+            {
+                return _innerEnumerator.MoveNext();
+            }
+
+            /// <devdoc>
+            /// Resets the enumerator back to its initial position before the first element in the collection.
+            /// </devdoc>
+            public void Reset()
+            {
+                _innerEnumerator.Reset();
+            }
+
+            object IEnumerator.Current => Current;
+        }
+
         /// <devdoc>
         /// Supports a simple ForEach-style iteration over a collection.
         /// </devdoc>
@@ -310,6 +353,7 @@ public bool MoveNext()
                     return false;
 
                 _currentResult = null;
+
                 if (!_initialized)
                 {
                     int hr = _results.SearchObject.GetFirstRow(_results.Handle);

src/libraries/System.DirectoryServices/tests/System.DirectoryServices.Tests.csproj

@@ -9,6 +9,7 @@
     <Compile Include="System\DirectoryServices\ActiveDirectory\DomainControllerTests.cs" />
     <Compile Include="System\DirectoryServices\DirectoryEntryTests.cs" />
     <Compile Include="System\DirectoryServices\DirectoryServicesPermissionTests.cs" />
+    <Compile Include="System\DirectoryServices\DirectorySearcherTests.cs" />
     <Compile Include="System\DirectoryServices\DirectoryServicesTests.cs" />
     <Compile Include="System\DirectoryServices\DirectorySynchronizationTests.cs" />
     <Compile Include="System\DirectoryServices\DirectoryVirtualListViewContextTests.cs" />
@@ -20,8 +21,7 @@
     <Compile Include="System\DirectoryServices\ActiveDirectory\ForestTests.cs" />
     <Compile Include="System\DirectoryServices\ActiveDirectory\ActiveDirectoryTests.cs" />
     <Compile Include="System\DirectoryServices\ActiveDirectory\TrustHelperTests.cs" />
-    <Compile Include="$(CommonTestPath)System\DirectoryServices\LdapConfiguration.cs"
-             Link="Common\DirectoryServices\LdapConfiguration.cs" />
+    <Compile Include="$(CommonTestPath)System\DirectoryServices\LdapConfiguration.cs" Link="Common\DirectoryServices\LdapConfiguration.cs" />
   </ItemGroup>
   <ItemGroup>
     <Content Include="$(CommonTestPath)System\DirectoryServices\LDAP.Configuration.xml">

src/libraries/System.DirectoryServices/tests/System/DirectoryServices/DirectorySearcherTests.cs

@@ -0,0 +1,83 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Runtime.InteropServices;
+using System.Collections.Generic;
+using System.Collections;
+using Xunit;
+using Xunit.Sdk;
+using System.Reflection;
+
+namespace System.DirectoryServices.Tests
+{
+    public partial class DirectorySearcherTests
+    {
+        internal static bool IsLdapConfigurationExist => LdapConfiguration.Configuration != null;
+        internal static bool IsActiveDirectoryServer => IsLdapConfigurationExist && LdapConfiguration.Configuration.IsActiveDirectoryServer;
+
+        private const int ADS_SYSTEMFLAG_CR_NTDS_NC = 0x1;
+        private const int ADS_SYSTEMFLAG_CR_NTDS_DOMAIN = 0x2;
+
+        [ConditionalFact(nameof(IsLdapConfigurationExist))]
+        public void DirectorySearch_IteratesCorrectly_SimpleEnumeration()
+        {
+            var e = GetDomains();
+            Assert.NotNull(e);
+
+            foreach (var result in e)
+            {
+                Assert.NotNull(result);
+            }
+        }
+
+        [ConditionalFact(nameof(IsLdapConfigurationExist))]
+        public void DirectorySearch_IteratesCorrectly_AfterCount()
+        {
+            var e = GetDomains();
+            Assert.NotNull(e);
+            Assert.NotEqual(0, e.Count);
+
+            foreach (var result in e)
+            {
+                Assert.NotNull(result);
+            }
+        }
+
+
+        [ConditionalFact(nameof(IsLdapConfigurationExist))]
+        public void DirectorySearch_IteratesCorrectly_MixedCount()
+        {
+            var e = GetDomains();
+            Assert.NotNull(e);
+            Assert.NotEqual(0, e.Count);
+
+            foreach (var result in e)
+            {
+                Assert.NotEqual(0, e.Count);
+                Assert.NotNull(result);
+            }
+        }
+
+        private static SearchResultCollection GetDomains()
+        {
+            using var entry = new DirectoryEntry("LDAP://rootDSE");
+            var namingContext = entry.Properties["configurationNamingContext"][0]!.ToString();
+            using var searchRoot = new DirectoryEntry($"LDAP://CN=Partitions,{namingContext}");
+            using var ds = new DirectorySearcher(searchRoot)
+            {
+                PageSize = 1000,
+                CacheResults = false
+            };
+            ds.SearchScope = SearchScope.OneLevel;
+            ds.PropertiesToLoad.Add("distinguishedName");
+            ds.PropertiesToLoad.Add("nETBIOSName");
+            ds.PropertiesToLoad.Add("nCName");
+            ds.PropertiesToLoad.Add("dnsRoot");
+            ds.PropertiesToLoad.Add("trustParent");
+            ds.PropertiesToLoad.Add("objectSid");
+            ds.Filter = string.Format("(&(objectCategory=crossRef)(systemFlags={0}))", ADS_SYSTEMFLAG_CR_NTDS_DOMAIN | ADS_SYSTEMFLAG_CR_NTDS_NC);
+
+            return ds.FindAll();
+        }
+    }
+}