Support trimming distributed transactions if unused, without a feature switch.

src/libraries/System.Transactions.Local/ref/System.Transactions.Local.cs

@@ -191,7 +191,7 @@ public static partial class TransactionManager
         [System.Diagnostics.CodeAnalysis.DisallowNullAttribute]
         public static System.Transactions.HostCurrentTransactionCallback? HostCurrentCallback { get { throw null; } set { } }
         public static System.TimeSpan MaximumTimeout { get { throw null; } set { } }
-        public static bool ImplicitDistributedTransactions { get; set; }
+        public static bool ImplicitDistributedTransactions { get; [System.Diagnostics.CodeAnalysis.RequiresUnreferencedCode("Distributed transactions support may not be compatible with trimming. If your program creates a distributed transaction via System.Transactions, the correctness of the application cannot be guaranteed after trimming.")] set; }
         public static event System.Transactions.TransactionStartedEventHandler? DistributedTransactionStarted { add { } remove { } }
         public static void RecoveryComplete(System.Guid resourceManagerIdentifier) { }
         public static System.Transactions.Enlistment Reenlist(System.Guid resourceManagerIdentifier, byte[] recoveryInformation, System.Transactions.IEnlistmentNotification enlistmentNotification) { throw null; }

src/libraries/System.Transactions.Local/src/System/Transactions/DtcProxyShim/DtcProxyShimFactory.cs

@@ -21,6 +21,11 @@ internal sealed class DtcProxyShimFactory
     // at the same time.
     private static readonly object _proxyInitLock = new();
 
+    // This object will perform the actual distributed transaction connection.
+    // It will be set only if TransactionManager.ImplicitDefaultTransactions
+    // is set to true, allowing the relevant code to be trimmed otherwise.
+    internal static ITransactionConnector? s_transactionConnector;
+
     // Lock to protect access to listOfNotifications.
     private readonly object _notificationLock = new();
 
@@ -39,6 +44,7 @@ internal DtcProxyShimFactory(EventWaitHandle notificationEventHandle)
 
     // https://docs.microsoft.com/previous-versions/windows/desktop/ms678898(v=vs.85)
     [DllImport(Interop.Libraries.Xolehlp, CharSet = CharSet.Unicode, ExactSpelling = true, PreserveSig = false)]
+    [RequiresUnreferencedCode(TransactionManager.DistributedTransactionTrimmingWarning)]
     private static extern void DtcGetTransactionManagerExW(
         [MarshalAs(UnmanagedType.LPWStr)] string? pszHost,
         [MarshalAs(UnmanagedType.LPWStr)] string? pszTmName,
@@ -47,7 +53,7 @@ private static extern void DtcGetTransactionManagerExW(
         object? pvConfigPararms,
         [MarshalAs(UnmanagedType.Interface)] out ITransactionDispenser ppvObject);
 
-    [RequiresUnreferencedCode("Distributed transactions support may not be compatible with trimming. If your program creates a distributed transaction via System.Transactions, the correctness of the application cannot be guaranteed after trimming.")]
+    [RequiresUnreferencedCode(TransactionManager.DistributedTransactionTrimmingWarning)]
     private static void DtcGetTransactionManager(string? nodeName, out ITransactionDispenser localDispenser) =>
         DtcGetTransactionManagerExW(nodeName, null, Guids.IID_ITransactionDispenser_Guid, 0, null, out localDispenser);
 
@@ -64,14 +70,15 @@ public void ConnectToProxy(
             throw new PlatformNotSupportedException(SR.DistributedNotSupportedOn32Bits);
         }
 
-        if (!TransactionManager.ImplicitDistributedTransactions)
+        if (s_transactionConnector is null)
         {
             throw new NotSupportedException(SR.ImplicitDistributedTransactionsDisabled);
         }
 
-        ConnectToProxyCore(nodeName, resourceManagerIdentifier, managedIdentifier, out nodeNameMatches, out whereabouts, out resourceManagerShim);
+        s_transactionConnector.ConnectToProxyCore(this, nodeName, resourceManagerIdentifier, managedIdentifier, out nodeNameMatches, out whereabouts, out resourceManagerShim);
     }
 
+    [RequiresUnreferencedCode(TransactionManager.DistributedTransactionTrimmingWarning)]
     private void ConnectToProxyCore(
         string? nodeName,
         Guid resourceManagerIdentifier,
@@ -82,9 +89,7 @@ private void ConnectToProxyCore(
     {
         lock (_proxyInitLock)
         {
-#pragma warning disable IL2026 // This warning is left in the product so developers get an ILLink warning when trimming an app using this transaction support
             DtcGetTransactionManager(nodeName, out ITransactionDispenser? localDispenser);
-#pragma warning restore IL2026
 
             // Check to make sure the node name matches.
             if (nodeName is not null)
@@ -371,4 +376,38 @@ internal ITransactionReceiver GetCachedReceiver()
 
     internal void ReturnCachedReceiver(ITransactionReceiver receiver)
         => _cachedReceivers.Enqueue(receiver);
+
+    internal interface ITransactionConnector
+    {
+        void ConnectToProxyCore(
+            DtcProxyShimFactory proxyShimFactory,
+            string? nodeName,
+            Guid resourceManagerIdentifier,
+            object managedIdentifier,
+            out bool nodeNameMatches,
+            out byte[] whereabouts,
+            out ResourceManagerShim resourceManagerShim);
+    }
+
+    [RequiresUnreferencedCode(TransactionManager.DistributedTransactionTrimmingWarning)]
+    internal sealed class DtcTransactionConnector : ITransactionConnector
+    {
+        public void ConnectToProxyCore(
+            DtcProxyShimFactory proxyShimFactory,
+            string? nodeName,
+            Guid resourceManagerIdentifier,
+            object managedIdentifier,
+            out bool nodeNameMatches,
+            out byte[] whereabouts,
+            out ResourceManagerShim resourceManagerShim)
+        {
+            proxyShimFactory.ConnectToProxyCore(
+                nodeName,
+                resourceManagerIdentifier,
+                managedIdentifier,
+                out nodeNameMatches,
+                out whereabouts,
+                out resourceManagerShim);
+        }
+    }
 }

src/libraries/System.Transactions.Local/src/System/Transactions/TransactionManager.cs

@@ -6,6 +6,9 @@
 using System.IO;
 using System.Threading;
 using System.Transactions.Configuration;
+#if WINDOWS
+using System.Transactions.DtcProxyShim;
+#endif
 using System.Transactions.Oletx;
 
 namespace System.Transactions
@@ -29,6 +32,10 @@ public static class TransactionManager
         private static TransactionTable? s_transactionTable;
 
         private static TransactionStartedEventHandler? s_distributedTransactionStartedDelegate;
+
+        internal const string DistributedTransactionTrimmingWarning =
+            "Distributed transactions support may not be compatible with trimming. If your program creates a distributed transaction via System.Transactions, the correctness of the application cannot be guaranteed after trimming.";
+
         public static event TransactionStartedEventHandler? DistributedTransactionStarted
         {
             add
@@ -391,7 +398,31 @@ public static TimeSpan MaximumTimeout
             }
         }
 
-        public static bool ImplicitDistributedTransactions { get; set; }
+#if WINDOWS
+        public static bool ImplicitDistributedTransactions
+        {
+            get => DtcProxyShimFactory.s_transactionConnector is not null;
+            [RequiresUnreferencedCode(DistributedTransactionTrimmingWarning)]
+            set
+            {
+                if (value)
+                {
+                    DtcProxyShimFactory.s_transactionConnector ??= new DtcProxyShimFactory.DtcTransactionConnector();
+                }
+                else
+                {
+                    DtcProxyShimFactory.s_transactionConnector = null;
+                }
+            }
+        }
+#else
+        public static bool ImplicitDistributedTransactions
+        {
+            get;
+            [RequiresUnreferencedCode(DistributedTransactionTrimmingWarning)]
+            set;
+        }
+#endif
 
         // This routine writes the "header" for the recovery information, based on the
         // type of the calling object and its provided parameter collection.  This information