Require global opt-in for distributed transactions

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

@@ -191,6 +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 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/Resources/Strings.resx

@@ -423,4 +423,7 @@
  <data name="DistributedNotSupportedOn32Bits" xml:space="preserve">
  <value>Distributed transactions are currently unsupported in 32-bit processes.</value>
  </data>
-</root>
+ <data name="ImplicitDistributedTransactionsDisabled" xml:space="preserve">
+ <value>Implicit distributed transactions have not been enabled. If you're intentionally starting a distributed transaction, set TransactionManager.ImplicitDistributedTransactions to true.</value>
+ </data>
+</root>

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

@@ -59,10 +59,14 @@ public void ConnectToProxy(
  out byte[] whereabouts,
  out ResourceManagerShim resourceManagerShim)
  {
- switch (RuntimeInformation.ProcessArchitecture)
+ if (RuntimeInformation.ProcessArchitecture == Architecture.X86)
  {
- case Architecture.X86:
- throw new PlatformNotSupportedException(SR.DistributedNotSupportedOn32Bits);
+ throw new PlatformNotSupportedException(SR.DistributedNotSupportedOn32Bits);
+ }
+
+ if (!TransactionManager.ImplicitDistributedTransactions)
+ {
+ throw new NotSupportedException(SR.ImplicitDistributedTransactionsDisabled);
  }
 
  ConnectToProxyCore(nodeName, resourceManagerIdentifier, managedIdentifier, out nodeNameMatches, out whereabouts, out resourceManagerShim);

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

@@ -391,6 +391,8 @@ public static TimeSpan MaximumTimeout
  }
  }
 
+ public static bool ImplicitDistributedTransactions { get; set; }
+
  // This routine writes the "header" for the recovery information, based on the
  // type of the calling object and its provided parameter collection. This information
  // we be read back by the static Reenlist method to create the necessary transaction

src/libraries/System.Transactions.Local/tests/OleTxTests.cs

@@ -469,6 +469,8 @@ private static void Test(Action action)
  return;
  }
 
+ TransactionManager.ImplicitDistributedTransactions = true;
+
  // In CI, we sometimes get XACT_E_TMNOTAVAILABLE; when it happens, it's typically on the very first
  // attempt to connect to MSDTC (flaky/slow on-demand startup of MSDTC), though not only.
  // This catches that error and retries.
@@ -524,7 +526,20 @@ public class OleTxFixture
  // this is likely due to on-demand slow startup of MSDTC. Perform pre-test connecting with retry
  // to ensure that MSDTC is properly up when the first test runs.
  public OleTxFixture()
- => Test(() =>
+ {
+ Test(() =>
+ {
+ TransactionManager.ImplicitDistributedTransactions = false;
+
+ using var tx = new CommittableTransaction();
+
+ Assert.Throws<NotSupportedException>(()
+ => tx.EnlistDurable(Guid.NewGuid(), new TestEnlistment(Phase1Vote.Done, EnlistmentOutcome.Committed), EnlistmentOptions.None));
+ });
+
+ TransactionManager.ImplicitDistributedTransactions = true;
+
+ Test(() =>
  {
  using var tx = new CommittableTransaction();
 
@@ -536,5 +551,6 @@ public OleTxFixture()
 
  tx.Commit();
  });
+ }
  }
 }