Add an overridable ADO BeginTransaction method

src/NHibernate/Async/Transaction/AdoTransaction.cs

@@ -12,6 +12,7 @@
 using System.Collections.Generic;
 using System.Data;
 using System.Data.Common;
+using NHibernate.Driver;
 using NHibernate.Engine;
 using NHibernate.Impl;
 

src/NHibernate/Driver/DriverBase.cs

@@ -46,6 +46,21 @@ protected bool IsPrepareSqlEnabled
 		public abstract DbConnection CreateConnection();
 		public abstract DbCommand CreateCommand();
 
+		/// <summary>
+		/// Begin an ADO <see cref="DbTransaction" />.
+		/// </summary>
+		/// <param name="isolationLevel">The isolation level requested for the transaction.</param>
+		/// <param name="connection">The connection on which to start the transaction.</param>
+		/// <returns>The started <see cref="DbTransaction" />.</returns>
+		public virtual DbTransaction BeginTransaction(IsolationLevel isolationLevel, DbConnection connection)
+		{
+			if (isolationLevel == IsolationLevel.Unspecified)
+			{
+				return connection.BeginTransaction();
+			}
+			return connection.BeginTransaction(isolationLevel);
+		}
+
 		/// <summary>
 		/// Does this Driver require the use of a Named Prefix in the SQL statement.  
 		/// </summary>

src/NHibernate/Driver/DriverExtensions.cs

@@ -1,16 +1,40 @@
+using System.Data;
 using System.Data.Common;
 using NHibernate.AdoNet;
 using NHibernate.SqlTypes;
-using NHibernate.Type;
 
 namespace NHibernate.Driver
 {
-	internal static class DriverExtensions
+	public static class DriverExtensions
 	{
 		internal static void AdjustParameterForValue(this IDriver driver, DbParameter parameter, SqlType sqlType, object value)
 		{
 			var adjustingDriver = driver as IParameterAdjuster;
 			adjustingDriver?.AdjustParameterForValue(parameter, sqlType, value);
 		}
+
+		// 6.0 TODO: merge into IDriver
+		/// <summary>
+		/// Begin an ADO <see cref="DbTransaction" />.
+		/// </summary>
+		/// <param name="driver">The driver.</param>
+		/// <param name="isolationLevel">The isolation level requested for the transaction.</param>
+		/// <param name="connection">The connection on which to start the transaction.</param>
+		/// <returns>The started <see cref="DbTransaction" />.</returns>
+		public static DbTransaction BeginTransaction(this IDriver driver, IsolationLevel isolationLevel, DbConnection connection)
+		{
+			if (driver is DriverBase driverBase)
+			{
+				return driverBase.BeginTransaction(isolationLevel, connection);
+			}
+
+			// So long for custom drivers not deriving from DriverBase, they will have to wait for 6.0 if they
+			// need the feature.
+			if (isolationLevel == IsolationLevel.Unspecified)
+			{
+				return connection.BeginTransaction();
+			}
+			return connection.BeginTransaction(isolationLevel);
+		}
 	}
 }

src/NHibernate/Transaction/AdoTransaction.cs

@@ -2,6 +2,7 @@
 using System.Collections.Generic;
 using System.Data;
 using System.Data.Common;
+using NHibernate.Driver;
 using NHibernate.Engine;
 using NHibernate.Impl;
 
@@ -146,14 +147,7 @@ public void Begin(IsolationLevel isolationLevel)
 
 				try
 				{
-					if (isolationLevel == IsolationLevel.Unspecified)
-					{
-						trans = session.Connection.BeginTransaction();
-					}
-					else
-					{
-						trans = session.Connection.BeginTransaction(isolationLevel);
-					}
+					trans = session.Factory.ConnectionProvider.Driver.BeginTransaction(isolationLevel, session.Connection);
 				}
 				catch (HibernateException)
 				{