Lazily create CancelAction only when needed.

Author: bgrainger

src/MySqlConnector/MySqlClient/CommandExecutors/TextCommandExecutor.cs

@@ -51,7 +51,7 @@ public virtual async Task<DbDataReader> ExecuteReaderAsync(string commandText, M
 			CommandBehavior behavior, IOBehavior ioBehavior, CancellationToken cancellationToken)
 		{
 			cancellationToken.ThrowIfCancellationRequested();
-			using (cancellationToken.Register(m_command.CancelAction))
+			using (m_command.RegisterCancel(cancellationToken))
 			{
 				m_command.Connection.Session.StartQuerying(m_command);
 				m_command.LastInsertedId = -1;

src/MySqlConnector/MySqlClient/MySqlCommand.cs

@@ -37,7 +37,6 @@ public MySqlCommand(string commandText, MySqlConnection connection, MySqlTransac
 			DbTransaction = transaction;
 			m_parameterCollection = new MySqlParameterCollection();
 			CommandType = CommandType.Text;
-			CancelAction = Cancel;
 		}
 
 		public new MySqlParameterCollection Parameters
@@ -51,8 +50,6 @@ public MySqlCommand(string commandText, MySqlConnection connection, MySqlTransac
 
 		public override void Cancel() => Connection.Cancel(this);
 
-		internal Action CancelAction { get; private set; }
-
 		public override int ExecuteNonQuery() =>
 			ExecuteNonQueryAsync(IOBehavior.Synchronous, CancellationToken.None).GetAwaiter().GetResult();
 
@@ -159,6 +156,23 @@ protected override void Dispose(bool disposing)
 
 		internal new MySqlConnection Connection => (MySqlConnection) DbConnection;
 
+		/// <summary>
+		/// Registers <see cref="Cancel"/> as a callback with <paramref name="token"/> if cancellation is supported.
+		/// </summary>
+		/// <param name="token">The <see cref="CancellationToken"/>.</param>
+		/// <returns>An object that must be disposed to revoke the cancellation registration.</returns>
+		/// <remarks>This method is more efficient than calling <code>token.Register(Command.Cancel)</code> because it avoids
+		/// unnecessary allocations.</remarks>
+		internal IDisposable RegisterCancel(CancellationToken token)
+		{
+			if (!token.CanBeCanceled)
+				return null;
+
+			if (m_cancelAction == null)
+				m_cancelAction = Cancel;
+			return token.Register(m_cancelAction);
+		}
+
 		private void VerifyNotDisposed()
 		{
 			if (m_parameterCollection == null)
@@ -183,5 +197,6 @@ private void VerifyValid()
 		MySqlParameterCollection m_parameterCollection;
 		CommandType m_commandType;
 		ICommandExecutor m_commandExecutor;
+		Action m_cancelAction;
 	}
 }

src/MySqlConnector/MySqlClient/MySqlDataReader.cs

@@ -108,7 +108,7 @@ private ValueTask<ResultSet> ScanResultSetAsync(IOBehavior ioBehavior, ResultSet
 
 		private async Task<ResultSet> ScanResultSetAsyncAwaited(IOBehavior ioBehavior, ResultSet resultSet, CancellationToken cancellationToken)
 		{
-			using (cancellationToken.Register(Command.CancelAction))
+			using (Command.RegisterCancel(cancellationToken))
 			{
 				try
 				{

src/MySqlConnector/MySqlClient/Results/ResultSet.cs

@@ -162,7 +162,7 @@ private ValueTask<Row> ScanRowAsync(IOBehavior ioBehavior, Row row, Cancellation
 			if (BufferState == ResultSetState.HasMoreData || BufferState == ResultSetState.NoMoreData || BufferState == ResultSetState.None)
 				return new ValueTask<Row>((Row)null);
 
-			using (cancellationToken.Register(Command.CancelAction))
+			using (Command.RegisterCancel(cancellationToken))
 			{
 				var payloadValueTask = Session.ReceiveReplyAsync(ioBehavior, CancellationToken.None);
 				return payloadValueTask.IsCompletedSuccessfully