Throw exception for misuse. Fixes #308

Author: bgrainger

src/MySqlConnector/MySqlClient/MySqlCommand.cs

@@ -78,9 +78,19 @@ public override void Prepare()
 		}
 
 		public override string CommandText { get; set; }
-		public new MySqlConnection Connection { get; set; }
 		public new MySqlTransaction Transaction { get; set; }
 
+		public new MySqlConnection Connection
+		{
+			get => m_connection;
+			set
+			{
+				if (m_connection?.HasActiveReader ?? false)
+					throw new MySqlException("Cannot set MySqlCommand.Connection when there is an open DataReader for this command; it must be closed first.");
+				m_connection = value;
+			}
+		}
+
 		public override int CommandTimeout
 		{
 			get => Math.Min(m_commandTimeout ?? Connection?.DefaultCommandTimeout ?? 0, int.MaxValue / 1000);
@@ -244,6 +254,7 @@ private bool IsValid(out Exception exception)
 
 		static int s_commandId = 1;
 
+		MySqlConnection m_connection;
 		MySqlParameterCollection m_parameterCollection;
 		int? m_commandTimeout;
 		CommandType m_commandType;

src/MySqlConnector/MySqlClient/MySqlConnection.cs

@@ -323,6 +323,8 @@ internal async Task<CachedProcedure> GetCachedProcedure(IOBehavior ioBehavior, s
 
 		internal MySqlSslMode SslMode => m_connectionSettings.SslMode;
 
+		internal bool HasActiveReader => m_activeReader != null;
+
 		internal void SetActiveReader(MySqlDataReader dataReader)
 		{
 			if (dataReader == null)