c# - Error: Object reference not set to an instance of an object./The connection was not closed. The connection's current state is open -

for longest time, code has been running, lately encounter error,object reference not set instance of object. don't know if related creation , usage of new database.

here code:

con2.open();     sqlcommand cmd = new sqlcommand();     cmd.connection = con;     cmd.commandtext = "select qtyinhand inventory productid=@productid";     cmd.parameters.add("@productid", sqldbtype.int).value = productid;     int existingqty = (int)cmd.executescalar();     cmd.parameters.clear();     cmd.commandtext = "update inventory set qtyinhand=@qtyinhand productid=@productid";     cmd.parameters.add("@qtyinhand", sqldbtype.int).value = existingqty - int.parse(quantity);     cmd.parameters.add("@productid", sqldbtype.int).value = productid;     cmd.executenonquery();     con2.close(); 

error on part: int existingqty = (int)cmd.executescalar();

when tried using other sqlconnection: con

con.open();     sqlcommand cmd = new sqlcommand();     cmd.connection = con;     cmd.commandtext = "select qtyinhand inventory productid=@productid";     cmd.parameters.add("@productid", sqldbtype.int).value = productid;     int existingqty = (int)cmd.executescalar();     cmd.parameters.clear();     cmd.commandtext = "update inventory set qtyinhand=@qtyinhand productid=@productid";     cmd.parameters.add("@qtyinhand", sqldbtype.int).value = existingqty - int.parse(quantity);     cmd.parameters.add("@productid", sqldbtype.int).value = productid;     cmd.executenonquery();     con.close(); 

i encounter error, the connection not closed. connection's current state open. error on con.open(); part. how should solve problem?

for first error, executescalar() call returning null value. either refine query - can check query running directly in database - or change logic deal null values.

for second error, if calling open() throwing error, it's because connection object in use before , not closed properly. it's considered bad practice reuse connections that, consider creating new connection instance when go opening one.

edit: tried imply in second paragraph there, feel must make explicit: don't forget deal connection left open there, may major performance hog application. specially since it's asp.net one. dispose of connections don't need them. when call dispose() connection, gets closed - added bonus of other finer memory management procedures well. read using statement , usage connections have time.