Check the actual string in the debugger: put a breakpoint on the line
con = new SqlConnection(str);
and look at exactly what
contains. At a guess, it's
or the empty string because the connection string read code hasn't been executed, or the web.config isn't what you think it is.
Either way, we can't check for you - it needs your code running to look at what is going on, and we can't do that for you - which means the debugger!
But don't do your code like that (particularly not in a web based system!)
Never concatenate strings to build a SQL command. It leaves you wide open to accidental or deliberate SQL Injection attack which can destroy your entire database. Always use Parameterized queries instead.
When you concatenate strings, you cause problems because SQL receives commands like:
SELECT * FROM MyTable WHERE StreetAddress = 'Baker's Wood'
The quote the user added terminates the string as far as SQL is concerned and you get problems. But it could be worse. If I come along and type this instead: "x';DROP TABLE MyTable;--" Then SQL receives a very different command:
SELECT * FROM MyTable WHERE StreetAddress = 'x';DROP TABLE MyTable;
Which SQL sees as three separate commands:
SELECT * FROM MyTable WHERE StreetAddress = 'x';
A perfectly valid SELECT
DROP TABLE MyTable;
A perfectly valid "delete the table" command
And everything else is a comment.
So it does: selects any matching rows, deletes the table from the DB, and ignores anything else.
So ALWAYS use parameterized queries! Or be prepared to restore your DB from backup frequently. You do take backups regularly, don't you?
And never store passwords in clear text - it is a major security risk. There is some information on how to do it here: Password Storage: How to do it.