Couple of things:
1) Never use Convert methods for user input: if they mistype - and they will, often - the Convert methods crash your app or (in your case) emit a pretty useless error message directly to the user.
Instead, use int.TryParse
] and it's related method, and do "proper" error reporting so he knows what is happening.
2) Do yourself a favour, and stop using Visual Studio default names for everything - you may remember that "TextBox9" is the users first name today, but when you have to modify it in three weeks time, will you then? Use descriptive names - "tbFirstName" for example - and your code becomes easier to read, more self documenting, easier to maintain - and surprisingly quicker to code because Intellisense can get to to "tbFirstName" in three keystrokes, where "TextBox9" takes thinking about and 8 keystrokes...
3) 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?
Fix that throughout your app, and the problem you have noticed will go away at the same time.