You are doing it wrong. Your SQL statement is composed by concatenation with strings taken from UI. Not only repeated string concatenation is inefficient (because strings are
immutable; do I have to explain why it makes repeated concatenation bad?), but there is way more important issue: it opens the doors to a well-known exploit called
SQL injection.
This is how it works:
http://xkcd.com/327[
^].
Richard Deeming is absolutely right in his comment to the question: you should use parametrized statements.
[EDIT]
Please also see our discussion with Richard in comments below. He convinced me that the protection from SQL injection is harder to perform on a desktop application. Such injection is still possible, but, in this case, not that simple; it can be done, say, via reverse-engineering of the client application, which is also pretty easy.
Still, parametrized statement makes sense; understanding of importance of parametrized statements is still… important. Possibility of SQL injection is not the only factor; there is a number of other important reasons: better performance, readability, maintenance.
[END EDIT]
What to do? Just read about this problem and the main remedy:
parametrized statements:
http://en.wikipedia.org/wiki/SQL_injection[
^].
With ADO.NET, use this:
http://msdn.microsoft.com/en-us/library/ff648339.aspx[
^].
Please see my past answers for some more detail:
EROR IN UPATE in com.ExecuteNonQuery();[
^],
hi name is not displaying in name?[
^].
Now, about the null exception you have:
You did not show where the exception with the message "Object reference not set to an instance of an object" is thrown.
Not to worry. This is one of the very easiest cases to detect and fix. It simply means that some member/variable of some reference type is dereferenced by using and of its instance (non-static) members, which requi8res this member/variable to be non-null, but in fact it appears to be null. Simply execute it under debugger, it will stop the execution where the exception is thrown. Put a break point on that line, restart the application and come to this point again. Evaluate all references involved in next line and see which one is null while it needs to be not null. After you figure this out, fix the code: either make sure the member/variable is properly initialized to a non-null reference, or check it for null and, in case of null, do something else.
Please see also:
want to display next record on button click. but got an error in if condition of next record function "object reference not set to an instance of an object"[
^].
Sometimes, you cannot do it under debugger, by one or another reason. One really nasty case is when the problem is only manifested if software is built when debug information is not available. In this case, you have to use the harder way. First, you need to make sure that you never block propagation of exceptions by handling them silently (this is a crime of developers against themselves, yet very usual). The you need to catch absolutely all exceptions on the very top stack frame of each thread. You can do it if you handle the exceptions of the type
System.Exception
. In the handler, you need to log all the exception information, especially the
System.Exception.StackTrace
:
http://msdn.microsoft.com/en-us/library/system.exception.aspx[
^],
http://msdn.microsoft.com/en-us/library/system.exception.stacktrace.aspx[
^].
The stack trace is just a string showing the full path of exception propagation from the throw statement to the handler. By reading it, you can always find ends. For logging, it's the best (in most cases) to use the class
System.Diagnostics.EventLog
:
http://msdn.microsoft.com/en-us/library/system.diagnostics.eventlog.aspx[
^].
Good luck,
—SA