Object reference not set to an instance of an object

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Hi I trying to get Data from my sql server tables. I have two comboboxes with a botton. I have fill the combobox with tables from the server and am now trying to retrieve some Infos within both tables BUT am getting the Message:
Object reference not sent to an instance of an objet or in german
Der objektverweis wurde nicht auf eine objektinstanz festgelegt

Here my click_btn code. Can somone help?

C#

SqlConnection con = new SqlConnection(CS);

            // TSQL Statement
            SqlCommand cmd = new SqlCommand();
            cmd.Connection = con;
            cmd.CommandText = (@" SELECT [GWU Planung mit AZ], [GWU Vorschau mit AZ], [Projektdefinition DB] FROM " + cbSelect1.SelectedValue.ToString() + 
                                "WHERE [Projektdefinition DB] = 'T.016002108' AND [Planungskostenelement (j/n)] = 'Nein' " +
                                " UNION SELECT [GWU Planung mit AZ], [GWU Vorschau mit AZ], [Projektdefinition DB] FROM " + cbSelect2.SelectedValue.ToString() + 
                               "WHERE [Projektdefinition DB] = 'T.016002108' AND [Planungskostenelement (j/n)] = 'Nein' " + "");


            SqlDataAdapter adapter = new SqlDataAdapter(cmd); // adapter is return the result in Datagridview

            // IF STATEMENT

            if (cbSelect1.SelectedValue.ToString() != null && cbSelect2.SelectedValue.ToString() != null)
            {
                try
                {
                    // Open connection
                    con.Open();
                    DataTable dtRecord = new DataTable();
                    adapter.Fill(dtRecord);
                    dataGridView1.AutoGenerateColumns = true;
                    dataGridView1.DataSource = dtRecord;
                    //dataGridView1.AutoResizeColumns(DataGridViewAutoSizeColumnsMode.AllCells);
                    con.Close();
                    
                }

                catch (Exception ex)
                {
                    MessageBox.Show(ex.Message);
                }
                finally
                {
                    con.Close();
                }

            }
            else
            {
                MessageBox.Show("ERROR");
            }

Posted 16-Oct-14 1:50am

mikybrain1

Updated 16-Oct-14 2:58am

Thanks7872

v2

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Comments

M.Farrukh Abbas 16-Oct-14 7:59am

first of all instead of cbSelect2.SelectedValue.ToString() used Convert.ToString() and !string.isNullorEmpty() function because .tostring() can not handle null value its always gives error on null or empty string.
try to change the code on above suggestion if not solve then specify exact location where you get this error.

Thanks7872 16-Oct-14 9:00am

At which line? Go through all the objects in the line where you faced the error, you will find one of the object as null. Try to make changes such that it should not be null.

Sinisa Hajnal 16-Oct-14 9:04am

And dispose of the command object too.

[no name] 16-Oct-14 9:10am

What line are you getting the error on above?

mikybrain1 16-Oct-14 9:13am

Hithere a no error in the code but if a run the app, it message me the error above and i don't know why

[no name] 16-Oct-14 9:31am

Here is what you do, attach your debugger application (Visual Studio) to the process of your running application and step through it to see where the code ends. Then tell us which line the code stops at.

M.Farrukh Abbas has touched on a possible cause for an error.

Also, try remove your Try Catch block or alter your exception to that of an NullReferenceException rather than just exception, and handle your exception in the catch phrase. That might give you more information on how it is crashing.

You can also read this amongst many articles explaining how to debug a running application created by Visual Studio.

Attach Process To Debugger

Richard MacCutchan 16-Oct-14 10:46am

Of course there is an error in the code, that's why you see this message. And if you show which line it occurs on, people may be able to help you to fix it.

ZurdoDev 16-Oct-14 9:33am

You'll have to debug it to see which object is null and then fix the code.

[no name] 16-Oct-14 9:52am

A further issue i have with your code; loose the string concatenation and start using parameters. What you're doing is bad practice, and you will face issues with your current method, not to mention you are subject to injection attacks.

Szymon Roslowski 16-Oct-14 12:25pm

if (cbSelect1.SelectedValue.ToString() != null && cbSelect2.SelectedValue.ToString() != null)

Could be replaced with

if (cbSelect1.SelectedValue != null && cbSelect2.SelectedValue != null)

and that should solve the problem.
No need to convert to string at this time.

Ps. a Quote from another post:

We can convert the integer “i” using “i.ToString()” or “Convert.ToString” so what’s the difference.
The basic difference between them is “Convert” function handles NULLS while “i.ToString()”
does not it will throw a NULL reference exception error. So as good coding practice using
“convert” is always safe.

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Sergey Alexandrovich Kryukov · Answer 1 · 2014-10-16T06:39:00

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