Why do I receive a object reference error

Question

I have a C# application and within the code I am checking whether a table exist:

// checking whether the table selected from the dataset exists in the database or not
string exists = null;
try
{
    SqlCommand cmd = new SqlCommand("SELECT * FROM sysobjects where name = '" + tbTableName.Text + "'", myConnection);
    exists = cmd.ExecuteScalar().ToString();
}
catch (Exception exce)
{
    MessageBox.Show(exce.Message, "Program Message2", MessageBoxButtons.OK, MessageBoxIcon.Error);
    exists = null;
}

Once that is done, if the table doesn't exist I create a new table, if the table exist I alter the columns if it is missing.

When I run the application and I click a button to check and the above code is run, I get the following error:

Object reference not set to an instance of an object

The table creation or modifying the table code works fine afterward but I am not sure why I am receiving the error.

Any idea how to fix it?

Did you debug your code? Maybe `cmd.ExecuteScalar()` returns `null`? Read http://ericlippert.com/2014/03/05/how-to-debug-small-programs/ And [What is a NullReferenceException and how do I fix it?](http://stackoverflow.com/questions/4660142/what-is-a-nullreferenceexception-and-how-do-i-fix-it) By the way, you should always use [parameterized queries](http://blog.codinghorror.com/give-me-parameterized-sql-or-give-me-death/). This kind of string concatenations are open for [SQL Injection](http://en.wikipedia.org/wiki/SQL_injection) attacks. — Soner Gönül, Jun 20 '14 at 14:05
What line do you get the error? What `Object` is the error referring to? — CDspace, Jun 20 '14 at 14:07
I think @C.Evenhuis is correct. You can't do a `.ToString()` on a null, check if it's null before trying to get a string from it — CDspace, Jun 20 '14 at 14:14

score 3 · Accepted Answer · answered Jun 20 '14 at 14:07

3

ExecuteScalar() returns null when no records are returned by the query. So effectively, if no records match the query, the code says:

exists = null.ToString()

answered Jun 20 '14 at 14:07

C.Evenhuis

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So should I check if cmd.ExecuteScalar is null or not first? – SearchForKnowledge Jun 20 '14 at 14:09
If you really want a `string` as a result, you could use `Convert.ToString(cmd.ExecuteScalar())`, but why don't you use a boolean instead? – C.Evenhuis Jun 20 '14 at 14:12
Because the rest of my code uses it as a string then boolean... rewriting code at the moment is not an option for me :/ – SearchForKnowledge Jun 20 '14 at 14:13
@SearchForKnowledge `Convert.ToString(...)` it is then, it will assign `null` to `exists` if no records matched, and otherwise the first field of the first match. – C.Evenhuis Jun 20 '14 at 14:15
Thank you. I edited my code and hopefully it works and I will accept your answer once it is confirmed working. :) – SearchForKnowledge Jun 20 '14 at 14:16
`Convert.ToString()` throws my entire app out of normal process. I wonder if there is another way :/ – SearchForKnowledge Jun 20 '14 at 15:37
@SearchForKnowledge I'm afraid I can't tell what values your app is expecting, what is the expected value of `exists` per different situation? – C.Evenhuis Jun 20 '14 at 17:24
I just used a TRY and CATCH without alerting any messagebox and it's working fine for me now. The table either exists or it doesn't. – SearchForKnowledge Jun 20 '14 at 17:54

Why do I receive a object reference error

1 Answers1