Object reference not set to an instance of an object. c#

Question

i can get record from database and take its into objects

for (int i = 0; i < selectedFishes.Length; i++)
{
    string stm = "SELECT * FROM Data WHERE name='" + selectedFishes[i] + "'";
    SQLiteCommand cmd = new SQLiteCommand(stm, conn);
    SQLiteDataReader rdr = cmd.ExecuteReader();
    while (rdr.Read())
    {
        fish[i] = new fish(rdr.GetString(0), rdr.GetDouble(1), rdr.GetDouble(2), rdr.GetDouble(3), rdr.GetDouble(4));
        Console.WriteLine(fish[i].name + " " + fish[i].value1 + " " + fish[i].value2 + " " + fish[i].value3 + " " + fish[i].value4);//it can be displayed into console
    }
}

MessageBox.Show(selectedFishes.Length.ToString()+" "+fish.Length.ToString());//results are same if the value >5 cant be displayed
for (int i = 0; i < fish.Length; i++)
{
   Console.WriteLine(fish[i].name + " " + fish[i].value1 + " " + fish[i].value2 + " " + fish[i].value3 + " " + fish[i].value4);

}

why cant be displayed? the error is"Object reference not set to an instance of an object" thanks before

Apart of *on which line is the error occuring*, the next question is *on which iteration number occurs the error?* — Matías Fidemraizer, May 26 '13 at 08:53
Irrelevant, but consider using the `Parameters` property of your `SQLiteCommand` instead of simply concatenating a `string`. It will make this part of your code a little safer against malicious input. — Theodoros Chatzigiannakis, May 26 '13 at 08:56
@TheodorosChatzigiannakis: Not just a *little* safer - much, **much** safer. — Jon Skeet, May 26 '13 at 08:57
@NadzarBagus See Björn's answer here: http://stackoverflow.com/questions/809246/adding-parameters-in-sqlite-with-c-sharp — Theodoros Chatzigiannakis, May 26 '13 at 09:04
thanks all, for your answer, i will debug myself.. maybe in the checkboxes for selectedfish there is a error... my checkboxes are devided into 5 columns,and if i select all 3nd and 5th columns, the error occurs — Nadzar Bagus, May 26 '13 at 09:18

Dirk · Answer 1 · 2013-05-26T09:02:30.730

3

I'm taking a wild guess and say that selectedFishes.Length is probably not equal to fish.Length. To be precise the former is most likely less than the latter.

Which means you only create objects for indices 0 to selectedFishes.Length-1 and then try to access index selectedFishes.Length which is still null and cause that exception.

To fix this you can change the upper bound of the second for loop or do a null check inside it.

Edit: and do you really want that while loop inside the first for loop? i doesn't change inside the while loop so you keep adding fish objects into the same array index.

edited May 26 '13 at 09:02

answered May 26 '13 at 08:52

Dirk

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Yes, this matches the described symptoms. I would say that `selectedFishes.Length < fish.Length` rather than just `selectedFishes.Length != fish.Length` – Matthew Watson May 26 '13 at 08:53
i've tried to change fish.lenght in to selectedfishes.lenght, but the result is same – Nadzar Bagus May 26 '13 at 08:55

score 1 · Answer 2 · answered Dec 05 '13 at 19:34

1

I'd say always in NullReferenceException try to ToString() each object that you expect to have a value.

answered Dec 05 '13 at 19:34

score 0 · Answer 3 · answered May 26 '13 at 08:55

0

Add a breakpoit to the second console writeline by pressing f9. Press f5. Let your program hit the breakpoint. Check ifname or value1-4 is null. Add code to handle null values.

answered May 26 '13 at 08:55

If you cannot reach the second console writeline where you told us the error occurs then then there must be something wrong earlier. Add abreakpoint some lines above and step through the code investigating the contents of your objects. You should read some introduction about debugging in visual studio. You will need debugging with a propability of 100%. – May 26 '13 at 09:33

Object reference not set to an instance of an object. c#

3 Answers3