Object reference not set to an instance of object C#

Question

1.25/5 (4 votes)

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.NET4.5

I work in windows form c# vs 2015 i get error i need to solve it

but i dont know how to solve it

i write function in textbox changed event but when i start writing in textbox

get error object reference not set to an instance of object .

so that why this problem happen and how to solve it .

in textbox changed event

C#

private void textBox3_TextChanged(object sender, EventArgs e)  
       {  
           try  
           {  
                
  
                   MatrixClass matrix = new MatrixClass();  
                   string value1 = matrix.GetAtcCode(textBox3.Text);  
                   textBox1.Text = value1;  
                    
                 
           }  
           catch (Exception ex)  
           {  
               MessageBox.Show(ex.Message);  
           }  
       }  
public string GetAtcCode(string GenericName)  
       {  
           SqlConnection con = new SqlConnection(connection);  
            
           con.Open();  
  
           SqlCommand comm = new SqlCommand("ATC_Code_ByGeneric_H", con);  
  
           comm.CommandType = CommandType.StoredProcedure;  
  
           comm.Parameters.Add(new SqlParameter("@Lvl_Name", GenericName));  
  
           string ret = comm.ExecuteScalar().ToString();   
           con.Close();  
  
           return ret;  
  
       }

SQL

ALTER PROCEDURE [dbo].[ATC_Code_ByGeneric_H]  
    @Lvl_Name nvarchar(500)  
AS  
    SELECT     Tbl_Lvl1.Code + Tbl_Lvl2.Code + Tbl_Lvl3.Code + Tbl_Lvl4.Code + Tbl_Lvl5.Code AS ATC_Code  
    FROM         Tbl_Lvl1  
                 INNER JOIN  
                 Tbl_Lvl2  
                 ON Tbl_Lvl1.S_ID = Tbl_Lvl2.UpLvl_ID  
                 INNER JOIN  
                 Tbl_Lvl3  
                 ON Tbl_Lvl2.S_ID = Tbl_Lvl3.UpLvl_ID  
                 INNER JOIN  
                 Tbl_Lvl4  
                 ON Tbl_Lvl3.S_ID = Tbl_Lvl4.UpLvl_ID  
                 INNER JOIN  
                 Tbl_Lvl5  
                 ON Tbl_Lvl4.S_ID = Tbl_Lvl5.UpLvl_ID  
    WHERE     (Tbl_Lvl5.Lvl_Name = @Lvl_Name) and Tbl_Lvl5.human=1  
    RETURN

my code above can get atc code without any problem
problem happen when start writing from first charachter in textbox
so that how to solve that problem ?

What I have tried:

object reference not set to an instance of object c#

Posted 27-Apr-17 5:16am

ahmed_sa

Updated 27-Apr-17 5:38am

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Comments

[no name] 27-Apr-17 11:23am

You use your debugger and find the null object then you fix your code.

ZurdoDev 27-Apr-17 11:33am

1. This is very easy to fix but only you can do it because we can't run your code.
2. No point showing your sql because the error is in C#.
3. Something is null that you are trying to access but since you did not tell us what line of code caused the error we cannot tell you what to look for.

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OriginalGriff · Answer 1 · 2017-04-27T05:38:00

This is one of the most common problems we get asked, and it's also the one we are least equipped to answer, but you are most equipped to answer yourself.

Let me just explain what the error means: You have tried to use a variable, property, or a method return value but it contains null - which means that there is no instance of a class in the variable.
It's a bit like a pocket: you have a pocket in your shirt, which you use to hold a pen. If you reach into the pocket and find there isn't a pen there, you can't sign your name on a piece of paper - and you will get very funny looks if you try! The empty pocket is giving you a null value (no pen here!) so you can't do anything that you would normally do once you retrieved your pen. Why is it empty? That's the question - it may be that you forgot to pick up your pen when you left the house this morning, or possibly you left the pen in the pocket of yesterdays shirt when you took it off last night.

We can't tell, because we weren't there, and even more importantly, we can't even see your shirt, much less what is in the pocket!

Back to computers, and you have done the same thing, somehow - and we can't see your code, much less run it and find out what contains null when it shouldn't.
But you can - and Visual Studio will help you here. Run your program in the debugger and when it fails, VS will show you the line it found the problem on. You can then start looking at the various parts of it to see what value is null and start looking back through your code to find out why. So put a breakpoint at the beginning of the method containing the error line, and run your program from the start again. This time, VS will stop before the error, and let you examine what is going on by stepping through the code looking at your values.

But we can't do that - we don't have your code, we don't know how to use it if we did have it, we don't have your data. So try it - and see how much information you can find out!