Nullreferenceexception when using INSERT into to enter data into database

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Hi, I have the problem as stated above and i hope that i can found the solution here. So here is the error:

C#

Line 86: 
Line 87: 
Line 88:                 if (txtTempIdSubject.Text.Trim() == "")
Line 89:                 {
Line 90:                     lblAttention.Text = "Please enter complete data!";

Here is the code that has the error from .aspx.cs:

C#

protected void dgSubject_ItemCommand(object sender, System.Web.UI.WebControls.DataGridCommandEventArgs e)
        {
            SubjectClass std = new SubjectClass();

            if (e.CommandName == "AddUser")
            {
                TextBox txtTempIdSubject = (TextBox)e.Item.Cells[0].FindControl("txtSubjectID");
                TextBox txtTempNameSubject = (TextBox)e.Item.Cells[1].FindControl("txtSubjectName");
                TextBox txtTempModifiedBy = (TextBox)e.Item.Cells[2].FindControl("txtModifiedBy");
                TextBox txtTempModifiedOn = (TextBox)e.Item.Cells[3].FindControl("txtModifiedOn");



                if (txtTempIdSubject.Text.Trim() == "" )-----> here is the error
                {
                    lblAttention.Text = "Please enter complete data!"; 
                }
                else
                {
                    if (std.AddSubject(txtTempIdSubject.Text,txtTempNameSubject.Text, txtTempModifiedBy.Text, txtTempModifiedOn.Text) == false)
                    {
                        lblAttention.Text = " *Data addition failed!";
                    }
                    else
                    {
                        Server.Transfer("Subject.aspx");
                    }
                }
            }

and here is the class:

C#

public bool AddSubject(string idSubject,string nameSubject, string modifiedBy, string modifiedOn)
        {
            try
            {
             if (this.SetIdSubject(Convert.ToInt32(idSubject)) && this.SetNameSubject(nameSubject) && this.SetModifiedBy(modifiedBy) && this.SetModifiedOn(System.DateTime.Now))
                {
                    DatabaseManager db = new DatabaseManager();
                    cmd = new SqlCommand();
                    cmd.CommandText = "INSERT INTO [dbo].[Subjects]" +
                        "(SubjectID,SubjectName, UpdatedBy, UpdatedOn)" +
                        "VALUES" +
                        "(@P1, @P2, @P3,@P4)";
                    cmd.Parameters.AddWithValue("@P1", this.GetIdSubject());
                    cmd.Parameters.AddWithValue("@P2", this.GetNameSubject());
                    cmd.Parameters.AddWithValue("@P2", this.GetModifiedBy());
                    cmd.Parameters.AddWithValue("@P4", System.DateTime.Now.ToString());
                    db.ExecuteNonQuery(cmd);
                    return true;
                }
                else
                {
                    return false;
                }
            }
            catch
            {
                return false;
            }
        }

What I have tried:

I have changed all the Subjects table data into not null because I found that one of the solutions said that I need to make sure that the data is not in NULL but it still gives this error.

Posted 29-Apr-19 7:06am

dyooshi

Updated 29-Apr-19 13:27pm

PIEBALDconsult

v2

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ZurdoDev 29-Apr-19 13:19pm

This is very easy to fix and only you can do it. Something is NULL. Debug your code, find out what it is, then fix the code.

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W∴ Balboos, *GHB* · Answer 1 · 2019-04-29T08:07:00

Quote:
I have changed all the Subjects table data into not null because I found that one of the solutions said that I need to make sure that the data is not in NULL but it still gives this error.

This sounds like you're copying solutions but really haven't any idea what you're doing.

Finding solutions on-line is not a bad thing if you know what you're doing and need a hint - but just fining hunks of code and hoping they'll work together for whatever you think you want is somewhat foolish. Relying on on-line help to make up for a lack of putting in the time to study and understand won't get you far for long.

Not understand the meaning of null references is a serious shortcoming and you should put maximum effort into understand the various null-errors that are sent to you.

OriginalGriff · Answer 2 · 2019-04-29T08:03:00

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!

Nullreferenceexception when using INSERT into to enter data into database

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