Im having some trouble with my code

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Hi im having a error with my database updating code.The error says "must declare the scalar variable @numara"
Here is the code that i had error with (This code belongs to my form.cs):

C#

IslemVeritabani ekleme = new IslemVeritabani();<
ekleme.GuncelleIslem("update tbl_temeldb  set ogrenci_adi='" + ad.Text + "',ogrenci_soyadi='" + soyad.Text + "',ders1='" + dersler.SelectedItem + "',ders2='" + dersler.SelectedItem + "',ders3='" + dersler.SelectedItem + "',ders4='" + dersler.SelectedItem + "',ders5='" + dersler.SelectedItem + "',ilk_dersin_ogretmeni='" + ogretmenler.SelectedItem + "',ikinci_dersin_ogretmeni='" + ogretmenler2.SelectedItem + "',ucuncu_dersin_ogretmeni='" + ogretmenler3.SelectedItem + "',dorduncu_dersin_ogretmeni='" + ogretmenler4.SelectedItem + "',besinci_dersin_ogretmeni='" + ogretmenler5.SelectedItem + "',notu='" + int.Parse(not.Text) + "'where id=@numara");
           ekleme.komut.Parameters.AddWithValue("@numara", dataGridView1.CurrentRow.Cells[0].Value.ToString());
            listeleme();
            MessageBox.Show("Kayıt Başarıyla Güncellenmiştir.");

Here is the problem code(Compiler shows the error at this code section) :

C#

class IslemVeritabani : TemelVeritabani
public void GuncelleIslem(string sorgu)
{
    Baglan();
    SqlCommand cmd = new SqlCommand(sorgu);
    cmd.Connection = baglanti;
    cmd.ExecuteNonQuery();
    baglanti.Close();
    baglanti.Dispose();
}

What I have tried:

I tried everything I know but i couldn't solve this problem

Posted 6-Jun-20 8:33am

Member 14855516

Updated 6-Jun-20 21:39pm

Richard MacCutchan

v2

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OriginalGriff · Answer 1 · 2020-06-06T08:58:00

Don't do it like that! Never concatenate strings to build a SQL command. It leaves you wide open to accidental or deliberate SQL Injection attack which can destroy your entire database. Always use Parameterized queries instead.

When you concatenate strings, you cause problems because SQL receives commands like:

SQL

SELECT * FROM MyTable WHERE StreetAddress = 'Baker's Wood'

The quote the user added terminates the string as far as SQL is concerned and you get problems. But it could be worse. If I come along and type this instead: "x';DROP TABLE MyTable;--" Then SQL receives a very different command:

SQL

SELECT * FROM MyTable WHERE StreetAddress = 'x';DROP TABLE MyTable;--'

Which SQL sees as three separate commands:

SQL

SELECT * FROM MyTable WHERE StreetAddress = 'x';

A perfectly valid SELECT

SQL

DROP TABLE MyTable;

A perfectly valid "delete the table" command

SQL

--'

And everything else is a comment.
So it does: selects any matching rows, deletes the table from the DB, and ignores anything else.

So ALWAYS use parameterized queries! Or be prepared to restore your DB from backup frequently. You do take backups regularly, don't you?

And the parameter you are using - @numara - has to be added before you call ExecuteNonQuery, not after - which is why you are getting the error.

Don't try to create a method to do the DB stuff - Create the Connection inside a using block, create the Command object inside another using block, and always use parameterised queries. Trying to get a "generic" method to work with parameters is a PITA - so use inline code!