Get user name from one form to another c#

Question

Get user name from one form to another

 

access rights

 

I have error in  line 28

 

{"Invalid column name 'a'."}   

 

a is username and pasword

 

private static int korisnikId;
private static int korisnickaGrupa;
private string korisnickoIme;

public void findsUserName()
        {
            SqlConnection myConnection = new SqlConnection(cs);
            String command = "SELECT ime_prezime FROM osobe_korisnici  where id = \"" + korisnikId + "\"";

            SqlCommand myCommand = new SqlCommand(command, myConnection);


            myCommand.Connection.Open();
            this.korisnickoIme = myCommand.ExecuteScalar().ToString();
        }

private void prijava()
        if (korisnikId > 0)
            {
                command = "SELECT osobe_korisnici.lozinka FROM osobe_korisnici where osobe_korisnici.id = " + korisnikId;
                SqlCommand myCommand2 = new SqlCommand(command, myConnection);
                String lozinka = myCommand2.ExecuteScalar().ToString();
                if (lozinka == lozinkaTextBox.Text)
                {
                    command = "SELECT korisnik_grupa.redni_broj FROM korisnik_grupa where korisnik_grupa.broj_grupe = \"" + korisnikId + "\"";

                    SqlCommand myCommand3 = new SqlCommand(command, myConnection);
                    korisnickaGrupa = Int32.Parse(myCommand3.ExecuteScalar().ToString());

                    if (korisnickaGrupa == 1)
                        {
                        findsUserName();
                        DialogResult = DialogResult.OK;
                    }
                    else
                    {
                        errorLabel.Text = "You dont have acces for this app";
                    }
                }
                else
                {
                    errorLabel.Text = "Wrong password";
                }
            }
            else
            {
                errorLabel.Text = "Wrong user name";
            }


        }