SqlDataReader for numeric data type

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C#

SQL

DataGridView

DataReader

Hello guys, I have this code:

public void chyt_data()
        {
        
            SqlCommand novyprikaz = new SqlCommand("SELECT * FROM table WHERE akce="+nc_zajezd_vyber, spojeni); 
            spojeni.Open();
            SqlDataReader precti = novyprikaz.ExecuteReader();

            if (precti.Read())
            {
...
                zcena1.Text = precti.GetDecimal(4).ToString();
                zcena2.Text = precti.GetDecimal(5).ToString();
                zcena3.Text = precti.GetDecimal(6).ToString();
            
            }
            spojeni.Close();
        }

It seelcts data and reads them from my SQL DB table, problém is that It doesnt read zcena1,zcena2 and zcena3, its because the data type in SQL table is numeric, on microsoft's website was written that i should read it with GetDecimal, but it doesn't work either.
Is there any solution?

Thanks in advance!