如何从SQL Server数据库中删除表行

我在试图删除行时遇到麻烦。

在ListBox控件上选择并显示所有行。
然后，当调用btnObrisi_Click事件(Obrisi = delete)时，从ListBox中选择的项将被删除。

该项实际上是从控件中删除，但该值并未从数据库中删除。

这是我使用的代码:

private string conString = @"Data Source=.'SQLEXPRESS;AttachDbFilename=|DataDirectory|'Database1.mdf;Integrated Security=True;User Instance=True";
private SqlConnection con;
private SqlCommand com;
private DataTable dt;
private SqlDataAdapter da;
private DataTable SelectAll()
{
    con = new SqlConnection(conString);
    com = new SqlCommand();
    dt = new DataTable();
    da = new SqlDataAdapter();
    com.Connection = con;
    com.CommandType = CommandType.Text;
    com.CommandText = "SELECT * FROM predmeti";
    da.SelectCommand = com;
    da.Fill(dt);
    return dt;
}
private void Obrisi(int id)
{
    con = new SqlConnection(conString);
    com = new SqlCommand();
    com.Connection = con;
    com.CommandType = CommandType.Text;
    com.CommandText = "DELETE FROM predmeti WHERE predmetID = @predmetID";
    com.Parameters.AddWithValue("@predmetID", id);
    try
    {
        con.Open();
        com.ExecuteNonQuery();
         MessageBox.Show("Obrisano");
    }
    catch(Exception ex)
    {
         MessageBox.Show(ex.Message);
    }
    finally
    {
         con.Close();
    }
}
private void Citaj()
{
    listBox1.ValueMember = "predmetID";
    listBox1.DisplayMember = "ime_predmeta";
    listBox1.DataSource = SelectAll();
}
private void btnOsvezi_Click(object sender, EventArgs e)
{
    Citaj();
}
private void Form1_Load(object sender, EventArgs e)
{
    Citaj();
}
private void btnObrisi_Click(object sender, EventArgs e)
{
    int id = Convert.ToInt32(listBox1.SelectedValue);
    Obrisi(id);
    Citaj();
}