SQL连接出错-C#

我在C#应用程序上连接到SQL服务器时遇到一些问题。我有一个代码，可以插入或更新我的数据库表。如果我更新表，一切都会正常进行，但如果我插入它，就会出现错误。

这是代码：

int ID;
conn = new SqlConnection(@"Data Source = SERVER'SQLEXPRESS; Initial Catalog = SQLFiat; User ID = sa; Password = secret");
conn.Open();            
if (cbCarros.SelectedIndex >= 0)
{
    ID = cbCarros.SelectedIndex + 1;
    cmd = new SqlCommand("UPDATE Carros SET ID_Carro=@ID_Carro,Nome=@Nome,Modelo=@Modelo,Combustivel=@Combustivel,Cambio=@Cambio,Portas=@Portas WHERE ID_Carro=@ID_Carro", conn);
    cmd.Parameters.AddWithValue("@ID_Carro", ID);
    cmd.Parameters.AddWithValue("@Nome", tbNome.Text);
    cmd.Parameters.AddWithValue("@Modelo", tbModelo.Text);
    cmd.Parameters.AddWithValue("@Combustivel", cbCombustivel.SelectedItem.ToString());
    cmd.Parameters.AddWithValue("@Cambio", cbCambio.SelectedItem.ToString());
    cmd.Parameters.AddWithValue("@Portas", Int32.Parse(tbPortas.Text));
    cmd.ExecuteNonQuery();
    cmd.Dispose();
    cmd = new SqlCommand("UPDATE Rel_Carro_Apertadeira SET ID_Carro=@ID_Carro,ID_Aper=@ID_Aper WHERE ID_Carro=@ID_Carro", conn);
    cmd.Parameters.AddWithValue("@ID_Carro", ID);
    int ID_Aper = cbApertadeiras.SelectedIndex + 1;
    cmd.Parameters.AddWithValue("@ID_Aper", ID_Aper);
    cmd.ExecuteNonQuery();                
}
else
{
    ID = retornaMaiorID() + 1;               
    cmd = new SqlCommand("INSERT INTO Carros (ID_Carro,Nome,Modelo,Combustivel,Cambio,Portas) Values (@ID_Carro,@Nome,@Modelo,@Combustivel,@Cambio,@Portas)", conn);
    cmd.Parameters.AddWithValue("@ID_Carro", ID);
    cmd.Parameters.AddWithValue("@Nome", tbNome.Text);
    cmd.Parameters.AddWithValue("@Modelo", tbModelo.Text);
    cmd.Parameters.AddWithValue("@Combustivel", cbCombustivel.SelectedItem.ToString());
    cmd.Parameters.AddWithValue("@Cambio", cbCambio.SelectedItem.ToString());
    cmd.Parameters.AddWithValue("@Portas", Int32.Parse(tbPortas.Text));
    cmd.ExecuteNonQuery();
    cmd.Dispose();
    cmd = new SqlCommand("INSERT INTO Rel_Carro_Apertadeira (ID_Carro,ID_Aper) Values (@ID_Carro,@ID_Aper)", conn);
    cmd.Parameters.AddWithValue("@ID_Carro", ID);
    int ID_Aper = cbApertadeiras.SelectedIndex + 1;
    cmd.Parameters.AddWithValue("@ID_Aper", ID_Aper);
    cmd.ExecuteNonQuery();
}

错误为：

"System.InvalidOperationException"类型的未处理异常发生在System.Data.dll中。executeonquery需要打开和可用连接。连接的当前状态为关闭

提前感谢