在C#上执行查询时出错

Question

我有问题.我想用这个C#在我的mysqldatabase上存储数据

private void btnSaveFilm_Click(object sender, EventArgs e)
    {
        try 
        {
            MySqlConnection conn = new MySqlConnection(connection.mysqlconnectionbuilder());
            conn.Open();
            MySqlCommand cmd = conn.CreateCommand();
            cmd.CommandText = "INSERT INTO film(judul,genre,asal,kondisi)"
                + "VALUES(@judul,@genre,@asal,@kondisi)";
            cmd.CommandType = CommandType.Text;
            cmd.Parameters.AddWithValue("@judul", textBoxJudul.Text);
            cmd.Parameters.AddWithValue("@genre", category(comboBoxGenre.SelectedValue.ToString()).ToString());
            cmd.Parameters.AddWithValue("@asal", asal(comboBoxAsal.SelectedValue.ToString()).ToString());
            cmd.Parameters.AddWithValue("@kondisi", checkedStatus());
            cmd.ExecuteNonQuery();
            conn.Close();
        }
        catch(Exception exe) 
        { 
            Console.Write("Error on Save Film : " + exe.ToString() + "\n" +exe.Message);
        }

    }

但它显示错误System.NullReferenceException:对象引用未设置为对象的实例.

这一行错误40:

   cmd.Parameters.AddWithValue("@genre",kategori(comboBoxGenre.SelectedValue.ToString()).ToString());

怎么解决？

Answer 1

将你的代码折射成这个,使用using声明,

string connString = connectionmysqlconnectionbuilder();
using (MySqlConnection conn = new MySqlConnection(connString)
{
    using (MySqlCommand cmd = new MySqlCommand())
    {
        cmd.Connection = conn;
        cmd.CommandText = @"INSERT INTO film(judul,genre,asal,kondisi) 
                             VALUES(@judul,@genre,@asal,@kondisi)";
        cmd.Parameters.AddWithValue("@judul", textBoxJudul.Text);
        cmd.Parameters.AddWithValue("@genre", kategori(comboBoxGenre.Text).ToString());
        cmd.Parameters.AddWithValue("@asal", asal(comboBoxAsal.Text).ToString());
        cmd.Parameters.AddWithValue("@kondisi", checkedStatus());
        try
        {
            comm.Open();
            cmd.ExecuteNonQuery();
        }
        catch(MySqlException ex)
        (
            Console.WriteLine(ex.ToString());
        )
    }
}

你也可以用

comboBoxGenre.Text instead of comboBoxGenre.SelectedValue