ERROR system.nullreferenceexception对象引用未设置为对象的实例 [英] ERROR system.nullreferenceexception object reference not set to an instance of an object

问题描述

这是我的代码 

Here's my code 

 private void Form1_Load(object sender, EventArgs e)
        {
            try
            {
                con.Open();
                cmd.Connection = con;
                string sql = "select Department from test";
                cmd.CommandText = sql;

                OleDbDataReader reader = cmd.ExecuteReader();
                while (reader.Read())
                {
                    comboBox1.Items.Add(reader[0].ToString());
                }

                con.Close();
            }
            catch(Exception ex)
            {
                MessageBox.Show("Error " + ex);
            }
        }

问题发生在上面加下划线的行

the problem occurs at the line that is underlined above

并显示此消息 

and this message appears 

system.nullreferenceexception对象引用未设置为对象的实例

system.nullreferenceexception object reference not set to an instance of an object

推荐答案

您需要实例化命令对象（无论您是否使用using语句都无关紧要）

You need to instantiate the command object (doesn't matter if you are using a using statement or not)

public void SelectDemo()
{
    using (OleDbConnection cn = new OleDbConnection("TODO"))
    {
        using (OleDbCommand cmd = new OleDbCommand())
        {
            cmd.Connection = cn;
            cmd.CommandText = "SELECT statement goes here";
            cn.Open();
            // ...
        }
    }
}

以上是OleDb，与SqlClient相同。

The above is OleDb, same for SqlClient.

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