Reference C#对象引用未设置为实例

对象引用未设置为下一行中对象的实例是什么意思

case "S Connected":
                        List dd6 = new List(textBox1.Text);
                        **dd6.textBox2.Text = id.ToString();**
                        dd6.ShowDialog();

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谢谢，伙计们。第一种情况工作正常，W连接和S连接检索错误：

  private void dataGridView1_CellDoubleClick(object sender, DataGridViewCellEventArgs e)
    {
        var id = dataGridView1.CurrentRow.Cells["Id"].Value;
        var BBB = dataGridView1.CurrentRow.Cells["Connected"].Value;
        string idd = id.ToString();
        string BBB2 = BBB.ToString();
        textBox1.Text = id.ToString();
                switch (BBB2)
                {
                    case "Standalone":
                        ListStandalone dd = new ListStandalone(textBox1.Text);
                        dd.textBox2.Text = id.ToString();
                        dd.ShowDialog();
                        break;
                    case "W Connected":
                        ListW da2 = new ListW(textBox1.Text);
                        da2.textBox2.Text = id.ToString();
                        da2.ShowDialog();
                        break;
                    case "S Connected":
                        ListS dd6 = new ListS(textBox1.Text);
                        dd6.textBox2.Text = id.ToString();
                        dd6.ShowDialog();
                        break;
                    case "Retail":
                        PrintRetail dd4 = new PrintRetail(textBox1.Text);
                        dd4.Show();

                        break;
                }

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可能重复的请发布完整的示例代码