winform中二级联动的Combobox
有个省 市的二级联动Combobox
private void prov_SelectedIndexChanged(object sender, EventArgs e)
{
string sql = "select * from city where prov_id=" + prov.SelectedValue.ToString();
// MessageBox.Show(sql);
OleDbDataAdapter ad = new OleDbDataAdapter(sql, conn);
ds.Tables["city"].Clear();
ad.Fill(ds,"city");
city.DataSource=ds.Tables["city"];
city.DisplayMember = "city_name";
city.ValueMember = "city_id";
}
当省SelectedIndexChanged的时候市(city)填充新的数据
ds.Tables["city"].Clear(); 提示"错误未将对象引用设置到对象的实例。"
什么原因啊