combobox的未将对象的引用设置到对象的实例错误

[Microtigers]

private void Form1_Load(object sender, EventArgs e){
provbox.Items.Add(new DictionaryEntry("请选择省",""));
provbox.SelectedIndex = 0;
OleDbCommand cmd = new OleDbCommand("select * from prov", conn);
OleDbDataReader reader = cmd.ExecuteReader();
while (reader.Read())
{
provbox.Items.Add(new DictionaryEntry(reader["prov_name"], reader["prov_id"]));
}
provbox.DisplayMember = "Key";
provbox.ValueMember = "Value";
}
在form加载的时候初始化 combobox控件provbox
private void provbox_SelectedIndexChanged(object sender, EventArgs e)
{
string sql = "select * from city where prov_id=" + provbox.SelectedValue.ToString();
}
string sql = "select * from city where prov_id=" + provbox.SelectedValue.ToString(); 提示未将对象的引用设置到对象的实例

是哪里出错吴了呢

[zllking]

设置断点跟踪进去调试看看

[Microtigers]

断点跟踪怎么做,不会.

[zllking]

在你CS页面行号的前面点一下，出来一个红色的亮圆就是断点，还是建议楼主去看看VS帮助文档先

[Fibona]

if(provbox.SelectedValue!=null)
{
string sql = "select * from city where prov_id=" + provbox.SelectedValue.ToString(); 
}

因为你没有选择的时候SelectedValue为null
null->ToString()当然报错
在前面加一个判断

[Microtigers]

但是我选择了一个,怎么能没有数据呢!

[zllking]

是不是provbox压根都没有绑定Value？

[deng520159]

你这个不能用selectvalue.
仔细研究一下selectitem selecttext,selectvalue 这三个有什么区别，
下面是绑定combox
/// <summary>
/// 绑定combobox
/// </summary>
/// <param name="cb"></param>
/// <param name="sql"></param>
/// <param name="textField">显示字段</param>
/// <param name="valueField">值字段</param>
/// <param name="conn"></param>
/// <returns></returns>
public static bool bindComboBox(ComboBox cb, string sql, string textField, string valueField, string conn_str)
{
bool result = false;
OleDbConnection conn = new OleDbConnection(conn_str);
try
{

conn.Open();
string tablename = "temp";
cb.DataSource = ReturnDataTable(conn_str, sql, tablename);
cb.DisplayMember = textField;
cb.ValueMember = valueField;
result = true;
}
catch (Exception err)
{
//StringCom.showInformation(err.Message);
}
finally
{
conn.Close();
}
return result;
}