1- 使用 Oracle.DataAccess 连接到 Oracle XE
2-执行命令向表中添加列：alter table TABLE add COLUMN b int;
3-执行命令以选择此列
4- 使用 DataReader 读取。应用程序引发 IndexOutOfRangeException：无法在结果集中找到指定的列
5-重新启动应用程序，查询正常运行
为什么 DataReader 无法访问我刚才创建的列？
这是一个大而简单的测试代码：
private void button1_Click(object sender, EventArgs e)
{
using (OracleConnection con = new OracleConnection(Settings.Default.CS))
{
con.Open();
try
{
using (OracleCommand com = new OracleCommand())
{
com.Connection = con;
// Create a test table
com.CommandText = "CREATE TABLE Test (a int)";
com.ExecuteNonQuery();
// Add one column
com.CommandText = "ALTER TABLE Test ADD b int";
com.ExecuteNonQuery();
com.CommandText = "SELECT * FROM Test";
using (DbDataReader dr = com.ExecuteReader())
{
MessageBox.Show(dr.FieldCount.ToString());
// Here is showing "2", thats ok
}
}
}
finally
{
con.Close();
}
}
}
private void button2_Click(object sender, EventArgs e)
{
using (OracleConnection con = new OracleConnection(Settings.Default.CS))
{
con.Open();
try
{
using (OracleCommand com = new OracleCommand())
{
OracleTransaction trans = con.BeginTransaction();
try
{
// Add a column to table already created
com.Connection = con;
com.CommandText = "ALTER TABLE Test ADD c int";
com.ExecuteNonQuery();
// Insert a value, ok
com.CommandText = "INSERT INTO TEST (a, b, c) VALUES (1, 2, 3)";
com.ExecuteNonQuery();
trans.Commit();
}
catch
{
trans.Rollback();
throw;
}
// Selecting only "c" column
com.CommandText = "SELECT c FROM Test";
using (DbDataReader dr = com.ExecuteReader())
{
if (dr.Read())
MessageBox.Show(Convert.ToInt32(dr["c"]).ToString());
// Showing correct value, ok
}
// Uncomment these lines to solve problem
//con.Close();
//OracleConnection.ClearAllPools();
//con.Open();
// Selecting all fields * from table
com.CommandText = "SELECT * FROM Test";
using (DbDataReader dr = com.ExecuteReader())
{
MessageBox.Show(dr.GetSchemaTable().Rows.Count.ToString() + " / " + dr.FieldCount.ToString());
// HERE IS THE PROBLEM: message are showing 2/2, but table haves 3 fields
if (dr.Read())
MessageBox.Show(Convert.ToInt32(dr["c"]).ToString());
// Here throws IndexOutOfRangeException: Unable to find specified column in result set
}
}
}
finally
{
con.Close();
}
}
}