OleDbDataReader.GetString异常 - 指定的强制转换无效

Question

当我的C#(.NET 3.5)应用程序尝试访问MS Access 2007数据库时,该OleDbReader.GetString()方法抛出异常:

指定演员表无效.

我究竟做错了什么？

OleDbCommand cmd = null;
OleDbDataReader reader = null;
String queryString = "SELECT ids.ENUM_H, bas.[BAS BACnet Object Type/Instance] FROM [OV2 BAS] AS bas INNER JOIN [OV2 RefID] AS ids ON bas.[Ref ID] = ids.[Ref ID]";

this.Open();

try
{
    cmd = new OleDbCommand(queryString, this._conn);
    reader = cmd.ExecuteReader();

    if (!reader.HasRows)
    {
        Exception e = new Exception("Read of mapping table returned no results.");
        throw e;
    }
    else
    {
        while (reader.Read())
        {
            Int32 index;
            String classTypeString = null; // = reader.GetString(reader.GetOrdinal(MappingTable.OBJECT_IDENTIFIER_COLUMN_NAME)).Substring(0, 2);
            int it = reader.GetOrdinal(MappingTable.OBJECT_IDENTIFIER_COLUMN_NAME);
            string st = reader.GetString( it );  // <-- **Exception is thrown here** <--
            st = st.Substring(0,2);
            String classIdString = reader.GetString(reader.GetOrdinal(MappingTable.OBJECT_IDENTIFIER_COLUMN_NAME)).Substring(2);

            index = Convert.ToInt32(classIdString);
            ClassIds[index, 0] = reader.GetString(reader.GetOrdinal("ENUM_H"));
            ClassIds[index, 1] = classTypeString;
        }
    }
}
catch (Exception e)
{
    Console.WriteLine("ERROR: " + e.Message);
    Console.WriteLine(e.ToString());
    throw e;
}

this.Close();

我知道这些Open()和Close()方法都有效.我的查询或处理结果的方式有问题.谢谢.

Answer 1

好的,reader.IsDBNull(1)返回true也是如此...这意味着该字段的特定行中没有数据.

你需要弄清楚这意味着什么,并妥善处理它.您可能希望修改查询不包括这样的行,或者使用reader.IsDBNull来检测这样的行和采取适当的行动,例如使用的字段的默认值.