OleDbDataReader.GetString 异常 - 指定的强制转换无效
当我的 C# (.NET 3.5) 应用程序尝试访问 MS Access 2007 数据库时，OleDbReader.GetString() 方法引发异常：
指定的演员阵容无效。
我做错了什么？
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 Open() 和 Close() 方法可以工作。我的查询或处理结果的方式有问题。谢谢。
When my C# (.NET 3.5) application attempts to access a MS Access 2007 database the OleDbReader.GetString() method is throwing an exception:
Specified cast is not valid.
What am I doing wrong?
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();
I know that the Open() and Close() methods work. Something is wrong with my query, or the way I am processing the results. Thanks.
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评论（2）
好的，所以
reader.IsDBNull(1)返回 true...这意味着该字段的特定行中没有数据。您需要弄清楚这意味着什么，并适当地处理它。您可能希望修改查询以不包含此类行，或使用
reader.IsDBNull来检测此类行并采取适当的操作，例如使用字段的默认值。Okay, so
reader.IsDBNull(1)is returning true... which means there's no data in that particular row for that field.You need to work out what that means, and handle it appropriately. You may want to modify the query to not include such rows, or use
reader.IsDBNullto detect such rows and act appropriately, e.g. using a default value for the field.有时会因为单元格格式而发生。如果您的单元格格式设置为常规，您应该将其更改为文本。
Sometimes it happens because of Cell format. If your Cell format is set to General you should change it to Text .