OleDbDataReader.GetString Exception - Specified cast is not valid

Question

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.

What's the type of that column? If you call reader.GetValue() instead, what happens? Might that value be null? — Jon Skeet
– Jon Skeet, Commented Nov 4, 2011 at 16:52
Yes, it does, but it returns values for ids.ENUM_H, which is expected. When I step through it does evaluate to 1 as expected. When I change it to string st = reader.GetString(1) I get the same error. It's like it's not pulling anything from the database for bas.[BAS BACnet Object Type/Instance]. — Jim Fell
– Jim Fell, Commented Nov 4, 2011 at 16:55

Jon Skeet · Accepted Answer · 2011-11-04 17:54:35Z

9

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.IsDBNull to detect such rows and act appropriately, e.g. using a default value for the field.

answered Nov 4, 2011 at 17:54

Jon Skeet

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1 Comment

Jim Fell Over a year ago

Thanks for the help! Now that I know what's happening I can handle it. I looked in the database and you were right; there is no data in that row for that field.

MojAmiri · Accepted Answer · 2014-08-12 08:43:16Z

0

Sometimes it happens because of Cell format. If your Cell format is set to General you should change it to Text .

answered Aug 12, 2014 at 8:43

MojAmiri

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