Question

I’m getting an exception from SQL Data Reader (MS SQL as datastore) and I’d like to know which column name causes this Exception to be thrown. But I cannot find it in the InnerException.. nowhere.

((System.InvalidOperationException)ex.InnerException).StackTrace:

System.Data.SqlClient.SqlDataReader.ReadColumnHeader(Int32 i)
System.Data.SqlClient.SqlDataReader.IsDBNull(Int32 i)
...

Where is it hidden please ?

Answer 1

You can’t. The data reader does not communicate that in its stack trace. What you can do is wrapping the use of the data reader in another class. In a project I’m working on we used extension methods for this. The class looks like this:

public static class DataRecordExtensions
{
    public static byte GetByte(this IDataRecord record, string name)
    {
        return Get<byte>(record, name);
    }

    public static short GetInt16(this IDataRecord record, string name)
    {
        return Get<short>(record, name);
    }

    public static int GetInt32(this IDataRecord record, string name)
    {
        return Get<int>(record, name);
    }

    private static T Get<T>(IDataRecord record, string name)
    {
        // When the column was not found, an IndexOutOfRangeException will be 
        // thrown. The message will contain the name argument.
        object value = record[name];

        try
        {
            return (T)value;
        }
        catch (InvalidCastException ex)
        {
            throw BuildMoreExpressiveException<T>(record, name, value, ex);
        }
    }

    private static InvalidCastException BuildMoreExpressiveException<T>(
        IDataRecord record, string name, 
        object value, InvalidCastException ex)
    {
        string exceptionMessage = string.Format(CultureInfo.InvariantCulture,
            "Could not cast from {0} to {1}. Column name '{2}' of {3} " + 
            "could not be cast. {4}",
            value == null ? "<null>" : value.GetType().Name, 
            typeof(T).Name, name, record.GetType().FullName, ex.Message);

        return new InvalidCastException(exceptionMessage, ex);
    }
}

You can use it as follows:

using (var reader = SqlHelper.ExecuteReader(...))
{
    while (reader.Read())
    {
        yield return new Order()
        {
            OrderId = reader.GetInt32("orderId"),
            ItemId = reader.GetInt32("itemId")
        };
    }
}

btw. Such an class also allows you to get Nullable<T> objects back and get rid of those manual DbNull conversions you else need to do.