try
{
    tmp.OptionalField = reader["optionalfield"].ToString();
}
catch (IndexOutOfRangeException ex)
{
    //do nothing
}

List<string> myCols = new List<string>();
DataTable schema = reader.GetSchemaTable();
foreach (DataRow row in schema.Rows)
{
    myCols.Add(row[schema.Columns["ColumnName"]]);
}

public bool ColumnExists(IDataReader reader, string columnName)
{
    for (int i = 0; i < reader.FieldCount; i++)
    {
        if (reader.GetName(i) == columnName)
        {
            return true;
        }
    }

    return false;
}

DataTable dataTable = new DataTable();
dataTable.Load(reader);
foreach (var item in dataTable.Rows)
{
    bool columnExists = item.Table.Columns.Contains("ColumnName");
}

private static bool ColumnExists(SqlDataReader reader, string columnName)
        {
            using (var schemaTable = reader.GetSchemaTable())
            {
                if (schemaTable != null)
                    schemaTable.DefaultView.RowFilter = String.Format("ColumnName= '{0}'", columnName);

                return schemaTable != null && (schemaTable.DefaultView.Count > 0);
            }
        }