/// <summary>
/// 输出一行内容。
/// </summary>
/// <param name="level">级别，为空时将默认为{Empty}</param>
/// <param name="message">消息</param>
public virtual void WriteLine(string level, string message)
{
if (string.IsNullOrEmpty(level))
{
level = "{Empty}";
}
if (_levels.Count == 0 || _levels.Contains(level))
{
OnWriteLine(level, message);
}
}