如何在. net实现了上传图片按比例自动缩小或放大

//// 

/// 按比例缩小图片，自动计算宽度
/// 
/// 源图文件名(包括路径)
/// 缩小后保存为文件名(包括路径)
/// 缩小至高度
public void SmallPicWidth(string strOldPic, string strNewPic, int intHeight)
{
  System.Drawing.Bitmap objPic, objNewPic;
  try
  {
 objPic = new System.Drawing.Bitmap(strOldPic);
 int intWidth = (intHeight / objPic.Height) * objPic.Width;
 objNewPic = new System.Drawing.Bitmap(objPic, intWidth, intHeight);
 objNewPic.Save(strNewPic);
  }
  catch (Exception exp) { throw exp; }
  finally
  {
 objPic = null;
 objNewPic = null;
  }
}
/**//// 

/// 缩小图片
/// 
/// 源图文件名(包括路径)
/// 缩小后保存为文件名(包括路径)
/// 缩小至宽度
/// 缩小至高度
public void SmallPic(string strOldPic, string strNewPic, int intWidth, int intHeight)
{
  System.Drawing.Bitmap objPic, objNewPic;
  try
  {
 objPic = new System.Drawing.Bitmap(strOldPic);
 objNewPic = new System.Drawing.Bitmap(objPic, intWidth, intHeight);
 objNewPic.Save(strNewPic);
  }
  catch (Exception exp)
 { throw exp; }
  finally
  {
 objPic = null;
 objNewPic = null;
  }
}