昨天花了点时间写了几个比较实用的函数，分享给大家
代码如下，因为觉得可能对大家有所帮助，所以不敢独享。
/// <summary>/// 获取一个图片按等比例缩小后的大小。/// </summary>/// <param name="maxWidth">需要缩小到的宽度</param>/// <param name="maxHeight">需要缩小到的高度</param>/// <param name="imageOriginalWidth">图片的原始宽度</param>/// <param name="imageOriginalHeight">图片的原始高度</param>/// <returns>返回图片按等比例缩小后的实际大小</returns>public static Size GetNewSize(int maxWidth, int maxHeight, int imageOriginalWidth, int imageOriginalHeight){ double w = 0.0; double h = 0.0; double sw = Convert.ToDouble(imageOriginalWidth); double sh = Convert.ToDouble(imageOriginalHeight); double mw = Convert.ToDouble(maxWidth); double mh = Convert.ToDouble(maxHeight); if (sw < mw && sh < mh) { w = sw; h = sh; } else if ((sw / sh) > (mw / mh)) { w = maxWidth; h = (w * sh) / sw; } else { h = maxHeight; w = (h * sw) / sh; } return new Size(Convert.ToInt32(w), Convert.ToInt32(h));}/// <summary>/// 对给定的一个图片（Image对象）生成一个指定大小的缩略图。/// </summary>/// <param name="originalImage">原始图片</param>/// <param name="thumMaxWidth">缩略图的宽度</param>/// <param name="thumMaxHeight">缩略图的高度</param>/// <returns>返回缩略图的Image对象</returns>public static System.Drawing.Image GetThumbNailImage(System.Drawing.Image originalImage, int thumMaxWidth, int thumMaxHeight){ Size thumRealSize = Size.Empty; System.Drawing.Image newImage = originalImage; Graphics graphics = null; try { thumRealSize = GetNewSize(thumMaxWidth, thumMaxHeight, originalImage.Width, originalImage.Height); newImage = new Bitmap(thumRealSize.Width, thumRealSize.Height); graphics = Graphics.FromImage(newImage); graphics.CompositingQuality = CompositingQuality.HighQuality; graphics.InterpolationMode = InterpolationMode.HighQualityBicubic; graphics.SmoothingMode = SmoothingMode.HighQuality; graphics.Clear(Color.Transparent); graphics.DrawImage(originalImage, new Rectangle(0, 0, thumRealSize.Width, thumRealSize.Height), new Rectangle(0, 0, originalImage.Width, originalImage.Height), GraphicsUnit.Pixel); } catch { } finally { if (graphics != null) { graphics.Dispose(); graphics = null; } } return newImage;}/// <summary>/// 对给定的一个图片文件生成一个指定大小的缩略图。/// </summary>/// <param name="originalImage">图片的物理文件地址</param>/// <param name="thumMaxWidth">缩略图的宽度</param>/// <param name="thumMaxHeight">缩略图的高度</param>/// <returns>返回缩略图的Image对象</returns>public static System.Drawing.Image GetThumbNailImage(string imageFile, int thumMaxWidth, int thumMaxHeight){ System.Drawing.Image originalImage = null; System.Drawing.Image newImage = null; try { originalImage = System.Drawing.Image.FromFile(imageFile); newImage = GetThumbNailImage(originalImage, thumMaxWidth, thumMaxHeight); } catch { } finally { if (originalImage != null) { originalImage.Dispose(); originalImage = null; } } return newImage;}/// <summary>/// 对给定的一个图片文件生成一个指定大小的缩略图，并将缩略图保存到指定位置。/// </summary>/// <param name="originalImageFile">图片的物理文件地址</param>/// <param name="thumbNailImageFile">缩略图的物理文件地址</param>/// <param name="thumMaxWidth">缩略图的宽度</param>/// <param name="thumMaxHeight">缩略图的高度</param>public static void MakeThumbNail(string originalImageFile, string thumbNailImageFile, int thumMaxWidth, int thumMaxHeight){ System.Drawing.Image newImage = GetThumbNailImage(originalImageFile, thumMaxWidth, thumMaxHeight); try { newImage.Save(thumbNailImageFile, ImageFormat.Jpeg); } catch { } finally { newImage.Dispose(); newImage = null; }}/// <summary>/// 将一个图片的内存流调整为指定大小，并返回调整后的内存流。/// </summary>/// <param name="originalImageStream">原始图片的内存流</param>/// <param name="newWidth">新图片的宽度</param>/// <param name="newHeight">新图片的高度</param>/// <returns>返回调整后的图片的内存流</returns>public static MemoryStream ResizeImage(Stream originalImageStream, int newWidth, int newHeight){ MemoryStream newImageStream = null; System.Drawing.Image newImage = Globals.GetThumbNailImage(System.Drawing.Image.FromStream(originalImageStream), newWidth, newHeight); if (newImage != null) { newImageStream = new MemoryStream(); newImage.Save(newImageStream, ImageFormat.Jpeg); } return newImageStream;}/// <summary>/// 将一个内存流保存为磁盘文件。/// </summary>/// <param name="stream">内存流</param>/// <param name="newFile">目标磁盘文件地址</param>public static void SaveStreamToFile(Stream stream, string newFile){ if (stream == null || stream.Length == 0 || string.IsNullOrEmpty(newFile)) { return; } byte[] buffer = new byte[stream.Length]; stream.Position = 0; stream.Read(buffer, 0, buffer.Length); FileStream fileStream = new FileStream(newFile, FileMode.OpenOrCreate, FileAccess.Write); fileStream.Write(buffer, 0, buffer.Length); fileStream.Flush(); fileStream.Close(); fileStream.Dispose();}/// <summary>/// 对一个指定的图片加上图片水印效果。/// </summary>/// <param name="imageFile">图片文件地址</param>/// <param name="waterImage">水印图片（Image对象）</param>public static void CreateImageWaterMark(string imageFile, System.Drawing.Image waterImage){ if (string.IsNullOrEmpty(imageFile) || !File.Exists(imageFile) || waterImage == null) { return; } System.Drawing.Image originalImage = System.Drawing.Image.FromFile(imageFile); if (originalImage.Width - 10 < waterImage.Width || originalImage.Height - 10 < waterImage.Height) { return; } Graphics graphics = Graphics.FromImage(originalImage); int x = originalImage.Width - waterImage.Width - 10; int y = originalImage.Height - waterImage.Height - 10; int width = waterImage.Width; int height = waterImage.Height; graphics.DrawImage(waterImage, new Rectangle(x, y, width, height), 0, 0, width, height, GraphicsUnit.Pixel); graphics.Dispose(); MemoryStream stream = new MemoryStream(); originalImage.Save(stream, ImageFormat.Jpeg); originalImage.Dispose(); System.Drawing.Image imageWithWater = System.Drawing.Image.FromStream(stream); imageWithWater.Save(imageFile); imageWithWater.Dispose();}/// <summary>/// 对一个指定的图片加上文字水印效果。/// </summary>/// <param name="imageFile">图片文件地址</param>/// <param name="waterText">水印文字内容</param>public static void CreateTextWaterMark(string imageFile, string waterText){ if (string.IsNullOrEmpty(imageFile) || string.IsNullOrEmpty(waterText) || !File.Exists(imageFile)) { return; } System.Drawing.Image originalImage = System.Drawing.Image.FromFile(imageFile); Graphics graphics = Graphics.FromImage(originalImage); graphics.SmoothingMode = SmoothingMode.HighQuality; graphics.TextRenderingHint = TextRenderingHint.ClearTypeGridFit; graphics.CompositingQuality = CompositingQuality.HighQuality; graphics.InterpolationMode = InterpolationMode.HighQualityBicubic; SolidBrush brush = new SolidBrush(Color.FromArgb(153, 255, 255, 255)); Font waterTextFont = new Font("Arial", 16, FontStyle.Regular); SizeF waterTextSize = graphics.MeasureString(waterText, waterTextFont); float x = (float)originalImage.Width - waterTextSize.Width - 10F; float y = (float)originalImage.Height - waterTextSize.Height - 10F; graphics.DrawString(waterText, waterTextFont, brush, x, y); graphics.Dispose(); brush.Dispose(); MemoryStream stream = new MemoryStream(); originalImage.Save(stream, ImageFormat.Jpeg); originalImage.Dispose(); System.Drawing.Image imageWithWater = System.Drawing.Image.FromStream(stream); imageWithWater.Save(imageFile); imageWithWater.Dispose();}/// <summary>/// 判断上传组件是否包含内容。/// </summary>/// <param name="fileUpload">ASP.NET 2.0标准上传组件</param>/// <returns>如果数据有效，则返回True，否则返回False</returns>public static bool IsAttachmentValid(FileUpload fileUpload){ if (fileUpload != null && fileUpload.PostedFile != null && !string.IsNullOrEmpty(fileUpload.PostedFile.FileName) && fileUpload.PostedFile.ContentLength > 0) { return true; } return false;}