你可以试试net-vips, libvips的c#绑定。它是一个懒惰的、流的、需求驱动的图像处理库，所以它可以做这样的操作，而不需要加载整个图像。

例如，它附带了一个方便的图像缩略图:

using Image image = Image.Thumbnail("image.jpg", 300, 300);
image.WriteToFile("my-thumbnail.jpg");

它还支持智能裁剪，这是一种智能确定图像中最重要的部分并在裁剪图像时保持对焦的方法。例如:

using Image image = Image.Thumbnail("owl.jpg", 128, crop: Enums.Interesting.Attention);
image.WriteToFile("tn_owl.jpg");

其中owl.jpg是一个偏离中心的合成:

给出如下结果:

首先，它将图像的垂直轴缩小到128像素，然后使用注意力策略将图像缩小到128像素。这一个搜索图像中可能吸引人眼的特征，详见Smartcrop()。

在这个问题中，你会得到一些答案，包括我的:

public Image resizeImage(int newWidth, int newHeight, string stPhotoPath)
 {
     Image imgPhoto = Image.FromFile(stPhotoPath); 

     int sourceWidth = imgPhoto.Width;
     int sourceHeight = imgPhoto.Height;

     //Consider vertical pics
    if (sourceWidth < sourceHeight)
    {
        int buff = newWidth;

        newWidth = newHeight;
        newHeight = buff;
    }

    int sourceX = 0, sourceY = 0, destX = 0, destY = 0;
    float nPercent = 0, nPercentW = 0, nPercentH = 0;

    nPercentW = ((float)newWidth / (float)sourceWidth);
    nPercentH = ((float)newHeight / (float)sourceHeight);
    if (nPercentH < nPercentW)
    {
        nPercent = nPercentH;
        destX = System.Convert.ToInt16((newWidth -
                  (sourceWidth * nPercent)) / 2);
    }
    else
    {
        nPercent = nPercentW;
        destY = System.Convert.ToInt16((newHeight -
                  (sourceHeight * nPercent)) / 2);
    }

    int destWidth = (int)(sourceWidth * nPercent);
    int destHeight = (int)(sourceHeight * nPercent);


    Bitmap bmPhoto = new Bitmap(newWidth, newHeight,
                  PixelFormat.Format24bppRgb);

    bmPhoto.SetResolution(imgPhoto.HorizontalResolution,
                 imgPhoto.VerticalResolution);

    Graphics grPhoto = Graphics.FromImage(bmPhoto);
    grPhoto.Clear(Color.Black);
    grPhoto.InterpolationMode =
        System.Drawing.Drawing2D.InterpolationMode.HighQualityBicubic;

    grPhoto.DrawImage(imgPhoto,
        new Rectangle(destX, destY, destWidth, destHeight),
        new Rectangle(sourceX, sourceY, sourceWidth, sourceHeight),
        GraphicsUnit.Pixel);

    grPhoto.Dispose();
    imgPhoto.Dispose();
    return bmPhoto;
}

本遗嘱-

调整宽度和高度，不需要循环 不超过图像的原始尺寸

//////////////

private void ResizeImage(Image img, double maxWidth, double maxHeight)
{
    double resizeWidth = img.Source.Width;
    double resizeHeight = img.Source.Height;

    double aspect = resizeWidth / resizeHeight;

    if (resizeWidth > maxWidth)
    {
        resizeWidth = maxWidth;
        resizeHeight = resizeWidth / aspect;
    }
    if (resizeHeight > maxHeight)
    {
        aspect = resizeWidth / resizeHeight;
        resizeHeight = maxHeight;
        resizeWidth = resizeHeight * aspect;
    }

    img.Width = resizeWidth;
    img.Height = resizeHeight;
}

这个代码与上面的一个答案相同。但将透明像素转换为白色而不是黑色…谢谢:)

    public Image resizeImage(int newWidth, int newHeight, string stPhotoPath)
    {
        Image imgPhoto = Image.FromFile(stPhotoPath);

        int sourceWidth = imgPhoto.Width;
        int sourceHeight = imgPhoto.Height;

        //Consider vertical pics
        if (sourceWidth < sourceHeight)
        {
            int buff = newWidth;

            newWidth = newHeight;
            newHeight = buff;
        }

        int sourceX = 0, sourceY = 0, destX = 0, destY = 0;
        float nPercent = 0, nPercentW = 0, nPercentH = 0;

        nPercentW = ((float)newWidth / (float)sourceWidth);
        nPercentH = ((float)newHeight / (float)sourceHeight);
        if (nPercentH < nPercentW)
        {
            nPercent = nPercentH;
            destX = System.Convert.ToInt16((newWidth -
                      (sourceWidth * nPercent)) / 2);
        }
        else
        {
            nPercent = nPercentW;
            destY = System.Convert.ToInt16((newHeight -
                      (sourceHeight * nPercent)) / 2);
        }

        int destWidth = (int)(sourceWidth * nPercent);
        int destHeight = (int)(sourceHeight * nPercent);


        Bitmap bmPhoto = new Bitmap(newWidth, newHeight,
                      PixelFormat.Format24bppRgb);

        bmPhoto.SetResolution(imgPhoto.HorizontalResolution,
                     imgPhoto.VerticalResolution);

        Graphics grPhoto = Graphics.FromImage(bmPhoto);
        grPhoto.Clear(Color.White);
        grPhoto.InterpolationMode =
            System.Drawing.Drawing2D.InterpolationMode.HighQualityBicubic;

        grPhoto.DrawImage(imgPhoto,
            new Rectangle(destX, destY, destWidth, destHeight),
            new Rectangle(sourceX, sourceY, sourceWidth, sourceHeight),
            GraphicsUnit.Pixel);

        grPhoto.Dispose();
        imgPhoto.Dispose();

        return bmPhoto;
    }

为什么不用System.Drawing.Image.GetThumbnailImage方法呢?

public Image GetThumbnailImage(
    int thumbWidth, 
    int thumbHeight, 
    Image.GetThumbnailImageAbort callback, 
    IntPtr callbackData)

例子:

Image originalImage = System.Drawing.Image.FromStream(inputStream, true, true);
Image resizedImage = originalImage.GetThumbnailImage(newWidth, (newWidth * originalImage.Height) / originalWidth, null, IntPtr.Zero);
resizedImage.Save(imagePath, ImageFormat.Png);

来源: http://msdn.microsoft.com/en-us/library/system.drawing.image.getthumbnailimage.aspx

如果你正在使用BitmapSource:

var resizedBitmap = new TransformedBitmap(
    bitmapSource,
    new ScaleTransform(scaleX, scaleY));

如果你想要更好的控制质量，首先运行这个:

RenderOptions.SetBitmapScalingMode(
    bitmapSource,
    BitmapScalingMode.HighQuality);

(默认为BitmapScalingMode。线性，相当于BitmapScalingMode.LowQuality。)