以下是在 ASP.NET（aspx）中获取指定目录下的所有图片并压缩到指定大小的示例代码：

一、使用 System.Drawing 和 System.IO 命名空间实现

1. 在页面的后台代码中添加以下方法：

using System.Drawing;
using System.Drawing.Imaging;
using System.IO;


public void CompressImagesInDirectory(string sourceDirectory, string destinationDirectory, int maxWidth, int maxHeight)
{
    if (!Directory.Exists(destinationDirectory))
    {
        Directory.CreateDirectory(destinationDirectory);
    }


    string[] imageFiles = Directory.GetFiles(sourceDirectory, "*.jpg");
    string[] pngImageFiles = Directory.GetFiles(sourceDirectory, "*.png");
    string[] allImageFiles = imageFiles.Concat(pngImageFiles).ToArray();


    foreach (string imagePath in allImageFiles)
    {
        using (Image image = Image.FromFile(imagePath))
        {
            int newWidth = image.Width;
            int newHeight = image.Height;


            if (image.Width > maxWidth || image.Height > maxHeight)
            {
                double ratioX = (double)maxWidth / image.Width;
                double ratioY = (double)maxHeight / image.Height;
                double ratio = Math.Min(ratioX, ratioY);


                newWidth = (int)(image.Width * ratio);
                newHeight = (int)(image.Height * ratio);
            }


            using (Bitmap resizedImage = new Bitmap(newWidth, newHeight))
            {
                using (Graphics graphics = Graphics.FromImage(resizedImage))
                {
                    graphics.DrawImage(image, 0, 0, newWidth, newHeight);
                }


                string fileName = Path.GetFileName(imagePath);
                string destinationPath = Path.Combine(destinationDirectory, fileName);


                ImageFormat format = GetImageFormat(imagePath);
                resizedImage.Save(destinationPath, format);
            }
        }
    }
}


private ImageFormat GetImageFormat(string imagePath)
{
    string extension = Path.GetExtension(imagePath).ToLower();
    if (extension == ".jpg" || extension == ".jpeg")
    {
        return ImageFormat.Jpeg;
    }
    else if (extension == ".png")
    {
        return ImageFormat.Png;
    }
    else
    {
        return ImageFormat.Jpeg;
    }
}

2. 然后在按钮的点击事件或其他触发事件中调用这个方法：

protected void CompressButton_Click(object sender, EventArgs e)
{
    string sourceDirectory = @"C:\YourSourceDirectory"; // 指定源目录
    string destinationDirectory = @"C:\YourDestinationDirectory"; // 指定目标目录
    int maxWidth = 800; // 设置最大宽度
    int maxHeight = 600; // 设置最大高度
    CompressImagesInDirectory(sourceDirectory, destinationDirectory, maxWidth, maxHeight);
}

请确保将源目录和目标目录替换为实际的路径，并根据需要调整最大宽度和高度。这个方法将遍历指定目录中的所有 JPEG 和 PNG 图像文件，将它们压缩到指定的最大尺寸，并保存到目标目录中。

以下是在上述代码中添加设置压缩质量参数的方法：

1. 修改 CompressImagesInDirectory 方法如下：

public void CompressImagesInDirectory(string sourceDirectory, string destinationDirectory, int maxWidth, int maxHeight, int jpegQuality)
{
    if (!Directory.Exists(destinationDirectory))
    {
        Directory.CreateDirectory(destinationDirectory);
    }


    string[] imageFiles = Directory.GetFiles(sourceDirectory, "*.jpg");
    string[] pngImageFiles = Directory.GetFiles(sourceDirectory, "*.png");
    string[] allImageFiles = imageFiles.Concat(pngImageFiles).ToArray();


    foreach (string imagePath in allImageFiles)
    {
        using (Image image = Image.FromFile(imagePath))
        {
            int newWidth = image.Width;
            int newHeight = image.Height;


            if (image.Width > maxWidth || image.Height > maxHeight)
            {
                double ratioX = (double)maxWidth / image.Width;
                double ratioY = (double)maxHeight / image.Height;
                double ratio = Math.Min(ratioX, ratioY);


                newWidth = (int)(image.Width * ratio);
                newHeight = (int)(image.Height * ratio);
            }


            using (Bitmap resizedImage = new Bitmap(newWidth, newHeight))
            {
                using (Graphics graphics = Graphics.FromImage(resizedImage))
                {
                    graphics.DrawImage(image, 0, 0, newWidth, newHeight);
                }


                string fileName = Path.GetFileName(imagePath);
                string destinationPath = Path.Combine(destinationDirectory, fileName);


                ImageFormat format = GetImageFormat(imagePath);


                if (format == ImageFormat.Jpeg)
                {
                    // 使用 JPEG 编码器并设置质量参数
                    EncoderParameters encoderParams = new EncoderParameters(1);
                    encoderParams.Param[0] = new EncoderParameter(System.Drawing.Imaging.Encoder.Quality, jpegQuality);
                    resizedImage.Save(destinationPath, GetJpegCodecInfo(), encoderParams);
                }
                else
                {
                    resizedImage.Save(destinationPath, format);
                }
            }
        }
    }
}


private ImageCodecInfo GetJpegCodecInfo()
{
    ImageCodecInfo[] codecs = ImageCodecInfo.GetImageEncoders();
    foreach (ImageCodecInfo codec in codecs)
    {
        if (codec.MimeType == "image/jpeg")
            return codec;
    }
    return null;
}


private ImageFormat GetImageFormat(string imagePath)
{
    string extension = Path.GetExtension(imagePath).ToLower();
    if (extension == ".jpg" || extension == ".jpeg")
    {
        return ImageFormat.Jpeg;
    }
    else if (extension == ".png")
    {
        return ImageFormat.Png;
    }
    else
    {
        return ImageFormat.Jpeg;
    }
}

2. 然后在调用该方法时传入压缩质量参数，例如：

protected void CompressButton_Click(object sender, EventArgs e)
{
    string sourceDirectory = @"C:\YourSourceDirectory";
    string destinationDirectory = @"C:\YourDestinationDirectory";
    int maxWidth = 800;
    int maxHeight = 600;
    int jpegQuality = 80; // 设置 JPEG 压缩质量为 80，可以根据需要调整
    CompressImagesInDirectory(sourceDirectory, destinationDirectory, maxWidth, maxHeight, jpegQuality);
}

这样就可以在压缩图像时设置 JPEG 图像的压缩质量参数了。对于 PNG 图像，目前没有直接设置压缩质量的方法，因为 PNG 是无损压缩格式。