C# 以特定大小显示图像，无需压缩即可获得特定大小的图像

我对图像尺寸有问题。 当我上传了500*500维或其他尺寸的图像时，在这种情况下，我希望输出1024*768维的图像而不进行压缩。表示在图像背景上自动添加其他空间（作为黑色背景或其他）。 我怎么办，有什么建议吗

首先，我尝试在图像控件上以固定大小显示图像

.aspx

在此期间，我还使用了此代码。此代码用于压缩图像大小

System.Drawing.Image fullSizeImg = System.Drawing.Image.FromFile(imagePath);
            var thumbnailImg = new Bitmap(1024, 768);
            var thumbGraph = Graphics.FromImage(thumbnailImg);
            thumbGraph.CompositingQuality = System.Drawing.Drawing2D.CompositingQuality.HighQuality;
            thumbGraph.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;
            thumbGraph.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.HighQualityBicubic;
            var imageRectangle = new Rectangle(0, 0, 1024, 768);
            thumbGraph.DrawImage(fullSizeImg, imageRectangle);
            string targetPath = imagePath.Replace(Path.GetFileNameWithoutExtension(imagePath), Path.GetFileNameWithoutExtension(imagePath) + "-resize");
            string SName = Path.GetFileName(targetPath);
            string SImgName = ("~/imgprev/") + SName;
            //string st = targetPath.ToString();
            //string stArr = st.Split('\\')[8];        

            thumbnailImg.Save(targetPath, System.Drawing.Imaging.ImageFormat.Jpeg); //(A generic error occurred in GDI+) Error occur here !
            Label1.Text = SImgName.ToString();

            thumbnailImg.Dispose();
            return targetPath;

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你能写出你到目前为止试过的东西吗？我已经添加了一些代码。

protected void btnsave_Click(object sender, EventArgs e)
{
    string filepath = ImagePreview1.ImageUrl;
    using (WebClient client = new WebClient())
    {
        client.DownloadFile(filepath, Server.MapPath("~/imgprev/pic.jpg"));
    }

}

System.Drawing.Image fullSizeImg = System.Drawing.Image.FromFile(imagePath);
            var thumbnailImg = new Bitmap(1024, 768);
            var thumbGraph = Graphics.FromImage(thumbnailImg);
            thumbGraph.CompositingQuality = System.Drawing.Drawing2D.CompositingQuality.HighQuality;
            thumbGraph.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;
            thumbGraph.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.HighQualityBicubic;
            var imageRectangle = new Rectangle(0, 0, 1024, 768);
            thumbGraph.DrawImage(fullSizeImg, imageRectangle);
            string targetPath = imagePath.Replace(Path.GetFileNameWithoutExtension(imagePath), Path.GetFileNameWithoutExtension(imagePath) + "-resize");
            string SName = Path.GetFileName(targetPath);
            string SImgName = ("~/imgprev/") + SName;
            //string st = targetPath.ToString();
            //string stArr = st.Split('\\')[8];        

            thumbnailImg.Save(targetPath, System.Drawing.Imaging.ImageFormat.Jpeg); //(A generic error occurred in GDI+) Error occur here !
            Label1.Text = SImgName.ToString();

            thumbnailImg.Dispose();
            return targetPath;