ASP.NET C# 上傳圖檔並產生縮圖

在縮圖前就先計算好維持比例的縮圖尺寸，例如 800x600 要縮成 200x200 時，實際上應該縮成 200x150 ，於是下參數就下 200x150 ，修改後的原始碼(.CS)：

using System;
using System.Drawing;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;

public partial class UploadPicture01 : System.Web.UI.Page
{
    // 設定縮圖後的檔案存放路徑
    private const string savePath = @"C:\Inetpub\wwwroot\200x200\";

    // 設定暫存路徑
    private const string tempPath = @"C:\Inetpub\wwwroot\temp\";

    protected void Button1_Click(object sender, EventArgs e)
    {
        if (FileUpload1.HasFile)
        {
            string tempName = tempPath + FileUpload1.FileName;
            string imageName = savePath + FileUpload1.FileName;

            // 儲存暫存檔
            FileUpload1.SaveAs(tempName);

            // System.Web.UI.WebControls 與 System.Drawing 同時擁有 Image 類別
            // 所以以下程式碼明確指定要使用的是 System.Drawing.Image

            System.Drawing.Image.GetThumbnailImageAbort callBack = 
             new System.Drawing.Image.GetThumbnailImageAbort(ThumbnailCallback);
            Bitmap image = new Bitmap(tempName);

            // 計算維持比例的縮圖大小
            int[] thumbnailScale = getThumbnailImageScale(200, 200, image.Width, image.Height);

            // 產生縮圖
            System.Drawing.Image smallImage = 
            image.GetThumbnailImage(thumbnailScale[0], thumbnailScale[1], callBack, IntPtr.Zero);

            // 將縮圖存檔
            smallImage.Save(imageName);

            // 釋放並刪除暫存檔
            image.Dispose();
            System.IO.File.Delete(tempName);
        }
    }

    // 計算維持比例的縮圖大小
    private int[] getThumbnailImageScale(int maxWidth, int maxHeight, int oldWidth, int oldHeight)
    {
        int[] result = new int[] { 0, 0 };
        float widthDividend, heightDividend, commonDividend;

        widthDividend = (float)oldWidth / (float)maxWidth;
        heightDividend = (float)oldHeight / (float)maxHeight;

        commonDividend = (heightDividend > widthDividend) ? heightDividend : widthDividend;
        result[0] = (int)(oldWidth / commonDividend);
        result[1] = (int)(oldHeight / commonDividend);

        return result;
    }

    private bool ThumbnailCallback()
    {
        return false;
    }
}