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

public static void ResizeFromStream(string ImageSavePath, int MaxSideSize, Stream Buffer)
{
int intNewWidth;
int intNewHeight;
Image imgInput = Image.FromStream(Buffer);

//Determine image format
ImageFormat fmtImageFormat = imgInput.RawFormat;

//get image original width and height
int intOldWidth = imgInput.Width;
int intOldHeight = imgInput.Height;

//determine if landscape or portrait
int intMaxSide;

if (intOldWidth >= intOldHeight)
{
intMaxSide = intOldWidth;
}
else
{
intMaxSide = intOldHeight;
}

if (intMaxSide > MaxSideSize)
{
//set new width and height
double dblCoef = MaxSideSize / (double)intMaxSide;
intNewWidth = Convert.ToInt32(dblCoef * intOldWidth);
intNewHeight = Convert.ToInt32(dblCoef * intOldHeight);
}
else
{
intNewWidth = intOldWidth;
intNewHeight = intOldHeight;
}
//create new bitmap
Bitmap bmpResized = new Bitmap(imgInput, intNewWidth, intNewHeight);

//save bitmap to disk
bmpResized.Save(ImageSavePath, fmtImageFormat);

//release used resources
imgInput.Dispose();
bmpResized.Dispose();
Buffer.Close();
}