[Photo by Spiralz]

ASP.NET makes managing uploaded images and creating thumbnails relatively straightforward. I have utilized the following process to accept an uploaded image, and resize it to a maximum size of 200×200 pixels.

1. Use HTML’s ‘file’ INPUT tag to upload a user-selected image
2. Capture the uploaded image’s data stream, and create an Image object based off the data stream
3. Determine an appropriate scale factor to resize the image to the maximum desired width and height of 200×200 pixels
4. Create and save a thumbnail of the user’s original uploaded image

HTML

In order to receive an uploaded file, you only need a single server control, which will produce a textbox with a ‘Browse…’ button.

<input type="file" id="FileInput" runat="server" />

Code-Behind

Once the page is created, the code-behind has to handle the image from the postback and save it accordingly. The following code accepts an uploaded image, converts it to a maximum of a 200×200 pixel thumbnail, and saves it to the server.

Dim imgStream As System.IO.Stream
Dim imgPhoto As System.Drawing.Image

'Capture the image's stream
imgStream = Me.FileInput.PostedFile.InputStream
'Create an image (which we will manipulate) based on the stream
imgPhoto = System.Drawing.Image.FromStream(imgStream)

'The image must fit within a 200x200 pixel box
Const maxHeight As Integer = 200
Const maxWidth As Integer = 200

Dim imgHeight As Integer = imgPhoto.Height
Dim imgWidth As Integer = imgPhoto.Width
Dim imgScaleFactor As Double = 1

If imgHeight > maxHeight Then
   imgScaleFactor = maxHeight / imgHeight
End If
If (imgWidth > maxWidth) And (maxWidth / imgWidth < imgScaleFactor) Then
   'The image is wider than it is tall
   imgScaleFactor = maxWidth / imgWidth
End If

'Resize the image based on the calculated scale factor
imgPhoto = imgPhoto.GetThumbnailImage(imgWidth * imgScaleFactor, imgHeight * _
imgScaleFactor, Nothing, Nothing)

Dim extension As String, fileName As String
fileName = Me.FileInput.PostedFile.FileName
'Retrieve the extension of the uploaded file
extension = Mid(fileName, InStrRev(fileName, ".") + 1, fileName.Length)

'Save the file to the server
'Note: you'll want to change the path and image name
imgPhoto.Save(Server.MapPath("/images/img." & extension))

Resizing the image
The imgScaleFactor variable is very important. The scale factor indicates the percent that the image needs to be reduced in order to fit in a 200×200 box. If the image is wider than it is tall, then imgScaleFactor will be based on the image’s width (and vice versa for a taller image). If the image fits inside a 200×200 box, then no resizing is needed.

A few caveats…

1. You might need to give ASP.NET write access to the ‘images’ folder.
2. Before saving, check Me.FileInput.PostedFile.FileName to make sure the user is uploading a valid file type (typically JPG, PNG, GIF) — you don’t want users uploading .EXE files, do you?
3. The filename for the saved image isn’t going to me ‘img’ — it will obviously be something dynamic.