在不预先保存图像的情况下使图片适合图片框？

Question

I have a picture box. The user can draw different rectangles on it, assuming that the user draws rectangles which go beyond the size of the picturebox , can I uniformly resize the contents to fit the picturebox , even though the image is not saved yet?.

I can only manage to resize content that has been loaded in from and already saved image file. The code that I am using to draw rectangles is as follows:

gr.FillRectangle(Brushes.Black, 0, 0, 2, 75)

Rectangles do appear on picturebox as would be expected. Also I am unable to save using the following code:

PictureBox1.Image.Save("C:\test\myimage.jpg", System.Drawing.Imaging.ImageFormat.Jpeg)

The error that I get is: Object reference not set to an instance of an object.

Answer 1

当我想用图像填充图片框而不保存到硬盘驱动器时，我使用System.IO.MemoryStream 。

PictureBox1.Image = New System.Drawing.Bitmap(New IO.MemoryStream(New System.Net.WebClient().DownloadData("http://upload.wikimedia.org/wikipedia/commons/e/ec/Banksy.on.the.thekla.arp.jpg")))

Answer 2

This is probably because your PictureBox has no "Image" object assigned to it.

You can find the solution here (first answer): getting image from a picturebox

[Also, you might face issues later on with regards to saving to your root folder (C:). Try to use a folder on your local Desktop, for example, to avoid any access-permission errors.]

Edit: This is taken from the above link. It appears to me that you have not tried it.

myPictureBox.Image =  New Bitmap(myPictureBox.ClientSize.Width, _
                                 myPictureBox.ClientSize.Height) 

'Then, when you want to perform some drawing, use a Graphics object from the Image instead of the PictureBox:

Using g As Graphics = Graphics.FromImage(myPictureBox.Image)
    ' do the drawing ' 
End Using