The Controls collection cannot be modified because the control contains code blocks (i.e. <% … %>).

I spent quite some time trying to figure it out. Finally after hours of searching I found out that Code Blocks Inside Master Pages Cause Trouble.

I was using code blocks inside the head tag of all the MasterPages to resolve paths at runtime:

<link rel="stylesheet" href="<%= ResolveUrl("~/myStyleSheet.css") %>" type="text/css" />
<script type="text/javascript" src="<%= ResolveUrl("~/myJavaScript.js") %>"></script>

Fortunately Milan Negovan proposed a quick workaround which saved a lot of time.

Basically, the workaround is to change the code blocks to data binding expressions (<%# ... %>).

<link rel="stylesheet" href="<%# ResolveUrl("~/myStyleSheet.css") %>" type="text/css" />
<script type="text/javascript" src="<%# ResolveUrl("~/myJavaScript.js") %>"></script>

and adding this to the master page code behind:

protected override void OnLoad (EventArgs e)
{
  base.OnLoad (e);
  Page.Header.DataBind ();
}

Since HtmlHead ultimately derives from Control and everything that derives from Control has a DataBind() method, adding that one line above would force the header to resolve data binding expressions.

Thats it, that does the trick!

Finally I figured out what the problem was. I was encapulatning the Bitmap and Graphics objects inside a “using” statement. So the Bitmap object was being prematurely disposed before I returned it back to the caller.

So in the end my solution looked like this. I just god rid of the “usings”.

public Bitmap CropImage(Image image, Rectangle cropRect)
{
    Bitmap bitmap = new Bitmap(cropRect.Width, cropRect.Height, PixelFormat.Format24bppRgb);
    bitmap.SetResolution(image.HorizontalResolution, image.VerticalResolution);
    Graphics graphics = Graphics.FromImage(bitmap);
    graphics.DrawImage(image, 0, 0, cropRect, GraphicsUnit.Pixel);
    return bitmap;
}

Usage -

Bitmap croppedBmp = CropImage(“~/uploads/test.jpg”, new Rectangle(x, y, width, height));
croppedBmp.Save();

Moral of the story: If you are getting this error while calling Bitmap.Save() then make sure you are not disposing your Bitmap object prematurely. Hope this helps.