Ben Reichelt’s Weblog

I recently bought a subscription to www.digitalblasphemy.com because I like the guys wallpapers and hes got a lot of good dual monitor wallpapers.  After I downloaded a bunch of them, I was having trouble settling on one to use for my desktop, so I wanted to write a little app that would pick one at random when I logged on and set it as my desktop background.

After googling for how to set your desktop background in C#, I saw tons of examples that used the Win32 api call SystemParametersInfo.  So I tried using that function in the same way that the examples were using it, but it just wouldn’t take, all I would get was a dark blue background and the function also returned 0, which indicated that an error had occurred.

After some more digging, I found out that SystemParametersInfo only works if you are supplying a .bmp file, whereas my files were .jpg’s.  So, I did some more digging and found that theres a different function to use for files other than bitmaps, the aptly named SetWallpaper function on the IActiveDesktop interface.

I could only find one example of someone who used this interface in c# to set the desktop background, but I couldn’t get it to work, I kept getting an error about wrong GIUD’s for the COM interfaces.  What I ended up doing is using the old SysteParametersInfo function, and after selecting a random jpeg, I would save it out as a bitmap (if the bitmap for that file didn’t already exist) and then pass that bitmap into the function.  Ugh, ugly but it gets the job done.  I could have simply found the registry key that controls the desktop background and set it that way, and maybe I will if I get too annoyed at these extra bitmaps laying around, but I wanted to do it using the appropriate api. 

The point of all this was simply to inform you that SystemParametersInfo only takes bitmaps, not jpegs.  Have a happy Thanksgiving, all you Americans.