Xcode won't open: "Mac OS X platform is missing"

Xcode won't open. I had it running fine a couple of weeks ago. Now I get the following error message when Xcode tries to open:

Internal Error  
File:  /SourceCache/DevToolsBase/.../XCPlatformSpecification.m开发者_如何学JAVA  
Line:  438  
Object:  <XCPlatformSpecification>  
Method:  loadAllPlatforms  
The Mac OS X platform is missing - cannot set a default platform.  

Any idea what caused this or how I can recover? Changes I have made to my system (Snow Leopard) in the last couple of weeks include (a) allowing Software Update to run and (b) installing Eclipse IDE. But the problem may or may not be related to these changes.

If /Developer/Platforms/MacOSX is missing, then there's really not much you can do with Xcode. Reinstall it from the Developer DVD or download.

After letting "Software Update" upgrade the operating system from OS X 10.6.2 to 10.6.3, it also wanted to run an upgrade package for XCode, from 3.2.1 to 3.2.2, but somehow I skipped that step. After letting "Software Update" upgrade XCode to 3.2.2, XCode appeared to be working again.

However, victory was short-lived as I next encountered strange build errors on new and existing projects, related to an unrecognized "product type."

Finally decided to blow XCode away and re-install 3.2.2 from scratch, using the full download from developer.apple.com. It took a while to download and install, but that seems to have done the trick. New and existing projects now build and run normally.