Build fails with "...Web.csproj" was not found

   Your build fails with single message such as "The project file "..\ModelExplorerWeb\PrismExplorerWeb.csproj" was not found. PrismExplorer.Shell"
In this example "Shell" is your Silverlight application project ... and you have no idea why it would even be looking for a Web project ... let along this one. You search and can't find any references to it ... and you say "That would be a .NET project anyway; my Silverlight project can't have a reference to a .NET project".

THINK: Did you install RIA Services ... perhaps to see how that works?

I did. The install modifies the wizards in Visual Studio and this has consequences for all of your existing Silverlight applications. Yup ... they could all fail this way and they will all build now with hidden junk you don't want. Read on.

I'm assuming you aren't using RIA Services. Well RIA Services is using you. Here are your clues.

1) Look at the Silverlight tab of your Silverlight applications properties view. There's something new at the bottom, The .NET RIA Services link, and it's pointing at a web project sort of like this:

You didn't choose this setting. RIA did it for you ... without asking. Thanks, RIA Services!

In fact, the project named could be detritus from who knows where and might describe a project you don't even have!

If you aren't using RIA Services in your application, reset the value to "<No Project Set>". Even if there is no apparent problem, change this to "<No Project Set>". If you don't, there is another side-effect. This is clue-to-problem #2

2) RIA Services is generating code into a hidden folder!

In the Solution Explorer, open up the Silverlight application node
Click the "Show all files button"
Notice the "Generated_Code" folder!

You certainly don't want to carry this around. Get rid of it:

Make sure the .NET RIA Services Link = "<No Project Set>"
Delete "Generated_Code"
Now rebuild. The problem should go away and the Generated_Code folder should stay gone.
Thanks, RIA Services!