Charles Petzold

More Gripes About XBAPs

March 7, 2007
New York, NY

Chris Sells created the excusegenexpress project in Chapter 10 of his and Ian Griffith's book Programming Windows Presentation Foundation on a computer named EMULH. His user name is csells.

Adam Nathan created the XBAP version of his PhotoGallery program in Chapter 7 of his book Windows Presentation Foundation Unleashed on a computer named NATHANKITCHEN. His user name is Adam.

Is it weird that I should know this information? It's not exactly the type of stuff involved in fraud and identify theft, but it still seems rather personal. After all, I wouldn't want anybody to know that my computer's name is SLUDGEBOX and my user name is SweetThing. Why should that be public information?

I learned about Chris and Adam's computer names and user names simply by opening these two projects from their books in Visual Studio, selecting Project Properties and clicking the Signing tab.

Is it just a coincidence that the two Visual Studio projects in question are both XBAPs? No, it is not.

In a previous blog entry I discussed how I really like the idea of a partial-trust binary executable that combines code and XAML, but I couldn't understand why this binary needed to be multiple files. When I make a XAML file available to the readers of my blog, I just copy it to my Web site. If I want to make an XBAP available, I have to publish it from Visual Studio. I would much rather just copy the .XBAP file to my Web site but I can't because that file is insufficient to run the app.

The situation with XBAPs is doubly complicated for those of us who write books and magazine articles about WPF, and thus share out source code with the public. The problem involves certifying the origin of the executable by digitally signing it.

When you publish a .EXE file to a Web site, it's a good idea to sign the file with a code-signing certificate. Then, when a user downloads the file, a dialog pops up informing the user who signed the .EXE. The user is given the option to allow the .EXE to be downloaded or not. When you're publishing a .EXE file to a Web site using the ClickOnce facility, for example, you sign the file at the time of publication.

But XBAPs are different. XBAPs want to be signed regardless whether they're going to be published or not. An XBAP needs to be signed just to be compiled.

Of course, supplying a code-signing certificate just to compile a program is a real hassle, and if it's a sandboxed partial-trust application, it doesn't need to be signed even if it's going to be published. For this reason, when you use Visual Studio to create a "XAML Browser Application" (i.e., XBAP), Visual Studio creates a "test certificate" for you that contains your computer name and your user name, but otherwise doesn't mean much. You can also create this test certificate manually by clicking the Create Test Certificate button in the Signing tab of Project Properties. The certificate is an actual .PFX file that becomes part of the Visual Studio project, and the information is also encoded into the .CSPROJ project file.

Those of us who write books and articles about WPF and distribute source code to the public are then faced with a question:

Do we give out Visual Studio project files that contain these test certificates (and hence, our computer names and user names) and say "Well, it's kind of weird but I guess it's OK because Microsoft knows best."?

Or do we strip out from the .CSPROJ file all references to the code-signing certificate and in particular remove the ManifestCertificateThumbprint tag that encodes the certificate information in hexadecimal form?

If we choose the first route as Chris and Adam did with the source code for their books, then someone getting the source code could modify it and even publish the XBAP, and the project would still have the original test certificate attached to it. This may not mean much in practice, but conceptually it's not quite right.

If we choose the second route, then we are distributing a .CSPROJ file that will not compile! We have to tell the programmers getting this project file that they first need to bring up Project Properties, select the Signing tab, and click "Create Test Certificate" to create a certificate containing their own computer name and user name.

I want to love XBAPs. I really do. But they make it so hard.