Build Warning: The file 'filename.ext' could not be added to the project...

I recently encountered a build warning (C# project, Visual Studio 2005) that was less than informative and somewhat difficult to resolve:

Warning: The file 'keyfile.snk' could not be added to the project. A file with the same path already exists in the project. MyProject.UI

(Note: keyfile.snk and MyProject.UI can be any file 0r project.)

Other than the terse warning, there was nothing to go on. I tried deleting the file from the OS, removing/re-adding to the project, turning off signing, etc. Nothing worked, the error persisted.

I finally opened up the offending project file (MyProject.UI.csproj in this case) in Notepad, and searched for this particular file (keyfile.snk). The source of the warning became immediately obvious -- within one of the <ItemGroup> blocks, I noticed the following:

...
<None Include="keyfile.snk" />
<None Include="keyfile.snk" />
...

So, even though the file was only being included/referenced/visible once in the solution explorer, it was actually being included twice in the project file. I have no idea how/why that happened.

Regardless, the solution is simple -- remove one of the lines, save, and reload your project.

Specifying an Alternate Assembly for StartupURI

By default, when creating a WPF application in Visual Studio 2008, the StartupURI value is appropriately set to the main window of the application (typically Window1.xaml).

It is simple enough to manually edit the StartupURI to point to another component, provided the code resides in the local assembly. Referencing a window in another assembly is not so intuitive. In order to specify an alternate assembly, the pack:// nomenclature must be used:

StartupUri="pack://application:,,,/assembly_name;component/path/file_name.xaml"
Where:

• assembly_name is the name of the referenced assembly, sans extension
• path is the subfolder in which the component resides; if the component is at the project root, this element is omitted
• file_name is the file name of the component

Examples:

pack://application:,,,/UI;component/CalculatorView.xaml
assembly - UI.dll
path - none (file at project root)
file_name - CalculatorView

pack://application:,,,/MyApp.UI;component/Views/CalculatorView.xaml
assembly - MyApp.UI.dll
path - Views
file_name - CalculatorView

pack://application:,,,/UI;component/Views/External/CalculatorView.xaml assembly - UI.dll
path - Views/External
file_name - CalculatorView