The Build Action property indicates what Visual Studio does with a file when a build is executed. Build Action can have one of several values:
None - The file is not included in the project output group and is not compiled in the build process. An example is a text file that contains documentation, such as a Readme file.
Compile - The file is compiled into the build output. This setting is used for code files.
Content - The file is not compiled, but is included in the Content output group. For example, this setting is the default value for an .htm or other kind of Web file.
Embedded Resource - This file is embedded in the main project build output as a DLL or executable. It is typically used for resource files.
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Build Action property is extensible. As a result, you may see additional options listed for this property that have been added by other products and features.
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The default value for Build Action depends on the extension of the file that you add to the solution. For example, if you add a Visual Basic project to Solution Explorer, the default value for Build Action is Compile. This is because the extension .vb indicates a code file that can be compiled. File names and extensions appear in Solution Explorer.
Note that the name of the file in your project will not be the identifier for the managed resource in the assembly manifest (see Assembly Manifest for more information). The identifier will be namespace.filename.extension, where namespace is the value of the DefaultNamespace property in a Visual C# project or RootNamespace property in a Visual Basic project. Filename and extension remain the same as their original designation. If the file is a .resx file, the project system will run resgen.exe on the file, creating a .resource file. The .resource file will be embedded in the assembly. Therefore, the assembly manifest will refer to the .resources file and not the .resx file.
For example, if you add the file MyFile.bmp to a project whose default namespace is MyProj, and set the build action to Embedded Resource, MyProj.MyFile.bmp will be the identifier in the assembly manifest. If you then add the file MyFile.resx to the project, the default build action will be Embedded Resource and MyProj.MyFile.resources will be the identifier in the assembly manifest.
Note that when the resource editor adds an image, it sets Build Action to None, because the .resx file references the image file. At build time, the image is pulled into the .resources file created out of the .resx file. The image can then easily be accessed by way of the strongly-typed class auto-generated for the .resx file. Therefore, you should not change this setting to Embedded Resource, because doing this would include the image two times in the assembly.
For more information about how to access resource files (compiled from .resx files) at run time, see ResourceManager Class. For more information about how to access all other embedded files and resources at run time, see Assembly.GetManifestResourceStream Method.