I have my project set to .NET Framework 4.0. When I add System.IO.Packaging, it says that it doesn\'t exist. It also doesn\'t show up when I try to add it as a
For a C# solution in Visual Studio 2010 with .NET 4.0:
C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.0\WindowsBase.dll
Save the solution (especially if you're compiling from the command-line with MSBuild) and you should now be able to add the using System.IO.Packaging directive to the top of your .cs file without an error appearing.