I have a few dll files in \\lib folder of my project folder. In the property page of dll, I have selected \"Build Action\" as \"Content\" and \"Copy to Output Directory\" as
I had the same problem with Visual Studio 2010 / C# Project.
For assemblies (i. e. having the .NET interface) use folder "References" under your project in the Solution Explorer. Right click it, choose "Add existing item" and locate your .dll assembly.
Common .dll files can be placed in a subfolder (as "\lib" was mentioned above) and in the properties select:
This worked for me exactly as desired - during build, the .DLLs are copied to the output directory without the "\lib" subfolder.
If your main intent is to include DLLs without cluttering up the project root directory, another solution is to move the DLLs to a separate Shared Project and add this as a reference in the original project.
(Note that this post doesn't directly answer this question as it doesn't preserve the folder and project structure, but I found this approach useful because I was able to restructure my project in my case and because I wanted to avoid some of the downsides of the other approaches here.)
Steps
Solution -> Add -> New Project -> Shared Project
Build Action: Content
and Copy to Output Directory: Copy Always
)References -> Add Reference -> Shared Projects
The setup looks like this: