Do NOT Change "Copy Local” project references to false, unless understand subsequences.

To optimize performance of visual studio build I've found multiple recommendations to change CopyLocal property for dependent dlls to false,

2. e.g. From http://stackoverflow.com/questions/690033/best-practices-for-large-solutions-in-visual-studio-2008 

• CopyLocal? For sure turn this off

http://stackoverflow.com/questions/280751/what-is-the-best-practice-for-copy-local-and-with-project-references

• Always set the Copy Local property to false and enforce this via a custom msbuild step

• My advice is to always set ‘Copy Local’ to false

Some time ago we've tried to change the setting to false, and found that it causes problem for deployment of top-level projects.

Recently I've followed the suggestion and changed the settings for middle-level projects. It didn't cause immediate issues, but I was warned by Readify Consultant  about possible errors during deployments

I haven't undone the changes immediately and we found a few issues during testing.

There are many scenarios, when you need to have Copy Local' left to True.

The concerns are highlighted in some StackOverflow answers, but they have small number of votes.The concerns are highlighted in some StackOverflow answers, but they have small number of votes.

Top-level projects:  set copy local = true**.**

First of all, it doesn't work correctly for top-level projects, i.e. executables or web sites.

As pointed in the answer http://stackoverflow.com/a/6529461/52277

• for all the references in the one at the top set copy local = true.

Alternatively you have to change output directory as it's described in http://www.simple-talk.com/dotnet/.net-framework/partitioning-your-code-base-through-.net-assemblies-and-visual-studio-projects/

If you set ‘ Copy Local = false’, VS will, unless you tell it otherwise, place each assembly alone in its own _.\bin\Debug_directory. Because of this, you will need to configure VS to place assemblies together in the same directory. To do so, for each VS project, go to VS > Project Properties > Build tab > Output path, and set the Ouput path to _..\bin\Debug_for debug configuration, and ..\bin\Release for release configuration.

Second-level  dependencies:  set copy local = true.

Another example when copylocal =false fails on run-time, is when top level assembly doesn't directly referenced one of indirect dependencies. E..g. Top-level assembly A has reference to assembly B with copylocal =true, but assembly B has reference to assembly C with copylocal =false. Most likely assembly C will be missing on runtime and will cause errors 

E.g. http://stackoverflow.com/questions/602765/when-should-copy-local-be-set-to-true-and-when-should-it-not?lq=1

Copy local is important for deployment scenarios and tools. As a general rule you should use CopyLocal=True

and http://stackoverflow.com/questions/602765/when-should-copy-local-be-set-to-true-and-when-should-it-not?lq=1

Unfortunately there are some quirks and CopyLocal won't necessary work as expected for assembly references in secondary assemblies structured as shown below.

• MainApp.exe
• MyLibrary.dll
• ThirdPartyLibrary.dll (if in the GAC CopyLocal won't copy to MainApp bin folder)

This makes xcopy deployments difficult . 

Reflection called DLLs  dependencies:  set copy local = true.

E.g user can see error "ISystem.Reflection.ReflectionTypeLoadException: Unable to load one or more of the requested types. Retrieve the LoaderExceptions property for more information."

The fix for the issue is recommended in http://stackoverflow.com/a/6200173/52277

"I solved this issue by setting the Copy Local attribute of my project's references to true."

In general, the problems with investigation of deployment issues may overweight the benefits of reduced build time.

Setting the Copy Local to false without considering deployment issues is not a good idea.