the dependency 'file' in project 'project' cannot be copied to the run directoy because it would conflict with the dependency 'file'

This error message is saying,
- there are two versions of the same DLL in your project,
- somewhere you have a reference to the old one (most likely this is the problem)
- but VS can't copy it to the run directory, because that would overwrite the new version that you have,
...I hope I didn't make matters worse, but you have to understand the problem so you know what to look for

When you are writing a project that uses an assembly (class library) that you are still working on, you are likely to find this problem when you make changes to the class library and copy the new DLL to the project that uses it, the error message doesn't always point you in the right direction, here's a list of things to look for

1) the first thing you would try is simply copy the new version of the DLL to the project path (or wherever you keep your DLLs for the project), but most likely you already tried that and that's why you're looking for some other solution

2) the Microsoft recommendation to fix this problem:
http://msdn2.microsoft.com/en-us/library/3b12we19.aspx
- Make one of the assemblies a direct reference of your project. A possible downside to this approach is that the assembly you choose is not guaranteed to work with assemblies that require some other version of the referenced assembly.
- or -
- Make sure that both copies of the assembly are strong-named and in the global assembly cache. This eliminates the need to copy the assemblies to the bin directory.

there are different reasons why you wouldn't/couldn't/wont do that, so if your problem is still not fixed, keep reading

3) you can also open in notepad the project files .csproj (or .vbproj for VB) and look for the name of the offending dependency, (you have to open a folder the directory where your project is located and find those files, i.e. if your project is called MyProject, you look for MyProject.csproj)

assemblyname="SomeAssembly"
hintpath="DLLs\SomeAssembly.dll">

that gives a hint to the IDE on where to find the reference, (just a hint), make sure that path is not the problem, if you don't see anything wrong there, keep going

4) open in Notepad the project file .csproj.user (or vbproj.user for VB) and look for these lines

(VisualStudioProject)
(CSHARP LastOpenVersion = "7.10.3077" )
(Build)
(Settings ReferencePath...
- note: the parenthesis should be <> instead but freaking blogger doesn't work right
make sure you don't have copies of the DLL in question in those folders that would cause the conflict.
If you still don't see anything wrong with the paths (you know that in those paths you have the right version of the DLL)

5) then perhaps the problem is not in the current project; another cause of this problem is when your main project uses a DLL that references the same DLL that is causing the problem, to illustrate look at this:

main project
- reference DLL-A
- reference DLL-B

DLL-A
- references...
DLL-B
- reference DLL-A <<<=== the problem is here! your project indirectly has a reference to the old DLL here

in this case, to solve the problem you have to copy the right version of DLL-A to the references of DLL-B (basically perform the steps above but for the DLL-B project), so that when your project uses DLL-B, it will not have the reference to the old DLL-A

uufff... I hope I made some sense and that this helps someone