Question: I have many web applications in a Visual Studio solution.

All have the same post build command:

```
xcopy "$(TargetDir)*.dll" "D:\Project\bin" /i /d /y

```

It would be useful to avoid replacing newer files with old ones (e.g. someone could accidentally add a reference to an old version of a DLL).

How can I use xcopy to replace old files only with **newer DLL files** generated by Visual Studio?

Answer: 
From typing "help xcopy" at the command line:

```
/D:m-d-y     Copies files changed on or after the specified date.
             If no date is given, copies only those files whose
             source time is newer than the destination time.

```

So you already are using xcopy to only replace old files with new ones. If that's not happening, you may have to swap the positions of the `/i` and `/d` switches.
User following command to copy all new and modified file from source to destination

xcopy c:\source d:\destination /D /I /E /Y

/D to check any modified file are there  

/I If the destination does not exist and copying more than one file, assumes that destination must be a directory.  

/E To copy directory and sub directories  

/Y to suppress any over write prompt.