Error rise When I build asp.net application 2012

Aug 6 2014 7:33 AM

Error 41 Could not copy "D:\Work Folder\*******\bin\Debug\BLL_DES.dll" to "bin\BLL_DES.dll". Exceeded retry count of 10. Failed. DES_ODS

and

Error 42 Unable to copy file "D:\Work Folde\*****\BLL_DES\bin\Debug\BLL_DES.dll" to "bin\BLL_DES.dll". The process cannot access the file 'bin\BLL_DES.dll' because it is being used by another process.

Please tell me about this error..............