File.Copy problem : Could not find a part of the path 'X'

Hi
in my sample asp.net app i need to copy file with a new name. to do this, i'm using this code :
private string _tempDicrectory = "~/temp"; private string _extenstion = ".docx"; private string _sourceFileName = "~/temp/Sample.docx"; protected void btnExtractWord_Click(object sender, EventArgs e) { string fileName = string.Format("tmp_{0}", Guid.NewGuid().ToString().ToLower().Substring(0, 8)); string fullPath = _tempDicrectory + "/" + fileName + _extenstion; File.Copy(_sourceFileName, fullPath); // cause error for _sourceFileName } but at runTime, i'm facing the error : Could not find a part of the path '~/temp/Sample.docx'
this error is related to _sourceFileName object. where is the problem and how to solve it ?
thanks in advance