SP url - Problem with alias

I have a problem when using an alias to access my Sharepoint (2013) site through code.

I want to retrieve a document from a SharePoint library.

In my browser address bar I can type either :

http://myserver.domain:port/mylibrary/mydocument.xlsx

http://myAliasTotheSiteCollection/mylibrary/mydocument.xlsx

Both are OK.

But if I want to access the same file through code, the link with the alias does not work.

Here is the code:

strLink= "http://myserver.domain:port/mylibrary/mydocument.xlsx";
System.Net.WebClient wcVar = new System.Net.WebClient();
wcVar.Credentials = new System.Net.NetworkCredential(myLogin, myPwd, myEnvironnement);
wcVar.DownloadFile(strLink, strDestination);

=> OK

strLink= "http://myAliasTotheSiteCollection/mylibrary/mydocument.xlsx";
System.Net.WebClient wcVar = new System.Net.WebClient();
wcVar.Credentials = new System.Net.NetworkCredential(myLogin, myPwd, myEnvironnement);
wcVar.DownloadFile(strLink, strDestination);

=> ERROR 401 Non Authorized

How can I do to use the alias instead of the "hard coded" address ?

Thank you