Getting relative virtual path from physical path

Maybe this question is what you're looking for. There they suggest:

String RelativePath = AbsolutePath.Replace(Request.ServerVariables["APPL_PHYSICAL_PATH"], String.Empty);

    public static string MapPathReverse(string fullServerPath)
    {            
        return @"~\" + fullServerPath.Replace(HttpContext.Current.Request.PhysicalApplicationPath,String.Empty);
    }

Request.ServerVariables["APPL_PHYSICAL_PATH"]

is fine, but not always. It is available only if there's a HTTP request.

On the other hand the call

HostingEnvironment.ApplicationPhysicalPath

is always available.

You could also do something like this:

string relativePath = absolutePath.Replace(HttpContext.Current.Server.MapPath("~/"), "~/").Replace(@"\", "/");

The advantage is, that you don't need HttpContext.Current.Request.