Reverse of HttpServerUtility.MapPath or HostingEnvironment.MapPath, maps a physical path to an absolute virtual path.

VirtualPathHelper.cs

using System;
using System.Web;
using System.Web.Hosting;

public static class VirtualPathHelper
{
	/// <summary>
	/// Returns the virtual path that corresponds to the specified physical file path.
	/// </summary>
	/// <param name="path"></param>
	/// <returns></returns>
	public static string MapVirtualPath(string path)
	{
		var relativePath = new Uri(HostingEnvironment.ApplicationPhysicalPath)
			.MakeRelativeUri(new Uri(path))
			.ToString();

		var appPath = VirtualPathUtility.AppendTrailingSlash(HostingEnvironment.ApplicationVirtualPath);

		return string.IsNullOrEmpty(relativePath)
			? appPath
			: VirtualPathUtility.Combine(appPath, relativePath);
	}
}