C#

/// <summary>
/// Base class for WebForm pages.
/// </summary>
public class PageBase : 
    System.Web.UI.Page
{
    // ...
 
    /// <summary>
    /// Replace the tilde ("~") at the beginning of a string with
    /// the application path of the currently running web application.
    /// If no web application is running, you can configure an
    /// application configuration value "replaceTildeFallback" used
    /// instead.
    /// </summary>
    /// <param name="path">The path with the tilde to replace. The tilde
    /// must be the first character inside the string.</param>
    /// <returns>Returns the path with the replaced tilde, or the original
    /// path if no tilde is contained or the tilde cannot be replaced.</returns>
    public static string ReplaceTilde( 
        string path )
    {
        if ( path==null || 
            path.Length==0 ||
            path.IndexOf( '~' )!=0 )
        {
            return path;
        }
        else if ( HttpContext.Current==null ||
            HttpContext.Current.Request==null )
        {
            // Try to lookup from config.
            string tilde = ConfigurationSettings.AppSettings["replaceTildeFallback"];
  
            if ( tilde!=null && tilde.Length>0 )
            {
                tilde = tilde.TrimEnd( '\\', '/' );
                return path.Replace( "~", tilde );
            }
            else
            {
                return path;
            }
        }
        else
        {
            string tilde;
            if ( HttpContext.Current.Request.ApplicationPath=="/" )
            {
                tilde = string.Empty;
            }
            else
            {
                tilde = HttpContext.Current.Request.ApplicationPath;
            }
    
            return path.Replace( "~", tilde );
        }
    }
        
    // ...
}