If your ASP.NET application requires SSL, then you should make sure that user uses https: instead of http: to access your application, and your ASP.NET application should have the ability to automatically switch to the secure mode (https) if user comes to the application from a non-secure mode (http).

The easiest way to implement this feature is to use Global.asax's Application_BeginRequest function, where it checks if the request comes from a "HTTPS" protocol, if not, then changes "http" to "https" and then redirecst the request to the secure location.

The code snippets for VB.NET and C# are attached.


Note:
If the application is running on a local machine during the development phase, we should not try to redirect the request to a secure link, that is why there are some checks in the first couple of lines of code.

 
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'VB.NET
Sub Application_BeginRequest(ByVal sender As Object, ByVal e As EventArgs)
  ' Fires at the beginning of each request
  'Require SSL
  If (Request.UserHostName <> "127.0.0.1" _
	AndAlso Request.UserHostName <> "localhost") Then
      If Request.ServerVariables("HTTPS") = "off" Then
        Dim redir As String = "https://"   Request.ServerVariables("SERVER_NAME")   Request.ServerVariables("SCRIPT_NAME")
        If Request.ServerVariables("QUERY_STRING") <> "" Then
          redir  = "?"   Request.ServerVariables("QUERY_STRING")
        End If
        Response.Redirect(redir)
      End If
  End If
End Sub 
//C#
public void Application_BeginRequest(object sender, EventArgs e)
{
  if (Request.UserHostName != "127.0.0.1" && Request.UserHostName != "localhost")
  {
    if (Request.ServerVariables["HTTPS"] == "off")
	{
	  string redir = "https://"   Request.ServerVariables["SERVER_NAME"]   Request.ServerVariables["SCRIPT_NAME"];
	  if (Request.ServerVariables["QUERY_STRING"] != "")
	  {
	    redir  = "?"   Request.ServerVariables["QUERY_STRING"];
	  }
	  Response.Redirect(redir);
	}
  }
}