Question

i want a handler to redirect to a web-forms page, pre-filling in the values of some controls on the form.

i tried setting my current Request.Form data:

if (theyWantToDoSomething)
{
   //pre-fill form values
   context.Request.Form["TextBox1"] = "test";
   context.Request.Form["ComboBox1"] = "test 2";
   context.Request.Form["TextBox2"] = GetTheTextForTheThing();

   //tell the client to go there
   context.Response.Redirect("~/SomeWebForm.aspx");
   return;        
}

But i get an exception that Form values are read only.

What would be a way to send the client to another page, pre-filling form data?

---

Answer

i used the Session state to store values. It’s important to note that by default a Handler doesn’t have access to Session (the Session object will be null). You have to tell IIS to give you the Session object by adding the IRequiresSessionState marker interface to your handler class:

public class Handler : IHttpHandler, System.Web.SessionState.IRequiresSessionState
{
   public void ProcessRequest(HttpContext context)
   {
      ...
      if (theyWantToDoSomething)
      {
         //pre-fill form values
         context.Session["thing1"] = "test";
         context.Session["thing2"] = "test 2";
         context.Session["thing3"] = GetTheTextForTheThing();

         //tell the client to go there
         context.Response.Redirect("~/SomeWebForm.aspx");
         return; //not strictly needed, since Redirect ends processing
      }
      ...
   }
}

Answer 1

You can only populate your Response, the Request is input data and is indeed read-only.

If you are using ASP.NET, there are a variety of ways you could accomplish what you need:

• The best way would probably be to pass the data you need to be pre-populated to SomeWebForm.aspx via the Session object, and on that pages Load method, populate your form. Keep in mind that when you do Response.Redirect, a 302 response is sent to the client with the URL the client should redirect to. The process is transparent to the user…but there is a full round trip involved.

• Another alternative to populating the users Session would be to add GET parameters via a query string to the redirect to SomeWebForm.aspx.

• If you need to transfer processing to the SomeWebForm.aspx page without round tripping, you could use Server.Transfer. This will transfer execution from the current page to the page you choose…however, this can cause some odd behavior on the client end because the URL does not update. As far as the user is concerned, it will still appear as though they are on the same page they started on.