Unable to cast object of type 'System.String' to type 'System.Web.UI.Control'

Question

I got stuck here,

i have dynamic table that i want to print. So i make session to pass it into web control. Unfortunately, it doesn't run smooth.

Here are my code :

protected void bt_print_click(object sender, EventArgs e)
{
    StringWriter sw = new StringWriter();
    HtmlTextWriter w = new HtmlTextWriter(sw);
    panelBilling.RenderControl(w);
    string s = sw.GetStringBuilder().ToString();
    Session["ctrl"] = s;

    ClientScript.RegisterStartupScript(this.GetType(), "onclick", "<script language=javascript>window.open('Print.aspx?rep=1','PrintMe','height=680px,width=1024px,scrollbars=1');</script>");

}

And the Print.aspx.cs code:

protected void Page_Load(object sender, EventArgs e)
{
    Control ctrl = (Control)Session["ctrl"];
    PrintHelper.PrintWebControl(ctrl);
}

I always got error message :

"Unable to cast object of type 'System.String' to type 'System.Web.UI.Control'."

on

(Control)Session["ctrl"]

part. I have use this method many time and no problems before. Anyone has any idea? Thanks.

What happens if you store the panel in the `Session`? `Session["ctrl"] = panelBilling;` — Tim Schmelter, Nov 28 '13 at 15:03
I have to make it clear that actualy that "panelBilling" is ID for Div, not Panel. I use Panel before but it also not work. It say that panel is not serializable (or something like that, i dont remember exactly) — arera, Nov 28 '13 at 15:52
So you added the runat="server" attribute to your div? You shouldn't do that... — Frederik Prijck, Nov 29 '13 at 12:58
why? please explain, all i know is i have to put that runat="server" on every asp tag :)) — arera, Dec 02 '13 at 02:20

Frederik Prijck · Accepted Answer · 2013-11-29T09:52:18.470

This just doesn't make sense, casting a string value to a Control.

As far as I can see, you are not inserting a Control into the Sessiosn, not even the Control.ToString() value, which also wouldn't be working.

Before continuing, it might be usefull to check which type is retrieved from the Session without casting anything (remark: this is only possible with .NET 3.5 or higher):

var sessionValue = Session["ctrl"];

When doing so, you will find out the value is of type object, containing the value:

"<div>\r\n\r\n</div>"

The above output is not a Control, but a string/html value.

There are two solution paths which u can follow:

Change the "SET" call on the Session
Change the "GET" call on the Session

In the second example, you keep to the fact that you are inserting a string and therefor you would like to read out that string value:

string sessionValueAsString = string.Empty;
Object sessionValue = Session["ctrl"];

if (sessionValue != null)
    sessionValueAsString = sessionValue.ToString();

The above example could suit ur needs, never the less I'm guessing you are in the need to save an object of type "Control" in the session. This can be achieved like so:

MyControl myControl = new MyControl {Title = "My custom control"};
Session["myControl"] = myControl;
// Read the Session value cross Postback
MyControl mySessionControl = (MyControl)Session["myControl"];

Apart from the above example, I'm not 100% sure what you are trying to do. My guess is you made a mistake inserting the wrong value in the Session.

One remark: I'm not a fan of storing the WebControls in the session, it would be cleaner to store your data objects in the Session (e.g. store a Customer class instead of the CustomerUserControl, this way you can read out the Session and populate the required controls using the data found in the Session).

i'll try ur suggestion. Btw, i try to print a dynamic table. So that i store the dynamic table into session then print it. — arera, Nov 28 '13 at 15:58
When i add "var" and build page, there's errror : "Error 5 The type or namespace name 'var' could not be found (are you missing a using directive or an assembly reference?)" . What kind of reference should i have? — arera, Nov 29 '13 at 01:58
Storing a dynamic table should not require to save the entire control on the session. nevertheless, to reply to ur problem: replace var by object. (i've edited my answer) Which .NET framework are you using? — Frederik Prijck, Nov 29 '13 at 09:49
I finally realize what you mean about realizing type of the session. Instead of using Control, i change to string type. Now it run smoothly. Nice help! — arera, Nov 29 '13 at 11:04

Unable to cast object of type 'System.String' to type 'System.Web.UI.Control'

1 Answers1