asked on

NullReferenceException, c#

Question, why do I get NullReferenceException error in this code?

How can I correct this.

Thank you.

 

        string r = (string)(" ");
        r = Session["Region"].ToString();     // stepping shows r = "Region 1"

      if (IsPostBack)
        {
            if (Session["Region"] != null)
            {
                MasterPage myMaster = Master as MasterPage;
                DropDownList regionDropDownList = (DropDownList)myMaster.FindControl("cboRegion");
                regionDropDownList.Text = Session["Region"].ToString();   //  <---  NullReferenceException  **********
            }
        }

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Kyle Abrahams, PMP

You're not finding regionDropDownList.

To resolve the error:

if (regionDropDownList != null)
regionDropDownList.Text = Session["Region"].ToString();

To find the drop down correctly:
The find control isn't recursive.
You need to do the find control on cboRegion's parent or create a recursive find.

Mike Eghtebas

ASKER

re:> You're not finding regionDropDownList.

How do I find it? I know you are telling "You need to do the find control on cboRegion's parent or create a recursive find."

The parent of the control is myMaster (see line 10).

Is it possible to give me some lines of code I can work with?

Thank you,

Mike

Kyle Abrahams, PMP

here's a recursive find:

    private Control RecursiveFind(Control root, string id)
    {
        
        if (root.ID == id)
            return root;

        Control c = null;

        foreach (Control ctrl in root.Controls)
        {
            c = RecursiveFind(ctrl);            
            if (c != null) 
                 break;
        }

        return c;

    }

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Instead of calling the findcontrol:
DropDownList regionDropDownList = (DropDownList) RecursiveFind(myMaster, "cboRegion");

the other way to do this:
assume your master has the following:

<body>
<asp:Panel runat="Server" id = "A">
<asp:DropDownList runat="server" ID="B" />
</asp:Panel>

master.Controls Only returns "A"
You would have to find control your way down.
eg:

Panel p = (Panel)Master.FindControl("A");
DropDownList ddl = (DropDownList)p.FindControl("B");

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kaufmed

Which line raises the exception? The one you highlighted in your code snippet should not have.

Mike Eghtebas

ASKER

kaufmed,

Yes, at line 12 (regionDropDownList.Text = Session["Region"].ToString(); // <--- NullReferenceException **********).

--------------
Kyle Abrahams,

First solution I tried, was not successful. I will try it again making sure this time around I implement it correctly.

For the second solution, instead of a panel, I have table cell (<td ID="Region"> tag), I will try it using:

MasterPage myMaster = Master as MasterPage;
TableCell td = (TableCell)Master.FindControl("Region");
DropDownList ddl = (DropDownList)td.FindControl("cboRegion");
ddl.Text = Session["Region"].ToString();

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Thanks to all,

Mike

kaufmed

Ah. Never mind, then. I missed that you were using the DropDownList in that line. I s'pose Kyle has you sorted out = )

Deja Anbu

If the control that you want to access is inside a ContentPlaceHolder control on the master page, you must first get a reference to the ContentPlaceHolder control, and then call its FindControl method to get a reference to the control.

http://msdn.microsoft.com/en-us/library/vstudio/c8y19k6h(v=vs.100).aspx

Gary Davis

Looking at your code...

      string r = (string)(" ");
      r = Session["Region"].ToString();     // stepping shows r = "Region 1"

      if (IsPostBack)
        {
            if (Session["Region"] != null)
              :
              :

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Line 1 sets r to a space but line 2 replaces it with the session variable. You could combine these into one line since r does not need to be initialized as a space.

If the session variable does not exist, it will be null. Doing the .ToString() will then cause the null reference trap.

In line 6, you actually test the session variable for null which is correct but too late.

You could set r to the session variable using a cast insteat of using ToString(). That way r will be the string or null:

    string r = (string)Session["Region"];

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Gary Davis

Mike Eghtebas

ASKER

Hi All,

Having lost my grounding with use of session variables for now, with apology, I have switched to solve the problem using Serialization/Deerialization.

https://www.experts-exchange.com/questions/28493754/using-Serialization-Deerialization-object-c.html

But, after I get a solution this way, I will come back later and learn more about session variables.

I appreciate for any comment on this new question if you have time for it.

Regards,

Mike

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Kyle Abrahams, PMP

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Mike Eghtebas

ASKER

That is great, thank you very much.

Mike