Getting Started

Re: Check null value for an object

sandeepthekka — Mon, 29 Oct 2007 11:26:44 GMT

memberdetails.OfficePhone = txtOfficePhone.Text == "" ? "" : txtOfficePhone.Text; memberdetails.ResidencePhone = txtResPhone.Text == "" ? "" : txtResPhone.Text; memberdetails.Mobile = txtMobile.Text == "" ? "" : txtMobile.Text;

Re: Check null value for an object

tgopi99 — Fri, 26 Oct 2007 20:49:57 GMT

Very useful one, Thank you very much.

Re: Check null value for an object

azamsharp — Fri, 26 Oct 2007 20:09:34 GMT

Here is a small example that will clear up the things further:

This is the code for my previousPage:

<asp:TextBox ID="txtName" runat="server" />
<asp:Button ID="Button1" runat="server" PostBackUrl="~/FirstPage.aspx" Text="Button" OnClick="Button1_Click" />

And here is the page that access the PreviousPage.

protected void Page_Load(object sender, EventArgs e)
        {
            if (PreviousPage != null)
                {
                    TextBox tb = PreviousPage.FindControl("txtName") as TextBox;
                    Response.Write(tb.Text);
                }

            if (!Page.IsPostBack)
            {

            }
        }

If you are not able to find the control in the PreviousPage then you can use properties in the Previous to access the value of the control. Like the following:

public class MyPreviousPage : ....

public string Name { return txtName.Text; }

Now, in the main page you can access like this:

MyPreviousPage.Name

You will also need to add the following directive in the main page (NOT Previous Page)

<%@ PreviousPageType VirtualPath="~/MyPreviousPage.aspx" %>

Re: Check null value for an object

azamsharp — Fri, 26 Oct 2007 19:52:53 GMT

First check that the PreviousPage property is not null and also make sure that the Previous Page consists of the element named textboxFirstName. You can also expose properties in the PreviousPage to access the controls instead of using FindControl method.

Re: Check null value for an object

tgopi99 — Fri, 26 Oct 2007 17:49:27 GMT

I even checked with the above code, but still it is giving the following error.

Object reference not set to an instance of an object.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

Source Error:

Line 39:                 TextBox tbox = new TextBox();
Line 40:                 DropDownList dlist = new DropDownList();
Line 41:                 tbox = (TextBox)Page.PreviousPage.FindControl("textboxFirstName") as TextBox;
Line 42:                 if (tbox != null)
Line 43:                 {

Re: Check null value for an object

tgopi99 — Fri, 26 Oct 2007 17:48:33 GMT

Hi,

I am getting the following error when i am checking null value of an object as you said. Here is my code

tbox = (TextBox)Page.PreviousPage.FindControl("textboxFirstName") as TextBox;if (tbox != null)

{

Session["semfname"] = tbox.Text;

txtfirstname.Text = tbox.Text;

}

Object reference not set to an instance of an object.

Line 39:                 TextBox tbox = new TextBox();
Line 40:                 DropDownList dlist = new DropDownList();
Line 41:                 tbox = (TextBox)Page.PreviousPage.FindControl("textboxFirstName") as TextBox;
Line 42:                 if (tbox != null)
Line 43:                 {

Re: Check null value for an object

KinjanShah — Fri, 26 Oct 2007 07:10:00 GMT

Hi tgopi99, In Order to check for object Reference in C#.NET, you need to use null, in VB.NET you need to use Nothing. if(tbox == null) { //code for nothing } else { //Code for presence } Hope this will help you out. Thanks and Regards, Kinjan Shah, MCAD

Re: Check null value for an object

impathan — Fri, 26 Oct 2007 06:56:04 GMT

tgopi99:
tbox = (TextBox)Page.PreviousPage.FindControl("textboxFirstName");

if( tbox!=null)

{

}

best rEgard

pAthan

Re: Check null value for an object

ramana123 — Fri, 26 Oct 2007 06:35:24 GMT

Use

if(tbox!=null)

{

//do your code if control finds.

}

else

{

//tbox is null/

}

Re: Check null value for an object

ravipahuja1 — Fri, 26 Oct 2007 05:38:26 GMT

Go for "as" as suggest by azam.

Re: Check null value for an object

mbanavige — Fri, 26 Oct 2007 01:37:05 GMT

like this:

TextBox tbox = (TextBox)Page.PreviousPage.FindControl("textboxFirstName");
if (tbox == null) {
    // it was null
}

Re: Check null value for an object

azamsharp — Fri, 26 Oct 2007 01:28:32 GMT

You can use the "as" keyword to try to cast the object. If the casting fails null is returned.

tbox =Page.PreviousPage.FindControl("textboxFirstName") as TextBox;

Check null value for an object

tgopi99 — Fri, 26 Oct 2007 01:24:05 GMT

Hi,

How to check the object is null in C#. For the code below

TextBox tbox = new TextBox();

tbox = (TextBox)Page.PreviousPage.FindControl("textboxFirstName");

i want to find whether the tbox is null or not. How can i achieve this in C#.

Please help.