Dynamically created controls causing Null reference

Question

I am trying to dynamically create controls and give them properties during run time.

I have put my code inside the Page_Init event, when I run my website I can see my controls but when I click on the submit button an error occurrs saying "Object reference not set to an instance of an object".

Here is the code I have used:

    //Creates instances of the Control    
    Label FeedbackLabel = new Label();
    TextBox InputTextBox = new TextBox();
    Button SubmitButton = new Button();
    // Assign the control properties

    FeedbackLabel.ID = "FeedbackLabel";
    FeedbackLabel.Text = "Please type your name: ";
    SubmitButton.ID = "SubmitButton";
    SubmitButton.Text = "Submit";
    InputTextBox.ID = "InputTextBox";
    // Create event handlers
    SubmitButton.Click += new System.EventHandler(SubmitButton_Click);

    // Add the controls to a Panel
    Panel1.Controls.Add(FeedbackLabel);
    Panel1.Controls.Add(InputTextBox);
    Panel1.Controls.Add(SubmitButton);
}

protected void SubmitButton_Click(object sender, EventArgs e)
{
    // Create an instance of Button for the existing control
    Button SubmitButton = (Button)sender;
    // Update the text on the Button
    SubmitButton.Text = "Submit again!";

    // Create the Label and TextBox controls
    Label FeedbackLabel = (Label)FindControl("FeedbackLabel");
    TextBox InputTextBox = (TextBox)FindControl("InputTextBox");
    // Update the controls
    FeedbackLabel.Text = string.Format("Hi, {0}", InputTextBox.Text);

How can I fix this error?

This Is the Stack Trace

[NullReferenceException: Object reference not set to an instance of an object.] _Default.Page_PreInit(Object sender, EventArgs e) in c:\Users\bilalq\Documents\Visual Studio 2010\WebSites\WebSite3\Default.aspx.cs:31 System.Web.Util.CalliHelper.EventArgFunctionCaller(IntPtr fp, Object o, Object t, EventArgs e) +14 System.Web.Util.CalliEventHandlerDelegateProxy.Callback(Object sender, EventArgs e) +35 System.Web.UI.Page.OnPreInit(EventArgs e) +8876158 System.Web.UI.Page.PerformPreInit() +31 System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +328

@Neil, he said the error occurs when clicking submit. I think it can't find the controls (because they're added to a panel, not directly to the page). — George Duckett
– George Duckett, Commented Sep 5, 2011 at 9:47
@qablan89, could you please give us some info from the stack trace? Which line is the exception being thrown from? — Doctor Jones
– Doctor Jones, Commented Sep 5, 2011 at 9:49

Steve B · Accepted Answer · 2011-09-05 09:49:29Z

3

As FindControl is not recursive, you have to replace this code :

Label FeedbackLabel = (Label)FindControl("FeedbackLabel");
TextBox InputTextBox = (TextBox)FindControl("InputTextBox");

by this code :

Label FeedbackLabel = (Label)Panel1.FindControl("FeedbackLabel");
TextBox InputTextBox = (TextBox)Panel1.FindControl("InputTextBox");

However, according other answers, you should move the declaration (not the instantiation) outside the method (at class level) in order to easily get an entry for your controls.

answered Sep 5, 2011 at 9:49

Steve B

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2 Comments

qablan89 Over a year ago

To DoctJones I tried to replace them but it did not work The error ocurred here Panel1.Controls.Add(FeedbackLabel); This is the stack trace

Steve B Over a year ago

is Panel1 != null at this time ?

Davide Piras · Accepted Answer · 2011-09-05 09:46:27Z

2

try to put your code in the Page_Load instead of Page_Init and also, check for null before using objects returned by FindControl.

I suspect the object InputTextBox is null and it crashes when you try to print its Text.

as a general rule just check for null and also for type when casting results of FindControl to something else.

answered Sep 5, 2011 at 9:46

Davide Piras

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Comments

TheCodeKing · Accepted Answer · 2011-09-05 10:00:25Z

2

The FindControl is failing because it can't find the control and causing a null reference.

Just reference it directly using FeedbackLabel as you already have it in your class. Just move the scope outside of your 'Init' method.

private Label feedbackLabel = new Label();
private TextBox inputTextBox = new TextBox();
private Button submitButton = new Button();

public void Page_Init(EventArgs e)
{
    feedbackLabel.ID = "FeedbackLabel";
}

protected void SubmitButton_Click(object sender, EventArgs e)
{
    feedbackLabel.Text =...;
}

edited Sep 5, 2011 at 10:00

answered Sep 5, 2011 at 9:47

TheCodeKing

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Comments

Samir Adel · Accepted Answer · 2011-09-05 09:49:01Z

1

I suggest that you declare your controls outside the page_int and do your initialization in the init then use them with their name instead of find control.

answered Sep 5, 2011 at 9:49

Samir Adel

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Comments

Valamas · Accepted Answer · 2011-09-05 09:52:11Z

0

    protected override void OnLoad(EventArgs e)
    {
        base.OnLoad(e);
        //-- Create your controls here
    }

answered Sep 5, 2011 at 9:52

Valamas

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1 Comment

qablan89 Over a year ago

Thank you All it works when i commented this statement FeedbackLabel.Text = string.Format("Hi, {0}", InputTextBox.Text);

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