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

Answer 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.