Dynamically created controls causing Null reference

问题

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

回答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.

回答2:

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.

回答3:

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.

回答4:

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 =...;
}

回答5:

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

来源：https://stackoverflow.com/questions/7306305/dynamically-created-controls-causing-null-reference

标签

ASP.NET

dynamic-controls