C# ASP.NET FindControl keeps returning null
Question
I have a TextBox and a Button in my webpage. The user enters a number in the TextBox and clicks the Button , which creates (n+1) TextBox es in a Panel .
As you can see in the code, I have assigned ID s to the TextBox instances , again when trying to access the TextBox instances using FindControl() , I keep getting a NullReferenceException for ONLY the last TextBox (ID : f1) , what am I doing wrong here ?
Initially Button1.Text is not "Find" .
Code
protected void Button1_Click(object sender, EventArgs e)
{
if (Button1.Text.Equals("Find"))
{
for (int i = 0; i < size; i++)
{
TextBox tb = (TextBox)Panel1.FindControl("Number" + i);
n[i] = Convert.ToInt32(tb.Text);
}
localhost.Search s = new localhost.Search();
resultLabel = new Label();
TextBox tb1 = (TextBox)Panel1.FindControl("f1");
int fNumber = Convert.ToInt32(tb1.Text); // tb1 is null
if (s.LinearSearch(n, fNumber))
resultLabel.Text = "FOUND!";
else
resultLabel.Text = "NOT FOUND!";
form1.Controls.Add(resultLabel);
}
else
{
size = Convert.ToInt32(TextBox1.Text);
n = new int[size];
TextBox1.Enabled = false;
boxes = new TextBox[size];
for (int i = 0; i < size; i++)
{
Label l = new Label();
l.Text = "Number " + (i + 1) + " : ";
boxes[i] = new TextBox();
boxes[i].ID = "Number" + i;
Panel1.Controls.Add(l);
Panel1.Controls.Add(boxes[i]);
}
Label l1 = new Label();
l1.Text = "Find Number : ";
Panel1.Controls.Add(l1);
findBox = new TextBox();
findBox.ID = "f1";
Debug.Write("[!D] ID : "+findBox.ID);
Panel1.Controls.Add(findBox);
Button1.Text = "Find";
}
}
ASPX code for the page
<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default.aspx.cs" Inherits="_Default" %>
<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
</head>
<body>
<form id="form1" runat="server">
<div>
How Many Numbers
<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
<asp:Button ID="Button1" runat="server" OnClick="Button1_Click" Text="OK" />
<br />
<asp:Panel ID="Panel1" runat="server">
</asp:Panel>
</div>
</form>
</body>
</html>
2 answers
solution1
0 2015-01-15 07:30:50
Try searching the whole page for ID f1 rather than searching in a panel.
TextBox tb1 = (TextBox)FindControl("f1");
Add the ID in your textbox
<asp:TextBox ID="f1" runat="server"></asp:TextBox>
In your aspx page there is nothing with f1 ID because of which you were getting the null reference exception when trying to get the element's Text.
solution2
0 ACCPTED 2015-01-15 08:36:59
Whenver you are creating dynamic controls you need to recreate it on postback. Hope this helps
protected void Button1_Click(object sender, EventArgs e)
{
CreateControls();
if (Button1.Text.Equals("Find"))
{
for (int i = 0; i < size; i++)
{
TextBox tb = (TextBox)Panel1.FindControl("Number" + i);
n[i] = Convert.ToInt32(tb.Text);
}
localhost.Search s = new localhost.Search();
resultLabel = new Label();
TextBox tb1 = (TextBox)Panel1.FindControl("f1");
int fNumber = Convert.ToInt32(tb1.Text); // tb1 is null
if (s.LinearSearch(n, fNumber))
resultLabel.Text = "FOUND!";
else
resultLabel.Text = "NOT FOUND!";
form1.Controls.Add(resultLabel);
}
else
{
Button1.Text = "Find";
}
}
protected void CreateControls()
{
var size = Convert.ToInt32(TextBox1.Text);
var n = new int[size];
TextBox1.Enabled = false;
var boxes = new TextBox[size];
for (int i = 0; i < size; i++)
{
Label l = new Label();
l.Text = "Number " + (i + 1) + " : ";
boxes[i] = new TextBox();
boxes[i].ID = "Number" + i;
Panel1.Controls.Add(l);
Panel1.Controls.Add(boxes[i]);
}
Label l1 = new Label();
l1.Text = "Find Number : ";
Panel1.Controls.Add(l1);
var findBox = new TextBox();
findBox.ID = "f1";
Debug.Write("[!D] ID : " + findBox.ID);
Panel1.Controls.Add(findBox);
}
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