Dynamic Creation of TextBox in Asp.Net

This will never be reliable. The controls will not work with viewstate, nor will they survive a page refresh. Your best bet with dynamic controls is no postbacks, and to handle sending data back using AJAX.

updated:
rahkan's answer is more accurate. Read his answer.

at your page load event, add this:

C#

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
                
    }
    AddTextBox();
}

void AddTextBox()
{
    Table tblTxtBox = new Table();
    for (int i = 0; i < 5; i++)
    {
        TableCell tblcell = new TableCell();
        TableRow tblrow = new TableRow();
        TextBox txtBox = new TextBox();
        txtBox.ID = "txtBox" + i;
        tblcell.Controls.Add(txtBox);
        tblrow.Cells.Add(tblcell);
        tblTxtBox.Rows.Add(tblrow);
    }
    Panel1.Controls.Add(tblTxtBox);
}

When dynamically creating controls you need to make sure that you are reloading the controls during any postback event. The controls need to be loaded with the same ID and in the same order as they were created. What @adriancs posted should work as long as the user has only clicked once on the button that adds the textboxes. What I generally do in a situation like this is save the count of the dynamically added controls using a session variable, then during a postback I can reload the controls using the count from the session variable.

public int TextBoxCount
{
    get
    {
        object temp = Session["TextBoxCount"];
        return temp == null ? 0 : (int)temp;
    }
    set
    {
        Session["TextBoxCount"] = value;
    }
}

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
                
    }
    AddTextBox();
}
 
void AddTextBox()
{
    Table tblTxtBox = new Table();
    
    for (int i = 0; i < TextBoxCount; i++)
    {
        TableCell tblcell = new TableCell();
        TableRow tblrow = new TableRow();
        TextBox txtBox = new TextBox();
        txtBox.ID = "txtBox" + i;
        tblcell.Controls.Add(txtBox);
        tblrow.Cells.Add(tblcell);
        tblTxtBox.Rows.Add(tblrow);
    }

    Panel1.Controls.Add(tblTxtBox);
}

protected void btnItem_Click(object sender, EventArgs e)
{
    Table tblTxtBox = new Table();
    int start = TextBoxCount;
    TextBoxCount += 5;

    for (int i = start; i < TextBoxCount; i++)
    {
 
        TableCell tblcell = new TableCell(); 
        TableRow tblrow = new TableRow(); 
        TextBox txtBox = new TextBox();
 
        txtBox.ID = "txtBox" + i;
            
        tblcell.Controls.Add(txtBox);          
        tblrow.Cells.Add(tblcell); 
        tblTxtBox.Rows.Add(tblrow); 
    }

    Panel1.Controls.Add(tblTxtBox);
}

That may need a little tweaking as I'm just working from memory right now, but hopefully it helps some