I'm adding a template column to a DataGrid dynamically:

private void CreateColumn()
{
   TemplateColumn col = new TemplateColumn();
   ...//set some col properties
   col.EditItemTemplate = new MyTemplate();
   MyGrid.Columns.Add(col);
}

The MyTemplate class implementes InstantiateIn() method as:

public void InstantiateIn(Control container)
{
   TableCell cell = container as TableCell;
   TextBox txt = new TextBox();
   txt.ID = "MyTextBox";
   container.Controls.Add(txt);
   RequiredFieldValidator req = new RequiredFieldValidator();
   req.ID = "MyReqVal";
   req.ControlToValidate = txt.ID;
   container.Controls.Add(req);
}

Inside DataGrid.ItemDataBound event handler I set initial value
of the above MyTextBox as:

private void MyGrid_ItemDataBound(object sender, DataGridItemEventArgs e)
{
   if (e.Item.ItemType == ListItemType.EditItem)
   {
       (e.Item.Cells[N].Controls[0] as TextBox).Text = SomeValue;
   }
}

And I call CreateColumn() from Page_Load() on the first visit
and from overrided LoadViewState() of the Page on every postback:

protected override void LoadViewState(object savedState)
{
   base.LoadViewState(savedState);
   CreateColumns();
}

And this works well. The MyGrid switches to edit mode normally and
renders the MyTextBox with a bounded value. The MyReqVal validates
MyTextBox properly at a client side.
But when I postback the page to save a text entered in MyTextBox,
the Page.IsValid is always FALSE. Also, MyTextBox is always EMPTY:

private void SaveBtn_Click(object sender, System.EventArgs e)
{
   //at this point the IsValid is false and MyTextBox.Text is blank
   if (IsValid)
   {
       ...// cannot reach here!
   }
}

What am I doing wrong?