Find the right value of textbox - asp.net

Question

I have a problem with inserting data inserting data into database.
<asp:Repeater ID="Repeater1" runat="server" DataSourceID="SqlDataSource1" OnItemDataBound="Repeater1_ItemDataBound">
<ItemTemplate>
<!-- Some other data (text...) -->
<asp:HiddenField ID="HiddenField1" runat="server" Value='<%# Eval("PostId") %>' />
<asp:TextBox ID="txtAddComment" runat="server" CssClass="textbox" Width="200px" />
<asp:Button ID="btnAddComment" runat="server" CssClass="button" Text="Comment" CausesValidation="false" OnClick="btnAddComment_Click"/>
</ItemTemplate>
</asp:Repeater>
Code behind:
Protected Sub btnAddComment_Click(sender As Object, e As EventArgs)
Dim HiddenField1 As HiddenField = DirectCast(Repeater1.Items(0).FindControl("HiddenField1"), HiddenField)
Dim txtAddComment As TextBox = DirectCast(Repeater1.Items(0).FindControl("txtAddComment"), TextBox)
Dim conn As New SqlConnection(ConfigurationManager.ConnectionStrings("ConnectionString").ConnectionString.ToString)
Try
If txtAddComment.Text = "" Then
Exit Sub
Else
conn.Open()
Dim cmd As SqlCommand = conn.CreateCommand()
cmd.CommandText = "INSERT INTO Comments (PostId, UserName, CommentText) VALUES (#PostId, #UserName, #Text)"
cmd.Parameters.Add("PostId", System.Data.SqlDbType.Int).Value = CInt(HiddenField1.Value)
cmd.Parameters.Add("UserName", System.Data.SqlDbType.NVarChar).Value = Context.User.Identity.Name
cmd.Parameters.Add("Text", System.Data.SqlDbType.NText).Value = MakeLink(HtmlRemoval.StripTagsCharArray(txtAddComment.Text))
cmd.ExecuteNonQuery()
End If
Catch ex As SqlException
Finally
conn.Close()
End Try
End Sub
I display some text, under the text there is textbox and button for comments. I am bounding textId in hiddenfield for inserting comments. The code works fine, but I can only add comment for the first row in database displayed by repeater. When I want to add comment for other rows (2, 3...), the page refreshes and the text in the TextBox stay there.
It is problem with this line of code:
Dim txtAddComment As TextBox = DirectCast(Repeater1.Items(0).FindControl("txtAddComment"), TextBox)
The ID from HiddenField is right. But the code doesn't perform because the line above references for the first textbox on the page and his value is null.
When I change the line of code like this:
Dim txtAddComment As TextBox = DirectCast(Repeater1.FindControl("txtAddComment"), TextBox)
It returns an error that Object reference not set to an instance of an object.
How to get the right textbox with right value?
Thanks for the answer

Answer 1

cast the sender to a button that sent it, then get the textbox from the parent. Example:
Protected Sub button_click(ByVal sender As Object, ByVal e As EventArgs)
Dim myButton As button= CType(sender, button)
Dim myTextBox as TextBox = CType(myButton Parent.FindControl("buttonName"), TextBox)
end sub