Question

I used code below to hide template field Imagebutton on pageload but it DOES NOT work,
Thanks in advance:

Protected Sub Page_Load(sender As Object, e As System.EventArgs) Handles Me.Load
            Dim ImageButton1 As ImageButton = DirectCast(GridView1.FindControl("ImageButton1"), ImageButton)
            If User.Identity.Name.Substring(InStr(User.Identity.Name, "\")).ToUpper = "User1" Then
                ImageButton1.Visible = False
            End If
        End Sub

Answer 1

Assuming you have binding grid before and it has rows. Find the ImageButton in some row of grid instead of find within gridview. The if condition you have seems to never get true, as you are comparing string after ToUpper with a string that is not in upper case, Change User1 to USER1 As you are using ToUpper.

Change

 Dim ImageButton1 As ImageButton = DirectCast(GridView1.FindControl("ImageButton1"), ImageButton) 
 If User.Identity.Name.Substring(InStr(User.Identity.Name, "\")).ToUpper = "User1" Then
            ImageButton1.Visible = False
 End If

To

   Dim ImageButton1 As ImageButton = DirectCast(GridView1.Rows(0).FindControl("ImageButton1"), ImageButton)
 If User.Identity.Name.Substring(InStr(User.Identity.Name, "\")).ToUpper = "USER1" Then
       ImageButton1.Visible = False
 End If

Iterating whole grid by loop

For Each row As GridViewRow In GridView1.Rows
 Dim ImageButton1 As ImageButton = DirectCast(row.FindControl("ImageButton1"), ImageButton)
     If User.Identity.Name.Substring(InStr(User.Identity.Name, "\")).ToUpper = "USER1" Then
           ImageButton1.Visible = False
     End If
Next