Nested Masterpages and .FindControl

have you tried this.Master.Master.FindControl("controlname"); ?

When you're nesting master pages, you'll get an extra container "Content" you need to look through.

As a result, if you're trying to use FindControl from a given child page the usual approach is something to the effect of:

Label myLabel = (Label)this.Master.FindControl("myLabel");
myLabel.Text = "Success!";

Since we have a nested master page, with "myLabel" in the child master, this control will be contained within a content control.

So, this changes the code to:

ContentPlaceHolder ph = (ContentPlaceHolder)this.Master.Master.FindControl("yourContentPane");

Label myLabel = (Label)ph.FindControl("myLabel");
myLabel.Text = "Success!";

and in VB.NET

Dim ph As ContentPlaceHolder = DirectCast(Me.Master.Master.FindControl("yourContentPane"), ContentPlaceHolder)

Dim myLabel As Label = DirectCast(ph.FindControl("myLabel"), Label)
myLabel.Text = "Success!"

The content from the child page is loaded into the first master page control, which is subsequently loaded into the grandparent master page.