Can content page use ContentPlaceHolderID of master parent of its master page (nested master pages)

Question

I have a 3 level nested master pages and a content page. parent1 is the top parent, parent2 is parent of parent3 and parent3 is the parent of the content page.

I get an error ' Cannot find ContentPlaceHolder xxx... ' where xxx is a ContentPlaceholder. It resides in parent2 and content page is trying to fill it.

Can content pages only use their direct parent ContentPlaceHolders or can they also use any of the higher master pages?

Answer 1

There is one way to do this but there is a slight problem with it under certain circumstances if you are relying on any default content from a placeholder.

In your example, you have Parent1.master :

<div id="content">
    <h1>Lorem Ipsum, from Parent1</h1>
    <asp:ContentPlaceHolder ID="cphContent" runat="server">
        <p>I am default content from Parent1...</p>
    </asp:ContentPlaceHolder>
</div>

And you also have a nested Parent2.master , which consumes the placeholder from Parent1:

<asp:Content ContentPlaceHolderID="cphContent" runat="server">
    <h2>I am some specific stuff from Parent2...</h2>
    <asp:ContentPlaceHolder ID="cphContent" runat="server">
        <p>I am default content from within Parent2!</p>
        <p>We want to create another, nested CPH so that Parent3 can use it!</p>
        <p>(It is seemingly OK that we can use the same ID for this CPH<br />
            in Parent2 that we did originally in Parent1.)</p>
    </asp:ContentPlaceHolder>   
</asp:Content>

And so now Parent3.master can consume the placeholder from Parent2. (And also provide another placeholder for eventual content page to consume!) Here it is:

<asp:Content ContentPlaceHolderID="cphContent" runat="server">
    <h3>Hello from Parent3!</h3>
    <asp:ContentPlaceHolder ID="cphContent" runat="server">
        <p>I am more default text in yet another nested placeholder</p>
    </asp:ContentPlaceHolder>   
</asp:Content>

Your rendered content page would look something like this:

<div id="content">
    <h1>Lorem Ipsum, from Parent1</h1>
    <h2>I am some specific stuff from Parent2...</h2>
    <h3>Hello from Parent3!</h3>
    <p>I am the plugged-in content, from the content page!</p>
</div>

One cool thing about this approach, and why we might want to use the same names for these nested CPHs throughout the inheritance chain, is that your eventual content page could change from using any of the Parent master pages 1 through 3 without having to change anything else, so long as they expected to find something called cphContent to consume.

OK, so now you have seen the fun part, but the only thing I mentioned might be a problem, is if you were trying to let any of the "default" text trickle down to any of the grand-children. By this, I mean that if your content page doesn't supply any Content for the "cphContent" placeholder, then only the default from the last master page would be used. The default from Parent1.master is essentially lost beyond Parent2. (Although you could certainly use the default from Parent3.) There may be a way to do this programmatically, but "out of the box" this seems to allow you to do what you asked, if you can live with this caveat.

Best of luck!

Answer 2

我相信内容页面只能使用直接父级的ContentPlaceHolder。

Answer 3

Getting the Values of Controls on the Master Page At run time, the master page is merged with the content page, so the controls on the master page are accessible to content page code. (If the master page contains controls in a ContentPlaceHolder control, those controls are not accessible if overridden by a Content control from the content page.) The controls are not directly accessible as master-page members because they are protected. However, you can use the FindControl method to locate specific controls on the master page. If the control that you want to access is inside a ContentPlaceHolder control on the master page, you must first get a reference to the ContentPlaceHolder control, and then call its FindControl method to get a reference to the control.

The following example shows how you can get a reference to controls on the master page. One of the controls being referenced is in a ContentPlaceHolder control and the other is not.

Visual Basic Copy Code ' Gets a reference to a TextBox control inside a ContentPlaceHolder

Dim mpContentPlaceHolder As ContentPlaceHolder
Dim mpTextBox As TextBox
mpContentPlaceHolder = _
    CType(Master.FindControl("ContentPlaceHolder1"), _
    ContentPlaceHolder)
If Not mpContentPlaceHolder Is Nothing Then
    mpTextBox = CType(mpContentPlaceHolder.FindControl("TextBox1"), _
        TextBox)
    If Not mpTextBox Is Nothing Then
        mpTextBox.Text = "TextBox found!"
    End If

Since you want to Find a nested Content place holder you may have to find the parent then use that instance to find the child