Why would 'this.ContentTemplate.FindName' throw an InvalidOperationException on its own template?

Question

Ok... this has me stumped. I've overridden OnContentTemplateChanged in my UserControl subclass. I'm checking that the value passed in for newContentTemplate does in fact equal this.ContentTemplate (it does) yet when I call this...

var textBox = this.ContentTemplate.FindName("EditTextBox", this);

...it throws the following exception...

"This operation is valid only on elements that have this template applied."

Per a commenter in another related question, he said you're supposed to pass in the content presenter for the control, not the control itself, so I then tried this...

var cp = FindVisualChild<ContentPresenter>(this);

var textBox = this.ContentTemplate.FindName("EditTextBox", cp);

...where FindVisualChild is just a helper function used in MSDN's example (see below) to find the associated content presenter. While cp is found, it too throws the same error. I'm stumped!!

Here's the helper function for reference...

private TChildItem FindVisualChild<TChildItem>(DependencyObject obj)
where TChildItem : DependencyObject {

    for(int i = 0 ; i < VisualTreeHelper.GetChildrenCount(obj) ; i++) {

        var child = VisualTreeHelper.GetChild(obj, i);

        if(child is TChildItem typedChild) {
            return typedChild;
        }
        else {
            var childOfChild = FindVisualChild<TChildItem>(child);
            if(childOfChild != null)
                return childOfChild;
        }
    }

    return null;
}

Answer 1

Explicitly applying the template before calling the FindName method will prevent this error.

this.ApplyTemplate(); 

Answer 2

As John pointed out, the OnContentTemplateChanged is being fired before it is actually applied to the underlying ContentPresenter. So you'd need to delay your call to FindName until it is applied. Something like:

protected override void OnContentTemplateChanged(DataTemplate oldContentTemplate, DataTemplate newContentTemplate) {
    base.OnContentTemplateChanged(oldContentTemplate, newContentTemplate);

    this.Dispatcher.BeginInvoke((Action)(() => {
        var cp = FindVisualChild<ContentPresenter>(this);
        var textBox = this.ContentTemplate.FindName("EditTextBox", cp) as TextBox;
        textBox.Text = "Found in OnContentTemplateChanged";
    }), DispatcherPriority.DataBind);
}

Alternatively, you may be able to attach a handler to the LayoutUpdated event of the UserControl, but this may fire more often than you want. This would also handle the cases of implicit DataTemplates though.

Something like this:

public UserControl1() {
    InitializeComponent();
    this.LayoutUpdated += new EventHandler(UserControl1_LayoutUpdated);
}

void UserControl1_LayoutUpdated(object sender, EventArgs e) {
    var cp = FindVisualChild<ContentPresenter>(this);
    var textBox = this.ContentTemplate.FindName("EditTextBox", cp) as TextBox;
    textBox.Text = "Found in UserControl1_LayoutUpdated";
}