Accessing the children of an expander control

Question

I have a textblock inside the ContentTemplate of an Expander. I want to access that textblock in my code behind file. This is what I have tried so far

Answer 1

Here is some of my try :

FAILED TRY

These two ways which msdn mentioned, after trying I just got null :

How to: Find DataTemplate-Generated Elements

How to: Find ControlTemplate-Generated Elements

.

And the way link below mention also just got null

Access a control from within a DataTemplate with its identifying name

.

Also simply this.FindName("name") just got null:

https://msdn.microsoft.com/en-us/library/system.windows.frameworkelement.findname(v=vs.110).aspx

.

SUCCESS TRY

Finally King King's method works,

In my case I want to find a RichTextBox in the DataTemplate of ContentTemplate, so I write

yourExpander.IsExpanded = true;
yourExpander.UpdateLayout();
//now use your method
var richTextBox = FindVisualChild<RichTextBox>(yourExpander);

(but I find it works only after the window is initialized and show and when user invoke the event such as mouse click, if I put it into constructor it get null, maybe I'll give it some other try later

20170328updated: I find use FindVisualChild in the constructor may not find the child, but if you use in the Window_OnLoaded it seems can find, maybe it's because that in the stage of Loaded the controls are become concrete)

Besides, I test if there're two RichTextBoxes in the DataTemplate of ControlTemplate, this way can only get the first one,

So, I think a way to modify FindVisualChild to FindRichTextBox so that you can find RichTextBox with specific Name, and before that you have to give name to the control you're finding:

(you can modify similarly to your finding control type):

    public static RichTextBox FindRichTextBox(DependencyObject obj, string name)
    {
        for (int i = 0; i < VisualTreeHelper.GetChildrenCount(obj); i++)
        {
            DependencyObject child = VisualTreeHelper.GetChild(obj, i);
            //                                            add the name condition
            if (child != null && child is RichTextBox && ((RichTextBox)child).Name == name)
                return (RichTextBox)child;
            else
            {
                RichTextBox childOfChild = FindRichTextBox(child, name);
                if (childOfChild != null)
                    return childOfChild;
            }
        }
        return null;
    }

Answer 2

I've tried your code, it's almost fine. I've tried testing it on such as a Button first, it works OK. However for the Expander, it's more complex. There are 2 notices here:

• Be sure the Expander is expanded (IsExpanded = true).
• Be sure the layout is updated (you can call UpdateLayout explicitly)

So the code should be:

yourExpander.IsExpanded = true;
yourExpander.UpdateLayout();
//now use your method
var textBlock = FindVisualChild<TextBlock>(yourExpander);

Your code can be shorten more like this:

private childItem FindVisualChild<childItem>(DependencyObject obj) 
                               where childItem : DependencyObject
{
    for (int i = 0; i < VisualTreeHelper.GetChildrenCount(obj); i++)
    {
        DependencyObject child = VisualTreeHelper.GetChild(obj, i);
        if(child is childItem) return (childItem)child;            
        childItem childOfChild = FindVisualChild<childItem>(child);
        if (childOfChild != null) return childOfChild;            
    }
    return null;
}

Note that child will never be null. Because the GetChildrenCount() already limits the range of existing children, so the child should exist at the specified index i.