找不到应该存在的元素？

Question

I am adding tabs to my tab control through code:

TabItem tab = new TabItem();

var stack = new StackPanel() { Orientation = Orientation.Horizontal };
stack.Children.Add(new TextBlock() { Text = header });
stack.Children.Add(new TextBlock() { Name = "extra" });
tab.Header = stack;

tabControl.Items.Add(tab);

As you can see, it creates the header of the tabItem with a stack panel. It adds two text blocks; one of which is empty, but I've assigned the name "extra". What I would like to do is, later in the code, edit the textBlock named "extra" and add some new text to it.

How would I find and edit this element? I have tried the following code, but its producing an error saying the element can not be found:

object test = Application.Current.FindResource("extra");

Answer 1

FindName is what you are looking for but your TextBlock is not in the correct WPF Namescope.

MSDN states :

If you add an object to an object tree at a point in time after the XAML that produced that tree was parsed, a Name or x:Name value on the new object does not automatically update the information in a XAML namescope. To add a name for an object into a WPF XAML namescope after XAML is loaded, must call the appropriate implementation of RegisterName on the object that defines the XAML namescope.

For example:

var textBlock = new TextBlock() { Name = "extra" };
stack.Children.Add(textBlock );
RegisterName(textBlock);

...

TextBlock textBlock = FindName("extra") as TextBlock;

Finally, Application.Current.FindResource("extra") is returning null because the element does not exist when project resources are created. More on FindResource.

Answer 2

Just use FrameworkElement.FindName method:

var control = tab.FindName("extra");
if(control is TextBlock){
    // your logic here
}

You don't need Application.Current.Resource dictionary here because it's different collection. If you want to use it then you should put user controls within Resource dictionary beforehand.

Answer 3

Because you are trying to find resource with the key "extra". It's wrong.

Try this:

TabItem tab = new TabItem();

var stack = new StackPanel() { Orientation = Orientation.Horizontal };
var textBlock = new TextBlock() { Name = "extra" }
stack.Children.Add(new TextBlock() { Text = header });
stack.Children.Add(textBlock);
tab.Header = stack;

tabControl.Items.Add(tab);

Now you can reach it with textBlock instance.

Answer 4

Here's some VB WPF code for what you need

    Dim test As TextBlock
    test = DirectCast(FindName("extra"), TextBlock)

I have no idea if it will work like that in C# WPF although if that doesn't work try looking up CType