Question

How can I can check by tag if a Canvas child exists?

For example I want to know if in my Canvas (canvas1) there exists a child that has tag “p”.

Answer 1

How can I can check by tag if a canvas child exist?

So I suppose a bool value is what you want

bool result = mycanvas.Children.Cast<FrameworkElement>()
                      .Any(x => x.Tag != null && x.Tag.ToString() == "p");

I used (x.Tag == null ? "" : x.Tag.ToString()) == "p") just to be sure that tag property is not null.

OR

If you want the Element then

FrameworkElement result = mycanvas.Children.Cast<FrameworkElement>()
                                           .Where(x => x.Tag != null && 
                                                  x.Tag.ToString() == "p")
                                           .First();