[WPF] problem analysis for setting ToolTip in style

Just opened to the wisdom of the "WPF" in the style Set ToolTip blog post, although eventually gave a temporary solution to the problem, but did not analyze and explain the problem, It is with him: But as for why it is not possible to place TextBlock directly in Setter.value is an unsolved mystery.

Take advantage of the noon gap, followed a bit, here I will give you a complete analysis.

To describe the problem, first of all, to the problematic XAML, of course, you can also go to the sage's blog to view the detailed description.

<textblock x:name= "Textblockcontainer" text= "ABC" margin= "ten" >
    <!--The following wording is not a problem-->
    <!--< tooltipservice.tooltip>
        <textblock 
                        text= "//Get text from somewhere by binding"
                        textwrapping= "Wrap"
                        width= " />
    </ToolTipService.ToolTip>-->
    <!--use style to assign values to TOOLTIP, error! Will throw exception-->
    <TextBlock.Style>
        <style targettype= "TextBlock" >
            <setter property= "Tooltipservice.tooltip" >
                <Setter.Value>
                    <textblock x:name= "Tooltipblock"
                        text= "//Get the text from somewhere by binding,"
                        textwrapping= "Wrap"
                        width= "/>
                </Setter.Value>
            </ setter>
        </Style>  
    </TextBlock.Style>
</TextBlock>

The information for the exception is:

Exception

System.Windows.Markup.XamlParseException occurred
  message= "Cannot add content of type" System.Windows.Controls.TextBlock ' to an object of type ' System.Object '.  Error at object ' System.Windows.Controls.TextBlock ' in markup file ' Wpfapplication1;component/window1.xaml ' line 17 Position. "
  Source= "PresentationFramework"
  linenumber=17
  lineposition=30
  namecontext= "Value"
  stacktrace: At
       System.Windows.Markup.XamlParseException.ThrowException (String message, Exception innerexception, Int32 LineNumber, Int32 lineposition, Uri BaseUri, Xamlobjectids currentxamlobjectids, Xamlobjectids contextxamlobjectids, Type objectType)  innerexception: