How Help.ShowHelp() Method works

Question

private void Window_Help_Executed(object sender, ExecutedRoutedEventArgs e)
{
string helpFile = "../../HelpFiles/abc.chm";
Help.ShowHelp(null, helpFile, HelpNavigator.TableOfContents);

}

 

 

I have written the above code its working fine but I want to know why we give null for first parameter If I try to pass the control name it gives me an error. Like This:-

 

Help.ShowHelp(TextBox, helpFile, HelpNavigator.TableOfContents);

 

1. Error 3 'System.Windows.Controls.TextBox' is a 'type' but is used like a 'variable'

2.Error 4 'TextBox' is an ambiguous reference between 'System.Windows.Controls.TextBox' and 'System.Windows.Forms.TextBox'

3.Error 2 Argument 1: cannot convert from 'System.Windows.Controls.TextBox' to 'System.Windows.Forms.Control'

4.Error 1 The best overloaded method match for 'System.Windows.Forms.Help.ShowHelp(System.Windows.Forms.Control, string, System.Windows.Forms.HelpNavigator)' has some invalid arguments