c# - WPF WebBrowserControl: removal of PDF is slow -

i'm using webbrowser control in wpf display pdf documents.

displaying pdf assigning local uri works within few 100 milliseconds.

webbrowser.source = new uri(path); 

but assigning null remove pdf ...

webbrowser.source = null; 

... takes second.

same reassigning new uri without setting source null before.

has idea, why assigning null webbrowser.source takes longer loading , displaying pdf document?

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example:

<window x:class="wpfapp1.mainwindow"         xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"         xmlns:d="http://schemas.microsoft.com/expression/blend/2008"         xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"         xmlns:local="clr-namespace:wpfapp1"         mc:ignorable="d"         title="mainwindow" height="350" width="525">     <dockpanel lastchildfill="true">         <stackpanel dockpanel.dock="top" orientation="horizontal">             <button click="button_click" >click</button>             <button click="button_click_1" >click2</button>         </stackpanel>         <webbrowser name="webbrowser" horizontalalignment="stretch" verticalalignment="stretch"/>     </dockpanel> </window>    public partial class mainwindow : window { public mainwindow() {   initializecomponent(); }  private void button_click(object sender, routedeventargs e) {   webbrowser.source = new uri(@"t:\xp_ocr_exchange\a010.pdf"); }  private void button_click_1(object sender, routedeventargs e) {   webbrowser.source = null; } } 

when click on first button, die pdf displayed immediately. when click on second button (null assignment) takes quite while until pdf vanishes browser window.