How Do I Insert Rtf File Data Into Teleriks Radrichtextbox At Cursor Position

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C#

WinForms

1. We have intgrated teleriks wpf control in winform
2. Now we want to insert rtf file from winform to wpf window's radrichtextbox.
3. I tried to send filename from constructor but it causes new instance wpf window created.but can't succeed.
4. So I directly tried to insert file stream from winform to existing instance of wpf window but it applied to starting position.here is code on winform
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RadDocument raddoc = new RadDocument();
RtfFormatProvider provider = new RtfFormatProvider();
String abc = lblsnippetpath.Text;
var filePath = Environment.ExpandEnvironmentVariables(abc);            
using (Stream stream = new FileStream(filePath, FileMode.Open))
{
   DocumentPosition startPosition = objbizsop.rch.Document.CaretPosition;
   raddoc = provider.Import(stream);                  
   raddoc.Selection.SelectAll();                                      
   DocumentFragment fragment = raddoc.Selection.CopySelectedDocumentElements();
   objbizsop.radRichTextBox1.Document = raddoc;                  
   raddoc.InsertFragment(fragment, objbizsop.radRichTextBox1.Document.CaretPosition);
}
objbizsop.Show();

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here objbizsop is instance of wpf window where radrichtextbox is kept.
Please give me idea .
Here is steps how process runs
a. wpf window calls winform
b. winform opens file and insert into wpf window
but it is applied at initial position .I want cursor position.