how to extract Range(25,45) of Word Document,Add to other Word Document?

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C#

interop

MS-Word

hi,
I want extract Range(25,45) of "doc1",and add to "doc2",don't use clipboard.

if I use clipboard when the software is running,this software will fail.

Application app=new Application();
Document doc1,doc2;
doc1=app.Documents.Open(...);
doc2=app.Documents.Open(...);
doc1.Activate();
object start=25,end=45;
doc1.Range(ref start, ref end).Copy();
doc2.Activate();
start=1;
doc2.Range(ref start, ref start).Paste();