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先在项目引用里添加上对Microsoft Word 9.0 object library的引用。

using System;

namespace DocConvert

{
class DoctoRtf
{
  static void Main()
  {

//创建一个word的实例
   Word.Application newApp = new Word.Application();

   // 指定源文件和目标文件
   object Source="c:\\abc\\Source.doc";
   object Target="c:\\abc\\Target.rtf";

   object Unknown =Type.Missing;

   // 打开要转换的Word文件
   newApp.Documents.Open(ref Source,ref Unknown,
    ref Unknown,ref Unknown,ref Unknown,
    ref Unknown,ref Unknown,ref Unknown,
    ref Unknown,ref Unknown,ref Unknown,
    ref Unknown );

   // 指定文档的类型
   object format = Word.WdSaveFormat.wdFormatRTF;

   //改变文档类型
   newApp.ActiveDocument.SaveAs(ref Target,ref format,
    ref Unknown,ref Unknown,ref Unknown,
    ref Unknown,ref Unknown,ref Unknown,
    ref Unknown,ref Unknown,ref Unknown);                   

   //关闭word实例
   newApp.Quit(ref Unknown,ref Unknown,ref Unknown);

  }
}
}