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);

}
}
}