Programmatically Open Word Document Located in the Computer in C#

Instead of adding interop in your reference, you may also consider to use this:

System.Diagnostics.Process.Start(@"C:\Test\NewDocument.docx");

first add the dll of Microsoft.Office.Interop.Word to your references then add this:

using Microsoft.Office.Interop.Word;

and use the following code:

Application ap = new Application(); 
Document document = ap.Documents.Open(@"C:\Test\NewDocument.docx");

This Application is not this.Application it's Microsoft.Office.Interop.Word.Application.
So you can use this code:

using System;
using Microsoft.Office.Interop.Word;

class Program
{
    static void Main()
    {
    // Open a doc file.
    Application application = new Application();
    Document document = application.Documents.Open("C:\\word.doc");

    //Do whatever you want

    // Close word.
    application.Quit();
    }
}