There are a several ways to create a DOCX files in .NET C#. Here are some of the most common methods:

• Using the OpenXml SDK: The OpenXml SDK is a free open-source library that provides classes for creating and manipulating OpenXML documents, including DOCX files.
• Using the Aspose.Words library: Aspose.Words is a commercial library that provides classes for creating and manipulating Microsoft Office documents such as DOCX files.
• Using the Microsoft.Office.Interop.Word library: The Microsoft.Office.Interop.Word library is a .NET library that provides classes for accessing Microsoft Word from .NET applications.

Here is a sample code that showing how to create a DOCX file using the OpenXml SDK:

C#

using OpenXml;
using OpenXml.Word;

namespace CreateDocxFile
{
    class Program
    {
        static void Main(string[] args)
        {
            // Create a new document.
            Document doc = new Document();

            // Add a paragraph to the document.
            Paragraph paragraph = doc.AddParagraph();
            paragraph.AppendText("This is a sample paragraph.");

            // Save the document to a file.
            doc.Save("myfile.docx");
        }
    }
}

Here is a sample code that showing how to create a DOCX file using the Aspose.Words library:

C#

using Aspose.Words;

namespace CreateDocxFile
{
    class Program
    {
        static void Main(string[] args)
        {
            // Create a new document.
            Document doc = new Document();

            // Add a paragraph to the document.
            Paragraph paragraph = doc.Paragraphs.Add();
            paragraph.Text = "This is a sample paragraph.";

            // Save the document to a file.
            doc.Save("myfile.docx");
        }
    }
}

Here is a sample code that shows how to create a DOCX file using the Microsoft.Office.Interop.Word library:

C#

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

namespace CreateDocxFile
{
    class Program
    {
        static void Main(string[] args)
        {
            // Create a new Word document.
            Word.Application wordApp = new Word.Application();
            Word.Document doc = wordApp.Documents.Add();

            // Add a paragraph to the document.
            Word.Paragraph paragraph = doc.Paragraphs.Add();
            paragraph.Range.Text = "This is a sample paragraph.";

            // Save the document to a file.
            doc.SaveAs("myfile.docx");

            // Close the document.
            doc.Close();

            // Quit Word.
            wordApp.Quit();
        }
    }
}

The best method on your specific needs. If you need to create a complex DOCX files with a lot of formatting, then the OpenXml SDK or Aspose.Words library may be a good choices. If you just need to create a simple DOCX file, then the Microsoft.Office.Interop.Word library might be a good choice.