how can i copy and write a file in csharp

Copying from one to another text box:
Use the solution 1 from RMC[^]:

// copying from one to another text box
textbox2.Text = textbox1.Text;

For copying files you can use this:

using System;
using System.IO;

namespace FileSystemIO
{
    class Program
    {
        static void Main(string[] args)
        {
            string sourceFile = "C:\Temp\test.txt";//Original file path
            string destinationFile = "C:\Temp\test_copied.txt";//path of the copied file

            try
            {
                File.Copy(sourceFile, destinationFile); //Copy the file

                Console.WriteLine("File copied successfully");//Write succesful message if no exception raised
            }
            catch(Exception ex)
            {
                Console.WriteLine("Copying file failed: " + ex.Message); //Write error message if no exception raised.
            }

            Console.ReadKey();//Wait for user pressing key before ending program
        }
    }
}

More info about the System.IO.File.Copy() method[^] can you find on the MSDN page.

cheers,
Marco Alessandro Bertschi