using System;
using System.IO;
using System.Text;



string[] files = Directory.GetFiles("../var/data");
string maxFilePath = "";
long maxFileSize = 0;

foreach (var filePath in files)
{
    FileInfo fileInfo = new FileInfo(filePath);

    if (fileInfo.Length > maxFileSize)
    {
        maxFileSize = fileInfo.Length;
        maxFilePath = filePath;
    }
}

using (FileStream sourceStream = File.OpenRead(maxFilePath))
{
        byte[] buffer = new byte[sourceStream.Length];
        await sourceStream.ReadAsync(buffer, 0, buffer.Length);
        string text = Encoding.Default.GetString(buffer);

        using (FileStream targetStream = new FileStream("../var/result/data.txt", FileMode.OpenOrCreate))
        {
            targetStream.SetLength(0);
            byte[] textBytes = Encoding.Default.GetBytes(text);
            await targetStream.WriteAsync(textBytes, 0, textBytes.Length);
        }
}

Console.WriteLine("Файл перемещен.");