using System.Text;

string[] files = Directory.GetFiles("/var/data");

long maxFileLength = 0;

string maxFileLengthPath = "";

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

    if(fileInfo.Length > maxFileLength)
    {
        maxFileLength = fileInfo.Length;

        maxFileLengthPath = file;
    }
}

using (FileStream fstream = File.OpenRead(maxFileLengthPath))
{
    byte[] buffer = new byte[fstream.Length];
    await fstream.ReadAsync(buffer, 0, buffer.Length);
    string text = Encoding.Default.GetString(buffer);
    using (FileStream fstream1 = new FileStream("/var/result/data.txt", FileMode.OpenOrCreate))
    {
        fstream1.SetLength(0);
        byte[] buffer1 = Encoding.Default.GetBytes(text);
        await fstream1.WriteAsync(buffer1, 0, buffer1.Length);
    }
}