How to use Stream.CopyTo on .NET Framework 3.5?

You can’t, basically. It’s only implemented in .NET 4. You can write a similar method yourself though… and even make it an extension method:

// Only useful before .NET 4
public static void CopyTo(this Stream input, Stream output)
{
    byte[] buffer = new byte[16 * 1024]; // Fairly arbitrary size
    int bytesRead;

    while ((bytesRead = input.Read(buffer, 0, buffer.Length)) > 0)
    {
        output.Write(buffer, 0, bytesRead);
    }
}