AttachConsole.cs

[DllImport("kernel32.dll")]
public static extern bool AttachConsole(uint dwProcessId);

[DllImport("kernel32.dll")]
public static extern bool FreeConsole();

private bool writeConsole( string msg ) {

    if ( !AttachConsole( System.UInt32.MaxValue ) ) {
        return false;
    }

    // stdoutのストリームを取得
    System.IO.Stream       stream = System.Console.OpenStandardOutput();
    System.IO.StreamWriter stdout = new System.IO.StreamWriter( stream );

    // 指定された文字列を出力
    stdout.Write( msg );
    stdout.Flush();

    FreeConsole();
    return true;
}