AttachConsoleSample.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace AttachConsoleSample
{
    class Program
    {
        [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;
        }

        static void Main(string[] args)
        {
            writeConsole("出力！");
        }
    }
}