using System.IO;
...

string path = @"C:MP_Upload";
if(!Directory.Exists(path))
{
    Directory.CreateDirectory(path);
}

using System.IO;
...

Directory.CreateDirectory(@"C:MP_Upload");

if(!System.IO.Directory.Exists(@"c:mp_upload"))
{
     System.IO.Directory.CreateDirectory(@"c:mp_upload");
}

if(!Directory.Exists(@"C:MP_Upload")) {
    Directory.CreateDirectory(@"C:MP_Upload");
}

using System;
using System.IO;
using System.Windows.Forms;

namespace DirCombination
{
    public partial class DirCombination : Form
    {
        private const string _Path = @"D:/folder1/foler2/folfer3/folder4/file.txt";
        private string _finalPath = null;
        private string _error = null;

        public DirCombination()
        {
            InitializeComponent();

            if (!FSParse(_Path))
                Console.WriteLine(_error);
            else
                Console.WriteLine(_finalPath);
        }

        private bool FSParse(string path)
        {
            try
            {
                string[] Splited = path.Replace(@"//", @"/").Replace(@"\", @"/").Replace(@"", "/").Split(':');
                string NewPath = Splited[0] + ":";
                if (Directory.Exists(NewPath))
                {
                    string[] Paths = Splited[1].Substring(1).Split('/');

                    for (int i = 0; i < Paths.Length - 1; i++)
                    {
                        NewPath += "/";
                        if (!string.IsNullOrEmpty(Paths[i]))
                        {
                            NewPath += Paths[i];
                            if (!Directory.Exists(NewPath))
                                Directory.CreateDirectory(NewPath);
                        }
                    }

                    if (!string.IsNullOrEmpty(Paths[Paths.Length - 1]))
                    {
                        NewPath += "/" + Paths[Paths.Length - 1];
                        if (!File.Exists(NewPath))
                            File.Create(NewPath);
                    }
                    _finalPath = NewPath;
                    return true;
                }
                else
                {
                    _error = "Drive is not exists!";
                    return false;
                }
            }
            catch (Exception ex)
            {
                _error = ex.Message;
                return false;
            }
        }
    }
}

String path = Server.MapPath("~/MP_Upload/");
    if (!Directory.Exists(path))
    {
        Directory.CreateDirectory(path);
    }

using System.IO;string path = "C:MP_Upload";if(!Directory.Exists(path)){
   Directory.CreateDirectory(path);}