chuỗi tên là : “A”

Bài 1:

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

namespace ConsoleApplication2

{

    class Program

    {

        static void Main(string[] args)

        {

            int i, kt = 0;

            Console.WriteLine("nhap chuoi");

            string chuoi = Console.ReadLine();

            for (i = 0; i < chuoi.Length; i++)

            {

                if (chuoi.Substring(i, 1) ==" ")

                {

                    kt += 1;

                }

            }

            Console.WriteLine("chuoi co {0} khong trang ", kt);

            Console.ReadKey();

        }

    }

}

Bài 2:

            int i, kt = 0;

            Console.WriteLine("nhap chuoi mot ");

            string chuoi1 = Console.ReadLine();

            Console.WriteLine("nhap chuoi hai ");

            string chuoi2 = Console.ReadLine();

            string s = string.Concat(chuoi1, chuoi2);

            Console.WriteLine(s);

            Console.WriteLine("chuoi mot la ");

            Console.WriteLine(chuoi1);

            Console.ReadKey();

        }

    }

}

Bai 3 

int i, kt = 0;

                Console.WriteLine("nhap chuoi mot chuoi ");

                string chuoi = Console.ReadLine();

                Console.WriteLine(chuoi.ToLower());

                Console.ReadKey();

Bai 4

            int i, kt = 0;

            Console.WriteLine("nhap vao mot chuoi ");

            string chuoi = Console.ReadLine();           

            Console.WriteLine(chuoi.ToUpper());

            Console.ReadKey();

        }

    }

}