题目是编程输出1000以内的所有素数。

namespace ConsoleApplication18
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("2是素数");

            //fun1
            int count1 = 0;
            for (int i = 3; i < 1000; ++i)
            {
                if (IsPrime(i))
                {  
                    Console.WriteLine("{0}是素数", i);
                    count1++;
                }
                else
                    Console.WriteLine("{0}不是素数", i);
            }

            Console.WriteLine("\n\n\n\n\n");

            //fun2
            int count2 = 1;
            int[] arr = new int[1000];
            for(int i = 0; i < 1000; ++i)
                arr[i] = i + 1;
            foreach (int temp in arr)
            {
                if (temp > 2)
                {
                    if (IsPrime(temp))
                    {
                        Console.WriteLine("{0}是素数", temp);
                        count2++;
                    }
                }
            }
            Console.WriteLine("1000内的素数有{0}个",count2);
        }

        public static bool IsPrime(int num)
        {
            for (int loop = 2; loop <= Math.Sqrt(num); ++loop)
            {
                if (0 == num % loop)
                    return false;
            }
            return true;
        }
    }
}

用fun2 foreach就可以把所有结果输出；只用fun1的话输出只有701到999；fun1和fun2都写的话fun1只有875到999，fun2完整输出；Console.WriteLine("2是素数")这句也没输出。

求 知

那为啥你的CMD能全显示出来

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using namespace ConsoleApplication18
{
    class Program
    {
        static void Main(string[] args)
        {
            FileStream fs = new FileStream("D:\\a.txt", FileMode.Create);
            StreamWriter myFile = new StreamWriter(fs);
            myFile.WriteLine("2是素数");

            //fun1
            int count1 = 0;
            for (int i = 3; i < 1000; ++i)
            {
                if (IsPrime(i))
                {
                    myFile.WriteLine("{0}是素数", i);
                    count1++;
                }
                else
                    myFile.WriteLine("{0}不是素数", i);
                }

                myFile.WriteLine("\n\n\n\n\n");

                //fun2
                int count2 = 1;
                int[] arr = new int[1000];
                for (int i = 0; i < 1000; ++i)
                    arr[i] = i + 1;
                foreach (int temp in arr)
                {
                    if (temp > 2)
                    {
                        if (IsPrime(temp))
                        {
                            myFile.WriteLine("{0}是素数", temp);
                            count2++;
                        }
                        else
                            myFile.WriteLine("{0}不是素数", temp);
                    }
                }
                myFile.WriteLine("1000内的素数有{0}个", count2);
        }
        public static bool IsPrime(int num)
        {
            for (int loop = 2; loop <= Math.Sqrt(num); ++loop)
            {
                if (0 == num % loop)
                    return false;
            }
            return true;
        }
    }
}

a.txt里面所有输出都正常，看见你说的，才学的文件写入。