I have 10 numbers and I get average of them.
But how can print out how many of the numbers that are smaller than average, and how many of the numbers that are larger than average?

If you include LINQ(using System.Linq;), it includes an 'Average' method for integer arrays to find the average of every element in the array. You can also extract specific elements that match specific criteria. This link gives a fairly good explanation.

I mean the users can specify an arbitrary number of integers. Then the program will calculate the average of the given numbers. At last, the program will print out how many of the numbers that are smaller than average, and how many of the numbers that are larger than average?
Actually I need to know how can print out how many of the numbers that are smaller than average, and how many of the numbers that are larger than average?

int min = arrNumbers[0];
int max = arrNumbers[0];
int sum = 0;
double average = 0;
for (int i = 0; i < n; i++)
{
sum += arrNumbers[i];
if (min > arrNumbers[i])
min = arrNumbers[i];
if (max < arrNumbers[i])
max = arrNumbers[i];
}

Bu I need to print out all nummber that are smaller and larger than average?

To figure out which information you want to print out, the link in my other message gives very easy to understand information with examples on how to program that.
To print out the information, to the screen, not to an actual printer. You can use a temporary pop-up called Message Box, which will display your information until the user clicks and OK button. You can also put a Text Box or a Label on your form in which you can display the information until you change it in your program. If you meant an actual printer you could use the Printer Dialog, which is in your toolbox. It will depend on what your requirements are. Information on how to use these is linked to each one.

There's a link in my last message about Text Boxes. They are fairly straight forward basically TextBox1.Text=AnyString. Like any string variable if you want to add to the string, TextBox1.Text = TextBox1.Text + AnyString.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace ConsoleApplication6
{
class Program
{
static void Main(string[] args)
{
double[] MyArray = new double[]{10,9,5,77,45,23,12,5,88,49};
double wrecked = MyArray.Average();
int lowerthanAv = 0, higherthanAv = 0;
for (int i = 0; i < MyArray.Length; i++)
{
if (MyArray[i] < wrecked)
lowerthanAv++;
else
higherthanAv++; // I do not take equal than av into acount here
}
Console.WriteLine("The average is: {0}", wrecked);
Console.WriteLine("Number lower than average: {0}", lowerthanAv);
Console.WriteLine("Number higher than average: {0}", higherthanAv);
Console.ReadLine();
}
}
}