generating integer random numbers in c#

If you want to be sure that you don't get duplicates use a Guid:

Guid guid = Guid.NewGuid();  // however a Guid is not a 4 digit integer

If you want a random number use Random:

Random rnd = new Random();
int randomInt = rnd.Next(1000, 10000)

But note that you should not create the random instance in a loop because it is seeded with the current time. Otherwise you get repeating values. So either pas the random instance as parameter to this method, use a field/property or use the same instance in a loop which was created outside.

The easiest approach to get unique random numbers is to create new numbers if one already exists.

Best way to do it is with the Random class:

Random random = new Random();
int result = random.Next(1000, 10000);

You could also use the RNGCryptoServiceProvider class, which gives you more secure random numbers:

RNGCryptoServiceProvider csp = new RNGCryptoServiceProvider();
byte[] bytes = new byte[4];
csp.GetBytes(bytes);
int yourNum = bytes.Select(x => x % 10).Aggregate(0, (a, b) => (a * 10) + b);

The Random class is used to create random numbers. (Pseudo-random that is of course.)

Example:

Random rnd = new Random();
int num = rnd.Next(1000, 10000); //creates number from 1000 to 9999

You can use HashSet for store generated number and check duplicate

The easiest way to do this is generate a list of all possible choices in order and shuffle it. Then after shuffling the list just go through the list "in order" and the sequence will be random and non repeating.

List<int> numbers = new List<int>(Enumerable<int>.Range(0,10000);

HelperFunctions.Shuffle(numbers); //There are plenty of examples of how to shuffle the list on this site.

foreach(var number in numbers)
{
    Console.WriteLine(number.ToString("D4"); //Displays random numbers from 0000 to 9999 but never repeats
}

Like this:

var random = new Random();

var numbers = new List<int>(0);

for (var i = 0; i < 4; i++)
{
    var number = random.Next(1000, 10000);

    if (numbers.Contains(number))
    {
        i--;
        continue;
    }

    numbers.Add(number);
}

tbx1.Text = numbers[0].ToString();
tbx2.Text = numbers[1].ToString();
tbx3.Text = numbers[2].ToString();
tbx4.Text = numbers[3].ToString();

This will give you a 4 digit random number.

Random random= new Random();   

int RnNum = random.Next(1000,9999);

If you want to be sure that it doesn't get duplicated, then store the generated random values in Session & compare if its already generated. Of course there are other ways, if you don't want to do it the session way, write to a text file or something

You need to keep a record yourself of the numbers that have been generated in Dictionary or Hashset. Like

 HashSet<int> generatedValues = new HashSet<int>();
 Random rnd = new Random();
 int randomInt = rnd.Next(1000,9999);
 while(generatedValues.Contains(randomInt))
 {
    randomInt = rnd.Next(1000,9999);
 }
  generatedValues.Add(randomint);