A random password is a combination of characters, numbers, and special characters. We can generate a random password by combining random numbers and random strings.
The code snippet in this article is an example of how to generate random numbers and random strings and combine them to create a random password using C# and .NET Core.
The Random class constructors have two overloaded forms. It takes either no value, or it takes a seed value.
The Random class has three public methods - Next, NextBytes, and NextDouble. The Next method returns a random number, NextBytes returns an array of bytes filled with random numbers, and NextDouble returns a random number between 0.0 and 1.0.
Generate a random number
The following code in Listing 1 returns a random number.
// Generate a random number
Random random = new Random();
// Any random integer
int num = random.Next();
Listing 1.
Generate a random string
The following code snippet in Listing 2 generates a random string with a given size. The second parameter of the RandomString method is used for setting if the string is a lowercase string.
// Generate a random string with a given size and case.
// If second parameter is true, the return string is lowercase
public string RandomString(int size, bool lowerCase)
{
StringBuilder builder = new StringBuilder();
Random random = new Random();
char ch;
for (int i = 0; i < size; i++)
{
ch = Convert.ToChar(Convert.ToInt32(Math.Floor(26 * random.NextDouble() + 65)));
builder.Append(ch);
}
if (lowerCase)
return builder.ToString().ToLower();
return builder.ToString();
}
Listing 2.
Creating a random password
A random password can simply be a combination of a random string and a random number. To make it more complex, you can add special characters and mix them up.
For us, we will combine the two methods - RandomNumber and RandomString.
The following code snippet in Listing 3 generates a password of length 10 with the first four letters lowercase, the next four letters numbers, and the last two letters as uppercase.
// Generate a random password of a given length (optional)
public string RandomPassword(int size = 0)
{
StringBuilder builder = new StringBuilder();
builder.Append(RandomString(4, true));
builder.Append(RandomNumber(1000, 9999));
builder.Append(RandomString(2, false));
return builder.ToString();
}
Listing 3.
All of the above functionality is listed here in Listing 4. First, create a .NET Core Console app in Visual Studio and use this code.
using System;
using System.Text;
class RandomNumberSample
{
static void Main(string[] args)
{
// Generate a random number
Random random = new Random();
// Any random integer
int num = random.Next();
// A random number below 100
int randomLessThan100 = random.Next(100);
Console.WriteLine(randomLessThan100);
// A random number within a range
int randomBetween100And500 = random.Next(100, 500);
Console.WriteLine(randomBetween100And500);
// Use other methods
RandomNumberGenerator generator = new RandomNumberGenerator();
int rand = generator.RandomNumber(5, 100);
Console.WriteLine($"Random number between 5 and 100 is {rand}");
string str = generator.RandomString(10, false);
Console.WriteLine($"Random string of 10 chars is {str}");
string pass = generator.RandomPassword();
Console.WriteLine($"Random password {pass}");
Console.ReadKey();
}
}
public class RandomNumberGenerator
{
// Generate a random number between two numbers
public int RandomNumber(int min, int max)
{
Random random = new Random();
return random.Next(min, max);
}
// Generate a random string with a given size and case.
// If second parameter is true, the return string is lowercase
public string RandomString(int size, bool lowerCase)
{
StringBuilder builder = new StringBuilder();
Random random = new Random();
char ch;
for (int i = 0; i < size; i++)
{
ch = Convert.ToChar(Convert.ToInt32(Math.Floor(26 * random.NextDouble() + 65)));
builder.Append(ch);
}
if (lowerCase)
return builder.ToString().ToLower();
return builder.ToString();
}
// Generate a random password of a given length (optional)
public string RandomPassword(int size = 0)
{
StringBuilder builder = new StringBuilder();
builder.Append(RandomString(4, true));
builder.Append(RandomNumber(1000, 9999));
builder.Append(RandomString(2, false));
return builder.ToString();
}
}
Listing 4.
The random string and random password look like in Figure 1.
![Generate A Random Password In C# And .NET Core]()
Figure 1.
Random password with given characters
Now, let’s say you want to create a password that allows some specific characters only. The following code snippet in Listing 5 has a string of valid characters. The code uses this string to pick one character at a time for the password and stops at the given length. The default length of the password is 15.
private static string CreateRandomPassword(int length = 15)
{
// Create a string of characters, numbers, and special characters that are allowed in the password
string validChars = "ABCDEFGHJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789!@#$%^&*?_-";
Random random = new Random();
// Select one random character at a time from the string
// and create an array of chars
char[] chars = new char[length];
for (int i = 0; i < length; i++)
{
chars[i] = validChars[random.Next(0, validChars.Length)];
}
return new string(chars);
}
Listing 5.
Note
You can modify validChars string with the characters you allowed in the password.
Reference
https://madskristensen.net/blog/generate-random-password-in-c/
Listing 6 is the complete program written in .NET Core.
using System;
class Program
{
static void Main(string[] args)
{
Console.WriteLine(CreateRandomPassword());
Console.WriteLine(CreateRandomPassword(10));
Console.WriteLine(CreateRandomPassword(30));
Console.WriteLine(CreateRandomPasswordWithRandomLength());
Console.ReadKey();
}
// Default size of random password is 15
private static string CreateRandomPassword(int length = 15)
{
// Create a string of characters, numbers, special characters that allowed in the password
string validChars = "ABCDEFGHJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789!@#$%^&*?_-";
Random random = new Random();
// Select one random character at a time from the string
// and create an array of chars
char[] chars = new char[length];
for (int i = 0; i < length; i++)
{
chars[i] = validChars[random.Next(0, validChars.Length)];
}
return new string(chars);
}
private static string CreateRandomPasswordWithRandomLength()
{
// Create a string of characters, numbers, special characters that allowed in the password
string validChars = "ABCDEFGHJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789!@#$%^&*?_-";
Random random = new Random();
// Minimum size 8. Max size is number of all allowed chars.
int size = random.Next(8, validChars.Length);
// Select one random character at a time from the string
// and create an array of chars
char[] chars = new char[size];
for (int i = 0; i < size; i++)
{
chars[i] = validChars[random.Next(0, validChars.Length)];
}
return new string(chars);
}
Listing 6.
Summary
This article and code sample taught us how to generate random passwords using C# in .NET Core.