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.
Generate A Random Password In C# And .NET Core. 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(); 


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(); 


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 even add special characters and mix it 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 first 4 letters lowercase, next 4 letters numbers, and the last 2 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(); 


All of the above functionality is listed here in Listing 4. 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(); 
    } 


The random string and random password looks like Figure 1.
Generate A Random Password In C# And .NET Core


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, special characters that allowed in the password 
    string validChars = "[email protected]#$%^&*?_-"; 
    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); 


Note
You can modify validChars string with the characters you allowed in the password.
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 = "[email protected]#$%^&*?_-"; 
        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 = "[email protected]#$%^&*?_-"; 
        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); 
    }