To generate random alphanumeric usernames, you can use the System.Random class and the Guid.NewGuid() method, along with some string manipulation. Here’s an example of how you could do this:

using System;

namespace RandomUsernameGenerator
{
    class Program
    {
        static void Main(string[] args)
        {
            Random random = new Random();

            // Generate a random alphanumeric string of length 8
            string username = GenerateRandomUsername(8, random);
            Console.WriteLine(username);

            // Generate another random alphanumeric string of length 8
            username = GenerateRandomUsername(8, random);
            Console.WriteLine(username);
        }

        static string GenerateRandomUsername(int length, Random random)
        {
            const string chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
            return new string(Enumerable.Repeat(chars, length)
              .Select(s => s[random.Next(s.Length)]).ToArray());
        }
    }
}

This code will generate two random alphanumeric strings of length 8 and print them to the console. You can adjust the length of the strings and the character set (uppercase letters, lowercase letters, digits, etc.) to suit your needs.

Alternatively, you could use the Guid.NewGuid() method to generate a unique identifier, and then use string manipulation to extract the alphanumeric characters from the identifier. Here’s an example of how you could do this:

using System;

namespace RandomUsernameGenerator
{
    class Program
    {
        static void Main(string[] args)
        {
            // Generate a random alphanumeric string of length 8
            string username = GenerateRandomUsername(8);
            Console.WriteLine(username);

            // Generate another random alphanumeric string of length 8
            username = GenerateRandomUsername(8);
            Console.WriteLine(username);
        }

        static string GenerateRandomUsername(int length)
        {
            // Generate a new GUID
            string guid = Guid.NewGuid().ToString();

            // Remove any hyphens
            guid = guid.Replace("-", "");

            // Take the specified number of characters from the beginning of the string
            return guid.Substring(0, length);
        }
    }
}

This code will also generate two random alphanumeric strings of length 8 and print them to the console. The advantage of using the Guid.NewGuid() method is that the strings it generates are guaranteed to be unique.

The code can be compiled and run using C# compilers, such as the one provided with the .NET framework or the open-source Mono compiler.

The first example uses the System.Random class to generate random characters from a predefined character set, and the second example uses the Guid.NewGuid() method to generate a unique identifier and then extracts the alphanumeric characters from the identifier. Both examples include a Main() method that demonstrates how the functions can be used to generate and print random alphanumeric strings to the console.