Occasional Runtime errors with PRNG?

[Smashbrosboy]

I have written a pseudo-random number generator with both my own algorithm and the pre-built algorithm. In both cases, when I enter a large number (maybe 9999999) for the length of the random string I want generated, it throws me an error (which I catch). However, on other occasions, this does not happen and the string is generated normally.

Here's my code to look at. I apologize for the comments, but I correspond with a C programmer and this makes things easier for both of us.:




BUILT-IN RANDOM CLASS:

using System;
using System.Text;

namespace RandomGenerator
{
    class Program
    {
        static void Main(string[] args)
    {
            //Declaring the variables
            string choice;
            int conversion;
            double conversion2;
            int sizeofstring;
            string charstouse = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890";
            ///<Intro>
            Console.WriteLine("==RANDOM CHARACTERS GENERATOR==\nWhich would you like?\nType your selection and hit Enter.\nTo exit, press Enter without entering anything else.\n1-Number\n2-Letters\n3-Decimal\n4-Credits\n");
            choice = Console.ReadLine();
            //While loop for multiple calculations
            while (choice != "")
            {
                if (choice == "1")
                {
                    //The Random class produces almost totally random (nothing can be totally random) numbers). I'm creating a new object in that class.
                    Random randomint = new Random();
                    //You can't print a Random so I'm converting it to an int. The Next method gives me the NEXT value of randomint.
                    conversion = randomint.Next();
                    Console.WriteLine("\nYour random number is: " + conversion);
                }
                else if (choice == "2")
                {
                    Console.WriteLine("\nHow long do you want your string of letters to be?\nType the length and then press Enter.\n");
                    ///<Exception Handling> This is telling the program try the code in the try block. If an error occurs, go the catch block.
                    try
                    {
                        //sizeofstring is converting what the user typed to an integer.
                        sizeofstring = Convert.ToInt32(Console.ReadLine());
                        //Calling a method and passing these parameters
                        RandomString(sizeofstring, charstouse);
                        
                        
                    }
                        //If you typed in a letter or something, it sends you here.
                    catch
                    {
                        Console.WriteLine("\nYou thought you'd be funny and try to break the program, eh? No funny business is allowed here.\nA person like you doesn't deserve to use this program.\nPress Enter to exit.");
                        Console.ReadLine();
                        //Environment.Exit tells the CONSOLE Application to close.
                        Environment.Exit(1);
                    }
                }
                else if (choice == "3")
                {
                    //Same process as "int" except we're replacing the ints with doubles.
                    Random randomdouble = new Random();
                    
                    conversion2 = randomdouble.NextDouble();
                    Console.WriteLine("\nYour random decimal is: " + conversion2);
                }
                else if (choice == "4")
                {
                    //Prints credits
                    Console.WriteLine("Created by Smashbrosboy\nCopyright 2009 Smashbrosboy\nContact me: sbboyjunkmail@gmail.com\n");
                }
                else if (choice == "42")
                {
                    Console.WriteLine("\nYeah, that's the answer to the ultimate question, but what IS the ultimate question?\nPress Enter to continue.");
                    Console.ReadLine();
                }
                //The || pipes act like the word or.
                else if (choice == "No" || choice == "no")
                {
                    Console.WriteLine("\nCome on man, I know you're bored but still. At least give me a chance to show off if nothing else.\nPress Enter to continue.");
                    Console.ReadLine();
                }
                else
                {
                    Console.WriteLine("\nYoda: Amazingly funny are you, yet not laughing am I /sarcasm\nPress Enter to continue.");
                    Console.ReadLine();
                }
                Console.WriteLine ("\nSelect again or press Enter without typing anything to exit.\n1-Number\n2-Letters\n3-Decimal\n4-Credits\n"); 
                //Resetting the value of choice to prevent infinite looping.
                choice = Console.ReadLine();
            }

        }
        //This method returns a string value.
        //This method has two parameters that were passed to it. One int, sizeofstring that was passed and will be now called size in the method
        //and one string which was passed to it, charstouse, which is now called legalChars in the method.
       public static void RandomString(int size, string legalChars)
            {
                //Declaring a new random object
                Random z = new Random();
                //String builder is self-explanatory
                StringBuilder sb = new StringBuilder();
                //A for loop is used here to write as many characters as sizeofstring (now called size) dictates.
                for (int i = 0; i < size; i++)
                {
                    
                    //This is the most complex line of the code:
                    //Within the string legalChars, we are calling upon a certain letter.
                    //This letter is determined by the random object z which only generates numbers.
                    //z is limited to only generate numbers that are smaller than or equal to the length of the string. Thus, when it generates a number, it will correspond to a letter.
                    //The stringbuilder sb is appending the letter which corresponds to this number within legalChars.

                    sb.Append(legalChars[z.Next(legalChars.Length)]);
                }
                Console.WriteLine("\nYour random string is:\n" + sb);
            }

    }
}

MY OWN ALGORITHM:

First, Program.cs

using System;
using System.Text;

namespace RandomGenerator
{
    class Program
    {
        static void Main(string[] args)
    {
            //Declaring the variables
            string choice;
            
            double conversion2;
            int sizeofstring;
            ///<Intro>
            Console.WriteLine("==RANDOM CHARACTERS GENERATOR==\nWhich would you like?\nType your selection and hit Enter.\nTo exit, press Enter without entering anything else.\n1-Number\n2-Letters\n3-Decimal\n4-Credits\n");
            choice = Console.ReadLine();
            //While loop for multiple calculations
            while (choice != "")
            {
                if (choice == "1")
                {
                    /*//The Random class produces almost totally random (nothing can be totally random) numbers). I'm creating a new object in that class.
                    Random randomint = new Random();
                    //You can't print a Random so I'm converting it to an int. The Next method gives me the NEXT value of randomint.
                    conversion = randomint.Next();
                    Console.WriteLine("\nYour random number is: " + conversion);*/
                    Class1 obj = new Class1();
                }
                else if (choice == "2")
                {
                    Console.WriteLine("\nHow long do you want your string of letters to be?\nType the length and then press Enter.\n");
                    ///<Exception Handling> This is telling the program try the code in the try block. If an error occurs, go the catch block.
                    try
                    {
                        //sizeofstring is converting what the user typed to an integer.
                        sizeofstring = Convert.ToInt32(Console.ReadLine());
                        DateTime var = DateTime.Now;
                        int seed = var.Millisecond;
                        //Calling a method and passing these parameters
                        //RandomString(sizeofstring, charstouse);
                        Class1 obj = new Class1(sizeofstring, seed);
                        
                        
                    }
                        //If you typed in a letter or something, it sends you here.
                    catch
                    {
                        Console.WriteLine("\nYou thought you'd be funny and try to break the program, eh? No funny business is allowed here.\nA person like you doesn't deserve to use this program.\nPress Enter to exit.");
                        Console.ReadLine();
                        //Environment.Exit tells the CONSOLE Application to close.
                        Environment.Exit(1);
                    }
                }
                else if (choice == "3")
                {
                    //Same process as int except we're replacing int with double.
                    DateTime v = DateTime.Now;
                    double s = v.Millisecond;
                    Class1 c1 = new Class1(s);
                }
                else if (choice == "4")
                {
                    //Prints credits
                    Console.WriteLine("Created by Smashbrosboy\nCopyright 2009 Smashbrosboy\nContact me: sbboyjunkmail@gmail.com\n");
                }
                //I'm not explaining the remaining else ifs and elses in detail.
                else if (choice == "42")
                {
                    Console.WriteLine("\nYeah, that's the answer to the ultimate question, but what IS the ultimate question?\nPress Enter to continue.");
                    Console.ReadLine();
                }
                //The || pipes act like the word or.
                else if (choice == "No" || choice == "no")
                {
                    Console.WriteLine("\nCome on man, I know you're bored but still. At least give me a chance to show off if nothing else.\nPress Enter to continue.");
                    Console.ReadLine();
                }
                else
                {
                    Console.WriteLine("\nYoda: Amazingly funny are you, yet not laughing am I /sarcasm\nPress Enter to continue.");
                    Console.ReadLine();
                }
                Console.WriteLine ("\nSelect again or press Enter without typing anything to exit.\n1-Number\n2-Letters\n3-Decimal\n4-Credits\n"); 
                //Resetting the value of choice to prevent infinite looping.
                choice = Console.ReadLine();
            }

        }
    }
}

Now Class1:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace RandomGenerator
{
    class Class1
    {
       string legalchars = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
        public Class1(int size, int seed)
        {
            StringAlgorithm(size,seed);
        }
        public Class1(double seed)
        {
            double result;
            Console.WriteLine("Would you like any random decimal or only one that is less than 1?\n1-Any\n2-Less Than 1");
            string read = Console.ReadLine();
            if (read == "1")
            {
                if ((seed % 2) == 0)
                {
                    result = seed * 12.5877 + 71 % 100 * 65 / 89 % 70;
                    Console.WriteLine(result);
                }
                else
                {
                    result = seed * 0.245602 + 84 % 100 * 4 / 98 % 87;
                    Console.WriteLine(result);
                }
            }
            else if (read == "2")
            {
                if ((seed % 2) == 0)
                {
                    result = seed * 12.5877 + 71 % 100 * 65 / 89 % 70;
                    while (result < 0)
                    {
                        result += 0.5;
                    }
                    while (result > 1)
                    {
                        result -= 0.5;
                    }
                    Console.WriteLine(result);
                }
                else
                {
                    result = seed * 0.245602 + 84 % 100 * 4 / 98 % 87;
                    while (result < 0)
                    {
                        result += 0.5;
                    }
                    while (result > 1)
                    {
                        result -= 0.5;
                    }
                    Console.WriteLine(result);
                }
            }
            else
            {
                Console.WriteLine("Invalid Choice. Stop messing with the program. Press Enter to Exit.");
                Console.ReadLine();
                Environment.Exit(1);
            }
            
        }
        public Class1()
        {
            DateTime var = DateTime.Now;
            int seed = var.Millisecond;
            Algorithm(seed);
        }
        void Algorithm(int seed)
        {
            int result;
            if ((seed % 2) == 0)
            {
                result = seed * 283 + 71 % 100;
            }
            else
            {
                result = seed * 1236599832 + 71 % 100;
            }
            Console.WriteLine(result);
        }
        void StringAlgorithm(int size, int seed)
        {
            StringBuilder sb = new StringBuilder();
            while (seed == 0)
            {
                DateTime var = DateTime.Now;
                seed = var.Millisecond;
            }

            int result = seed * 2 + 7 % 100+97/5*100%40;
            for (int i = 0; i < size; i++)
            {
                result = seed * 2 + 7 % 100+97/5*100%40;
                try
                {
                    while (result > 61)
                    {
                        result -= 50;
                        result += 1;
                    }
                    while (result < 0)
                    {
                        result += 50;
                        result -= 1;
                    }
                }
                catch
                {
                    result = 23;
                }
                sb.Append(legalchars[result]);
                seed = result;
            }
            Console.WriteLine("\nYour random string is:\n" + sb);

        }
    }
}

[lobo521]

I tried like that and it's not crashing ;/

using System;

namespace RandomGenerator
{
	class Program
	{
		static void Main(string[] args)
		{
			//While loop for multiple calculations
			while (true)
			{
				new Class1(9999999, GetSeed());
			}
		}

		private static int GetSeed()
		{
			return DateTime.Now.Millisecond;
		}
	}
}

[Smashbrosboy]

I tried like that and it's not crashing ;/

using System;

namespace RandomGenerator
{
	class Program
	{
		static void Main(string[] args)
		{
			//While loop for multiple calculations
			while (true)
			{
				new Class1(9999999, GetSeed());
			}
		}

		private static int GetSeed()
		{
			return DateTime.Now.Millisecond;
		}
	}
}

Why would this stop the occasional crashes? I'm getting the seed in Class1 anyway.