Method to randomise a string's case

Question

I'm an amateur at programming and have created the following method with which I pass a lowercase string with the idea it is then returned either in it's original lowercase format, Capitalised or UPPERCASE. I'm not sure this is the best way or is truly "random" but does work up to a point but would like pointers on any improvements or different ways to achieve this.

        //==============================================================================
        // Randomise string case
        //==============================================================================
        private string randomCase (string word)
        {
            string changed;
            Random r = new Random(Guid.NewGuid().GetHashCode());
            int n = r.Next(1, 29);

            if (n >= 1 && n <= 9)
            {
                // First letter capatalised
                changed = char.ToUpper(word[0]) + word.Substring(1);
                return changed;
            }
            else if (n >= 10 && n <= 19)
            {
                // Word capatalised
                changed = word.ToUpper();
                return changed;
            }
            else if (n >= 20 && n <= 29)
            {
                // Left as lower case
                return word;
            }

            return null;
        }

Answer 1

Here are my observations:

• string randomCase: I would suggest to start your method name with a verb and use CamelCase. For example: RandomizeCase
• (string word): I've got your intent, but nothing prevents the consumer of your API to put a sentence into your word parameter. In such a general purpose method input can be considered as a fine parameter name.
• string changed: You don't need this variable at all. You use this as an intermediate container. If you return right after you set it then you can combine these two statements into one, like: return word.ToUpper();
• Random r / int n: Please try to avoid to use single letter for variable names (exceptions are the loop variables).
• Random(Guid.NewGuid().GetHashCode()): I've seen so many different ways how to seed a Random. For me the most convenient is the Random((int)DateTime.Now.Ticks). But do you really need to provide a seed at all?
• r.Next(1, 29): I get it that you want to achieve something like even or fair distribution. This is unnecessary. Your current implementation is hard to maintain and hard to change in the future. Also future reader of your code (even yourself) might ask: Why do we use 29 her as the upper-bound? Why not 316464616?
  • r.Next(1,3) or r.Next() % 3 could be a viable alternative.
• if ... else if ... return null: Even though you have restricted the range of n you need to provide a return null at the end to make it compile. Even worse this piece of code is never reached. If you want to stick with this approach a better alternative would be to use a single return:

string changed
if (n >= 1 && n <= 9)
{
    changed = char.ToUpper(word[0]) + word.Substring(1);
}
else if (n >= 10 && n <= 19)
{
    changed = word.ToUpper();
}
else if (n >= 20 && n <= 29)
{
    changed = word;
}

return changed;

• // First letter capatalised: Here you have made a huge assumption: The provided word is in lower case. You can't trust your input. The simplest way to ensure this is to make it lowercase by yourself:

if(string.IsNullOrEmpty(word))
   return string.Empty; //Or throw ArgumentException if you wish

string loweredWord = word.ToLower();

• The same assumption is present in each of your if-else branches.

---

UPDATE: Random

It is not crystal clear for me why do you need this method at all, but based on the provided comment I would like highly two things regarding random:

• Random is not thread-safe, which means that if the same instance is shared amongst different threads then Next may return zeros. (Related SO)
• Even though RNGCryptoServiceProvider is slower it is stronger from cryptographic point of view. There are several really good articles and samples out there, for example:
  • how to generate random number between a range
  • how to use it for seed generation
  • how to use it during hash generation

Answer 2

• This entire method should be static, as it clearly does not require an object
• new Random(Guid.NewGuid().GetHashCode()) is needlessly complicated. new Random() is initialized based on time by default
• A simpler range from [0,2] can be used.
• The changed variable can be entirely replaced by a switch and return structure

private static string Capitalized(string word) {
    return char.ToUpper(word[0]) + word.Substring(1);
}

private static string RandomCase(string word)
{
    int choice = new Random().Next(3);
    switch(choice) {
        case 0: return Capitalized(word);
        case 1: return word.ToUpper();
        case 2: default: return word;
    }
}