A method which, given a string, returns a string in which each character is separated by an asterisk

Use recursion in the solution.

I would really appreciate if you can help me to find a better way to do it.

public class AppendCharacterAfterEachCharacterInString
{
    int index =0;
    StringBuilder outputString = new StringBuilder();

    public string AppendInputCharacterAfterEachCharacterInString(string sInputString, string inputRepeatCharacter)
    {            
        if (string.IsNullOrEmpty(sInputString))
        {
            Console.WriteLine("Empty string !");
        }
        else
        {
            BuildNewString(sInputString, inputRepeatCharacter);
        }

        return outputString.ToString();
    }

    private void BuildNewString(string inputString, string repeatCharacter)
    {
        if (index < inputString.Length)
        { 
            outputString.Append(inputString[index]);

            if(!(inputString[index].ToString().Trim() == string.Empty) && !(index + 1 == inputString.Length))
              outputString.Append(repeatCharacter);                  

            index++;
            BuildNewString(inputString, repeatCharacter);  
        }

    }

Well, you've used recursion, but I wouldn't say that's a recursive solution because you have variables outside your functions that control what your functions do.

A recursive solution will have a function that is repeatedly called with smaller and smaller sub-problems, terminating in a base case, that together will accomplish your goal. A key aspect of recursive solutions is that the recursive function returns a value - your method is void and so returns no value.

As a first step, discard your index variable entirely. At each recursive step, call BuildNewString with a smaller version of the same problem. Combine the results in such a way as to produce the desired result.

StackExchange Code Review Q#51359, answer score: 10