Extension method splitting string on each capital letter

With regex it's virtually a one-liner:

var words = 
    Regex.Matches("SmallFireBall", @"([A-Z][a-z]+)")
    .Cast<Match>()
    .Select(m => m.Value);

var withSpaces = string.Join(" ", words);

• Regex.Matches - searches an input string for all occurrences of a regular expression and returns all the matches. MSDN
• [A-Z][a-z]+ - matches strings that begin with a capital letter and are followed by one or more lowercase letters

Output:

Small Fire Ball

Let's review your code anyway and optimize it a little bit. Even without regex it still can be very short.

• Use a StringBuilder for building string dynamically

• You can use a foreach loop for strings too

  public static string SplitOnCapitalLetters2(this string inputString)
  {
      var result = new StringBuilder();
  
      foreach (var ch in inputString)
      {
          if (char.IsUpper(ch) && result.Length > 0)
          {
              result.Append(' ');
          }
          result.Append(ch);
      }
      return result.ToString();
  }
  

The 3rd alternative would be only LINQ:

public static string SplitOnCapitalLetters3(this string inputString)
{
    // starts with an empty string and accumulates the new string into 'result'
    // 'next' is the next character
    return inputString.Aggregate(string.Empty, (result, next) =>
    {
        if (char.IsUpper(next) && result.Length > 0)
        {
            result += ' ';
        }
        return result + next;
    });
}