StringService.cs

using System;

namespace StringProcessingApp.Services
{
    public class StringService
    {
        private string _originalText = string.Empty;
        private string _currentText = string.Empty;

        public void SetText(string text)
        {
            _originalText = text;
            _currentText = text;
        }

        public string GetCurrentText() => _currentText;

        public void ToUpper() => _currentText = _currentText.ToUpper();

        public void ToLower() => _currentText = _currentText.ToLower();

        public int GetLength() => _currentText.Length;

        public bool ContainsWord(string word) => _currentText.Contains(word, StringComparison.OrdinalIgnoreCase);

        public void ReplaceWord(string oldWord, string newWord) => 
            _currentText = _currentText.Replace(oldWord, newWord);

        public string ExtractSubstring(int startIndex, int length)
        {
            if (startIndex < 0 || startIndex + length > _currentText.Length)
                return "Error: Out of range.";
            
            return _currentText.Substring(startIndex, length);
        }

        public void TrimSpaces() => _currentText = _currentText.Trim();

        public void Reset() => _currentText = _originalText;
    }
}