VanilaSpirit

static int EvaluateExpression(string formula)
        {
            string[] numbers = formula.Split(new char[] { '+', '-' }).Where(e=>e.Trim()  != "").ToArray();
            string[] symbols = Regex.Matches(formula, "[+-]").Cast<Match>().Select(e => e.Value).ToArray();
			if(numbers.Length != symbols.Length+1)
				throw new FormatException("Не верное количество знаков");
            int result;
            int i = -1;
            try
            {
                result = int.Parse(numbers[0]);
				for (i = 0; i < symbols.Length; i++){
						switch(symbols[i])
						{
							case "+":
								result += int.Parse(numbers[i+1]);
								break;
							case "-":
								result -= int.Parse(numbers[i+1]);
								break;
						}
				}
			}catch(FormatException e){
                throw new FormatException(String.Format("Не верное число '{0}'", numbers[i+1]));
			}
 
            return result; 
        }