public static class BracketsChecker {

    public class BracketMismatchException : ApplicationException {
    }

    public class ExtraOpeningBracketException : ApplicationException {
    }

    public class ExtraClosingBracketException : ApplicationException {
    }

    private class Bracket {

        char[] openChars;

        char[] closeChars;

        public Bracket(char[] openChars, char[] closeChars) {
            this.openChars = openChars;
            this.closeChars = closeChars;
        }

    }

    private static readonly Bracket[] Brackets = new[] {
        new Bracket(new[] { '(' }, new[] { ')' }),
        new Bracket(new[] { '[' }, new[] { ']' }),
        new Bracket(new[] { '{' }, new[] { '}' }),
    }

    private static readonly Dictionary<char, Bracket> BracketsByOpenChar = (from bracket in Brackets from openChar in bracket.openChars select new KeyValuePair<char, Bracket>(openChar, bracket)).ToDictionary();

    private static readonly HashSet<char> CloseChars = new HashSet<char>(from bracket in Brackets from closeChar in bracket.CloseChars select closeChar);

    public static void CheckString(string str) {
        Stack<Bracket> currentState = new Stack<Bracket>();
        for(int i=0; i<str.length; i++) {
            if(BracketsByOpenChar.ContainsKey(str[i])) {
                currentState.Push(BracketsByOpenChar[str[i]]);
            } else if(CloseChars.Contains(str[i])) {
                if(currentState.IsEmpty) {
                    throw new ExtraClosingBracketException();
                }
                if(!currentState.Peek().closeChars.Contains(str[i])) {
                    throw new BracketsMismatchException();
                }
                currentState.Pop();
            }
        }
    }

}