TheAlgorithms/C-Sharp · error · InvalidOperationException

Mismatched parentheses.

Error message

Mismatched parentheses.

What it means

EmptyRemainingStack throws InvalidOperationException when leftover items on the operator stack include a parenthesis, meaning a '(' was never closed. The infix expression has an unmatched opening parenthesis.

Solutions

1. Validate bracket balance before conversion (e.g. BalancedParenthesesChecker.IsBalanced).
2. Ensure every '(' has a matching ')' in the input string.
3. Catch InvalidOperationException around the conversion call and treat as input validation failure.

Example fix

// before
string postfix = converter.InfixToPostfixConversion("(a+b");
// after
string postfix = converter.InfixToPostfixConversion("(a+b)");

if (expr.Count(c => c == '(') != expr.Count(c => c == ')')) throw new ArgumentException("Unbalanced parentheses.");

try { var postfix = converter.InfixToPostfixConversion(expr); } catch (InvalidOperationException) { /* unmatched '(' left on stack */ }

When it happens

Trigger: Calling InfixToPostfixConversion with more '(' than ')', e.g. InfixToPostfixConversion("(a+b") — the trailing '(' remains when draining the stack.

Common situations: Expressions assembled programmatically where a branch forgot to emit the closing bracket; partial user input.

Understand the failure class

Background: "Invalid ... format", "must be in format X", "does not look like a ..." — invalid argument format errors across CLI tools and libraries — this error's family across 17 libraries.

Related errors

• Mismatched parentheses in expression.
• Invalid character .
• Invalid character ' ' found in the expression.
• Insufficient operands
• Stack is empty

AI-assisted analysis of TheAlgorithms/C-Sharp@96e2905cab (2026-09-13). Data as JSON: /api/errors/69b80bf4710b275f. Report an issue: GitHub.

Appendix: source

Thrown at Algorithms/Stack/InfixToPostfix.cs:145

        }

        private static void ProcessOperator(char c, Stack<char> stack, StringBuilder postfixExpression)
        {
            while (stack.Count > 0 && stack.Peek() != '(' && Precedence(stack.Peek()) >= Precedence(c))
            {
                postfixExpression.Append(stack.Pop());
            }

            stack.Push(c);
        }

        private static void EmptyRemainingStack(Stack<char> stack, StringBuilder postfix)
        {
            while (stack.Count > 0)
            {
                if (stack.Peek() is '(' or ')')
                {
                    throw new InvalidOperationException("Mismatched parentheses.");
                }

                postfix.Append(stack.Pop());
            }
        }

        private static void EvaluateOperator(Stack<int> stack, char op)
        {
            if (stack.Count < 2)
            {
                throw new InvalidOperationException("Insufficient operands");
            }

            int b = stack.Pop();
            int a = stack.Pop();

            if (op == '/' && b == 0)
            {

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