TheAlgorithms/C-Sharp · error · InvalidOperationException

Mismatched parentheses in expression.

Error message

Mismatched parentheses in expression.

What it means

ProcessClosingParenthesis throws InvalidOperationException when a ')' is encountered but the operator stack is empty, meaning there is no matching '('. The infix expression has an unbalanced closing parenthesis.

Solutions

1. Balance-check the infix expression (e.g. with BalancedParenthesesChecker.IsBalanced) before conversion.
2. Fix the expression so every ')' has a preceding '('.
3. Catch InvalidOperationException and report mismatched parentheses to the user.

Example fix

// before
string postfix = converter.InfixToPostfixConversion("(a+b))");
// after
string expr = "(a+b))";
if (checker.IsBalanced(expr)) { var postfix = converter.InfixToPostfixConversion(expr); }

int depth = 0; foreach (var c in expr) { if (c == '(') depth++; else if (c == ')' && --depth < 0) throw new ArgumentException("Unbalanced ')'."); } if (depth != 0) throw new ArgumentException("Unbalanced '('.");

try { var postfix = converter.InfixToPostfixConversion(expr); } catch (InvalidOperationException) { /* mismatched parentheses */ }

When it happens

Trigger: Calling InfixToPostfixConversion with more ')' than '(', e.g. InfixToPostfixConversion("a+b)").

Common situations: Manually typed expressions missing an opening bracket; string building code that appends closing parens conditionally.

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.
• 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/b9da9d6d4161853a. Report an issue: GitHub.

Appendix: source

Thrown at Algorithms/Stack/InfixToPostfix.cs:123

            if (c == ')')
            {
                ProcessClosingParenthesis(stack, postfixExpression);
                return;
            }

            ProcessOperator(c, stack, postfixExpression);
        }

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

            if (stack.Count == 0)
            {
                throw new InvalidOperationException("Mismatched parentheses in expression.");
            }

            stack.Pop();
        }

        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)

View on GitHub (pinned to 96e2905cab)