{"record":{"id":"bba7d7648dc999f8","repo":"TheAlgorithms/C-Sharp","slug":"invalid-character-in-expression-ch","errorCode":null,"errorMessage":"Invalid character in expression: {ch}","messagePattern":"Invalid character in expression: (.+?)","errorType":"exception","errorClass":"InvalidOperationException","httpStatus":null,"severity":"error","filePath":"Algorithms/Stack/InfixToPostfix.cs","lineNumber":80,"sourceCode":"            {\n                if(char.IsWhiteSpace(ch))\n                {\n                    continue;\n                }\n\n                if(char.IsDigit(ch))\n                {\n                    stack.Push(ch - '0');\n                    continue;\n                }\n\n                if (IsOperator(ch))\n                {\n                    EvaluateOperator(stack, ch);\n                    continue;\n                }\n\n                throw new InvalidOperationException($\"Invalid character in expression: {ch}\");\n            }\n\n            if (stack.Count != 1)\n            {\n                throw new InvalidOperationException(\"Invalid postfix expression: Leftover operands.\");\n            }\n\n            return stack.Pop();\n        }\n\n        private static void ProcessInfixCharacter(char c, Stack<char> stack, StringBuilder postfixExpression)\n        {\n            if (IsOperand(c))\n            {\n                postfixExpression.Append(c);\n                return;\n            }\n","sourceCodeStart":62,"sourceCodeEnd":98,"githubUrl":"https://github.com/TheAlgorithms/C-Sharp/blob/96e2905cab7bc6b33ac0a34ee5bb82ddccbcbb6c/Algorithms/Stack/InfixToPostfix.cs#L62-L98","documentation":"PostfixExpressionEvaluation throws InvalidOperationException when a character in the postfix string is neither an operand (digit/letter) nor a known operator, so the evaluator cannot classify it. This signals a malformed or non-postfix expression.","triggerScenarios":"Evaluating a postfix string containing characters outside the operand/operator set, e.g. PostfixExpressionEvaluation(\"2 3 + #\") or an infix string accidentally passed to the postfix evaluator.","commonSituations":"Feeding infix notation (\"2+3\") to the postfix evaluator; expressions with spaces, commas, or decimal points that are not recognized as operands.","solutions":["Verify the input is valid postfix notation before calling the evaluator (use InfixToPostfixConversion to produce it).","Strip or reject unsupported characters (spaces, commas) before evaluation.","Catch InvalidOperationException and report the offending character position to the user."],"exampleFix":"// before\nint r = evaluator.PostfixExpressionEvaluation(\"2 3 + ,\");\n// after\nstring cleaned = \"2 3 + ,\".Replace(\",\", \"\").Replace(\" \", \"\");\nint r = evaluator.PostfixExpressionEvaluation(cleaned);","handlingStrategy":"validation","validationCode":"if (postfix.Any(c => !char.IsLetterOrDigit(c) && !\"+-*/^\".Contains(c))) throw new ArgumentException(\"Postfix contains non-operand/operator characters.\");","typeGuard":"bool IsValidPostfixAlphabet(string s) => s.All(c => char.IsLetterOrDigit(c) || \"+-*/^\".Contains(c));","tryCatchPattern":"try { int r = evaluator.PostfixExpressionEvaluation(postfix); } catch (InvalidOperationException ex) { /* report ex.Message */ }","preventionTips":["Only evaluate postfix produced by InfixToPostfixConversion","Remove spaces/commas before evaluation","Distinguish infix vs postfix APIs clearly in your code"],"tags":["validation","postfix","evaluation"],"backgroundTag":"invalid-argument-format","analyzedSha":"96e2905cab7bc6b33ac0a34ee5bb82ddccbcbb6c","analyzedAt":"2026-09-13T17:04:01.438Z","contentChangedAt":"2026-09-13T17:04:01.438Z","schemaVersion":2},"datasetVersion":"2026-09-16T04:17:20.429Z"}