Question

I need to parse complex expresion from string to bool.

It can only contain:
* boolean values (true/false),
* parenthesis,
* AND/OR operands (&&, ||)

Eg:

bool.Parse('((true || false) && (false || false)) || (true || false)' 

Any idea how to achieve this?

Answer 1

Here’s a cunning evaluator class that gives you the JScript.NET Eval function within C# code:

static public class Evaluator {     private const string _jscriptSource =         @'package Evaluator         {            class Evaluator            {               public function Eval(expr : String) : String                {                   return eval(expr);                }            }         }';      static private object _evaluator;     static private Type _evaluatorType;      [SuppressMessage('Microsoft.Performance', 'CA1810:InitializeReferenceTypeStaticFieldsInline',         Justification = 'Can't be done inline - too complex')]     static Evaluator()     {         InstantiateInternalEvaluator();     }      static private void InstantiateInternalEvaluator()     {         JScriptCodeProvider compiler = new JScriptCodeProvider();          CompilerParameters parameters;         parameters = new CompilerParameters();         parameters.GenerateInMemory = true;          CompilerResults results;         results = compiler.CompileAssemblyFromSource(parameters, _jscriptSource);          Assembly assembly = results.CompiledAssembly;         _evaluatorType = assembly.GetType('Evaluator.Evaluator');          _evaluator = Activator.CreateInstance(_evaluatorType);     }      static public int EvaluateToInteger(string statement)     {         string s = EvaluateToString(statement);         return int.Parse(s);     }      static public double EvaluateToDouble(string statement)     {         string s = EvaluateToString(statement);         return double.Parse(s);     }      static public decimal ForceEvaluateToDecimal(string statement)     {         decimal result;         bool s = Decimal.TryParse(statement, out result);         return result;     }      static public decimal EvaluateToDecimal(string statement)     {         string s = EvaluateToString(statement);         return decimal.Parse(s);     }      static public string EvaluateToString(string statement)     {         object o = EvaluateToObject(statement);         return o.ToString();     }      static public bool EvaluateToBool(string statement)     {         object o = EvaluateToObject(statement);         return (bool)o;     }      static public object EvaluateToObject(string statement)     {         try         {             return _evaluatorType.InvokeMember(                 'Eval',                 BindingFlags.InvokeMethod,                 null,                 _evaluator,                 new object[] {statement}                 );         }         catch (Exception)         {             InstantiateInternalEvaluator();             return null;         }     } } 

You just then call Evaluator.EvaluateToBool(string). Lifted from an existing project, so you may want to tweak!