Assert boolean conditions saved in a string

Question

I have a string with conditions like this:

string conditions = "true && true || false || true ";

How do I assert that the entire string is either true or false?

For example, something like this:

if( Assert(conditions) == true ){
    // passed
}

I need something that is readily available in C# without resorting to a 3rd party library.

He's interested in parsing a *string* representation of a boolean statement. That link won't do that. — Eric J., Feb 13 '16 at 00:11
Indeed, I have a string representation of boolean statements. — Bill Software Engineer, Feb 13 '16 at 00:12
I think it is duplicated: http://stackoverflow.com/questions/8476422/logic-evaluator-in-c-sharp-evaluate-logical-expressions — RafaelCaballero, Feb 13 '16 at 00:16
The grammar as exemplified in your question seems simplistic and not very computationally-powerful. Do you have any operator-precedence rules? What about bracketed sub-expressions? — Dai, Feb 13 '16 at 00:20

score 1 · Answer 1 · answered Feb 13 '16 at 00:19

1

Parse the string with a trivial tokeniser unless you know you'll have bracketized operations (in which case use a finite state automaton parser) - this will convert the string into an object representation, then use the shunting-yard algorithm to evaluate the string.

The grammar as exemplified in your question seems simplistic, and adopting equal-precedence operators without brackets will severely restrict the kinds of expressions possible.

answered Feb 13 '16 at 00:19

Dai

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I think System.Data.DataTable can do the trick without writing a parser – RafaelCaballero Feb 13 '16 at 00:22
I could indeed write a parser, but is there something readily available? – Bill Software Engineer Feb 13 '16 at 00:31

score 1 · Accepted Answer · edited May 23 '17 at 12:15

This is a static method that will do what you want, but it's SLOOOW!!

public static object Eval(string booleanExpression) {

  var c = new Microsoft.CSharp.CSharpCodeProvider();
  var icc = c.CreateCompiler();
  var cp = new System.CodeDom.Compiler.CompilerParameters();

  cp.CompilerOptions = "/t:library";
  cp.GenerateInMemory = true;

  StringBuilder sb = new StringBuilder("");
  sb.Append("using System;\n" );

  sb.Append("namespace BooleanEvaluator{ \n");
  sb.Append("public class BooleanEvaluator{ \n");
  sb.Append("public bool Evaluate(){\n");
  sb.Append("return "+booleanExpression+"; \n");
  sb.Append("} \n");
  sb.Append("} \n");
  sb.Append("}\n");

  var cr = icc.CompileAssemblyFromSource(cp, sb.ToString());

  System.Reflection.Assembly a = cr.CompiledAssembly;
  object o = a.CreateInstance("BooleanEvaluator.BooleanEvaluator");

  Type t = o.GetType();
  MethodInfo mi = t.GetMethod("Evaluate");

  return (bool) mi.Invoke(o, null);
}

However, the tokenizer and parser that Dai suggests can be easily generated with ANTLR and a boolean expression grammar. That will be much faster! See: ANTLR v3 grammar for boolean/conditional expression

Excellent, this will do. I can't add any 3rd party software, and I don't expect the expression to be longer than maybe maxim of 10 statements, so it's fine. Thanks a lot. — Bill Software Engineer, Feb 16 '16 at 17:57

Assert boolean conditions saved in a string

2 Answers2