Evaluating Expression at Runtime

Question

I have a C# Console Application project.

I have a logical expression that is stored in database as nvarchar.

For example, the stored expression is: ((34 > 0) || (US == ES)) && (4312 = 5691)

While my application running, I want to retrieve the expression and evaluate it so that the result will be true or false.

How can I do it at runtime?

Answer 1

Here's a rather unusual solution, involving JScript:

• Create a JScript class with the following code:

  public class JsMath {
      public static function Eval(expression:String) : Object {
          return eval(expression);
      }
  }
  

• Compile it into a DLL:

  jsc /target:library /out:JsMath.dll JsMath.js
  

• In your C# project, reference JsMath.dll and Microsoft.JScript.dll

• Now you can use the Eval method as follows:

  string expression = "((34 > 0) || ('US' == 'ES')) && (4312 == 5691)";
  bool result = (bool)JsMath.Eval(expression);
  

Benefits:

• no work required to parse the expression, the JScript engine does it for you
• no need to compile arbitrary code (which can be a big security hole if the code is entered by the user)
• should work with any simple mathematical or logical expression, as long as it follows the JScript syntax

Drawbacks:

• no way to pass variables (as far as I know)
• requires a reference to the JScript assembly (not a big issue in most cases, but I'm not sure this assembly is available in the Client Profile or in Silverlight)