Dynamic Expresso Customizing Parser

The Dynamic Expresso library allows you to customize the expression parser. You can use different options to customize the parser, such as EnableReflection, EnableAssignment, etc.

Let's consider the following simple example with the default settings.

public static void Example1()
{
    try
    {
        Interpreter interpreter = new Interpreter();

        string expression = "typeof(double).GetMethods()";
        var result = interpreter.Eval(expression);
        Console.WriteLine("{0} = {1}", expression, result);

        expression = "typeof(double).Assembly";
        result = interpreter.Eval(expression);
        Console.WriteLine("{0} = {1}", expression, result);
    }
    catch (ParseException e)
    {

        Console.WriteLine(e.Message);
    }
}

Let's execute the above code, and you will see the following exception.

Reflection expression not allowed. To enable reflection use Interpreter.EnableReflection(). (at index 0).

From version 1.3, some reflection methods are blocked to prevent malicious users to call unexpected types or assemblies within an expression.

You can allow the parser to handle the reflection methods by calling the interpreter.EnableReflection(); method before evaluation.

public static void Example2()
{
    try
    {
        Interpreter interpreter = new Interpreter();
        interpreter.EnableReflection();

        string expression = "typeof(double).GetMethods()";
        var result = interpreter.Eval(expression);
        Console.WriteLine("{0} = {1}", expression, result);

        expression = "typeof(double).Assembly";
        result = interpreter.Eval(expression);
        Console.WriteLine("{0} = {1}", expression, result);
    }
    catch (ParseException e)
    {

        Console.WriteLine(e.Message);
    }
}

Let's execute the above code, and you will see the following output.

typeof(double).GetMethods() = System.Reflection.MethodInfo[]
typeof(double).Assembly = System.Private.CoreLib, Version=5.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e

By default, assignment operators are enabled, to ensure that the user cannot change some values that you don't expect, you can disable assignment operators by using the interpreter.EnableAssignment(AssignmentOperators.None);.

public static void Example3()
{
    try
    {
        Interpreter interpreter = new Interpreter();
        interpreter.EnableAssignment(AssignmentOperators.None);
        int x = 10;
        interpreter.SetVariable("x", x);

        string expression = "x = 13;";
        var result = interpreter.Eval(expression);
        Console.WriteLine("After evaluation, x = {0}", x);
    }
    catch (ParseException e)
    {
        Console.WriteLine(e.Message);
    }
}

Let's execute the above code, and you will see the following output.

Assignment operator '=' not allowed (at index 2).