// C# Eval Expression

// Doc: https://eval-expression.net/register-unregister

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// @nuget: Z.Expressions.Eval

​

/*

## RegisterGlobalConstant

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The RegisterGlobalConstant method registers a global constant value. The constant value is shared across all evaluations but cannot be modified (not writeable).

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In this example, we will first register the constant 'x' with the value equal to 1. First, we will execute a simple addition expression that will work. Then, we will try to assign a new value to our constant 'x', which will fail since a constant is not writeable. Finally, we will use the `IsRegisteredGlobalConstant` and `UnregisterGlobalConstant` methods to show the constant was registered and successfully unregistered.

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> NOTE: A global constant has the same behavior as creating a constant in the code, such as `public const int x = 1;`. You can use the constant value, but you cannot change the value. However, like a `const` you can still add/remove items from a list or change the property values of an entity but not re-assign a new entity itself.

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See more: https://eval-expression.net/register-unregister

*/

​

using System;

using Z.Expressions;

​

public class Program

{

    public static void Main()

    {

        // Global Context: EvalManager.DefaultContext.RegisterGlobalConstant("x", 1)

        var context = new EvalContext();

        context.RegisterGlobalConstant("x", 1);

        var r1 = context.Execute<int>("x + y", new { y = 2 });

        Console.WriteLine("1 - Result: " + r1);

        try

        {

            // Try to change the constant value

            var fail = context.Execute("x = 2; return x;");

        }

        catch(Exception ex)

        {

            Console.WriteLine("2 - Exception: " + ex.Message);

        }

        // Check if the constant "x" is registered

        var r3 = context.IsRegisteredGlobalConstant("x");

        Console.WriteLine("3 - Result: " + r3);

        // Unregister the constant "x"

        context.UnregisterGlobalConstant("x");

        // Check if the constant "x" has been succesfully unregistered

        var r4 = context.IsRegisteredGlobalConstant("x");

        Console.WriteLine("4 - Result: " + r4);

    }

}