C# Eval Expression My First Option

In this article, we will only see a few options, but you can find all the options here. Understanding how register methods work will also help you understand better.

We will mix how options are added by either the default context, a local context, or a new static context. All three techniques are valid, and which one is recommended depends on what you want to achieve.

Use Option for Safe Mode

The Safe Mode ensures that only members and types registered can be used for the expression.

To use safe mode, we recommend:

  • A local or static context
  • Unregistering all types first
  • Limiting the maximum number of loops with the MaxLoopIteration option
  • Setting the SafeMode option to true
  • Registering only what you want the user to be able to use

For example, we will demonstrate how easy it is to configure options in the C# Eval Expression library:

var evalContext = new EvalContext();
evalContext.MaxLoopIteration = 5;
evalContext.SafeMode = true;

var r1 = evalContext.Execute("Math.Min(1, 2)");
Console.WriteLine("1 - Result: " + r1);

	// `List` is no longer registered, so it cannot be used
	var r2 = evalContext.Execute("new List<int>()");
catch(Exception ex)
	Console.WriteLine("Cannot use `List` exception: " + ex.Message);

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Use Option to Add Keyword

In the C# Eval Expression library, you can add your own keyword. You cannot override existing keywords, but you can create new ones, such as isin, that will check if the current item is in the list.

In this example, we will create an extension method named IsIn that checks if an item is part of a list. Then, we will create the keyword isin that will be bound to this extension method and can be used directly in the expression:

using System;
using System.Collections.Generic;
using Z.Expressions;

public class Program
	public static void Main()
		EvalManager.DefaultContext.RegisterKeyword("isin", "IsIn");
		var list = new List<int>() { 1, 2, 3 };
		Console.WriteLine(Eval.Execute("2 isin list", new { list }));
		Console.WriteLine(Eval.Execute("4 isin list", new { list }));

public static class CustomExtensions
	public static bool IsIn<T>(this T x, List<T> list)
		return list.Contains(x);

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Here are some suggestions for improving the grammar and simplifying the vocabulary in your article:

Use Option to Add Method

In the C# Eval Expression library, it's possible to add your own method. If you can create the method in a non-dynamic way, you should always prefer this method and register your method over creating a method dynamically.

In this example, we will create a method dynamically that is supposed to compare three numbers and use the formula from user input to determine if they are equal or not.

string userInputFormula = "A == B || A == -1 || B == -1";

var evalContext = new EvalContext();
evalContext.UseCache = false;		

evalContext.AddMethod($$"""public bool IsEqualsCustom(int A, int B) { return {{userInputFormula}}; }""");

Console.WriteLine(evalContext.Execute<bool>("IsEqualsCustom(1, 1)")); 
Console.WriteLine(evalContext.Execute<bool>("IsEqualsCustom(1, 2)")); 
Console.WriteLine(evalContext.Execute<bool>("IsEqualsCustom(1, -1)"));

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Adding options in the C# Eval Library is easy, but mastering all the available options will surely take more time. In most cases, you only need to register types, but people with advanced scenarios might need to better understand those advanced options.

Last updated: 2024-03-22