C# Eval Expression My First Evaluation

C# is a powerful programming language but lacks the eval method that some other languages have, such as JavaScript.

The C# Eval Expression library is the best solution to overcome this limitation and lets you quickly evaluate code at runtime with the minimum code required on your side.

In this tutorial, you will learn how to use our library to:

Evaluate a C# expression

The simplest way to execute a C# expression at runtime is by using the Eval.Execute method. Under the hood, it calls the Execute method from the DefaultContext that you will learn later in this tutorial.

The Execute method takes as the first parameter the code to execute.

In this example, we will dynamically create a list of int and filter it using LINQ to return only items greater than 2.

// NOTE: The returned list contains "3" and "4"
var list = Eval.Execute(@"
var list = new List<int>() { 1, 2, 3, 4 };
return list.Where(x => x > 2).ToList();
");

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Remember that the C# Eval library supports more than this basic statement. However, we will keep it very simple in our getting started section.

Evaluate a C# expression with variables

In the last example, we created the list dynamically in the Execute method and hardcoded the value 2, which we used to filter. However, those values will be from existing variables and user input in most real-life scenarios.

The Execute method takes the variables to use in our expression (or all remaining parameters) as the second parameter.

There are multiple different ways of passing variables, such as:

  • Anonymous Type
  • Class Member
  • Dictionary
  • Expando Object
  • Values

In this example, we will create the list and greaterThan variables and show different ways to pass those variables to our expression.

var list = new List<int>() { 1, 2, 3, 4 };
var greaterThan = 2;

// Passing parameters with an Anonymous Type
{
	var rList = Eval.Execute("list.Where(x => x > greaterThan)", new { list, greaterThan });
}		

// Passing parameters with an Anonymous Type using named members
{
	var rList = Eval.Execute("listName.Where(x => x > greaterThanName)", new { listName = list, greaterThanName = greaterThan });
}		

// Passing parameters with a class instance and using class members
// NOTE: When only using 1 parameter, you can directly use member names, "Item1" and "Item2" in the case of a Tuple<,>
{
	var rList = Eval.Execute("Item1.Where(x => x > Item2)", new Tuple<List<int>, int>(list, greaterThan));
}		

// Passing parameters with a Dictionary and using key names
// NOTE: When only using 1 parameter, you can directly use dictionary key names in the expression
{
	var dictionary = new Dictionary<string, object>();
	dictionary.Add("list", list);
	dictionary.Add("greaterThan", greaterThan);
	var rList = Eval.Execute("list.Where(x => x > greaterThan)", dictionary);
}

// Passing parameters using an Expando Object and using member names
// NOTE: When only using 1 parameter, you can directly use member names of the Expando Object
{
	dynamic expandoObject = new ExpandoObject();
	expandoObject.list = list;
	expandoObject.greaterThan= greaterThan;
	var rList = Eval.Execute("list.Where(x => x > greaterThan)", expandoObject);
}

// Passing parameters directly with the Values
// NOTE: Because parameters are not named, you need to use the position as our library is not aware of the name "list" and "greaterThan"
{
	var rList = Eval.Execute("{0}.Where(x => x > {1})", list, greaterThan);
}

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Evaluate a C# expression with a return type

In the last two examples, we learned how to execute code dynamically and use variables in our expressions.

The last remaining part is how to specify a return type. If no return type is specified, the value is of type object.

The Execute method takes as the generic type the type to return.

In this example, we will continue with our getting started example by using our list and greaterThan variables but this time, specify the List<int> return type.

var list = new List<int>() { 1, 2, 3, 4 };
var greaterThan = 2;

list = Eval.Execute<List<int>>("list.Where(x => x > greaterThan)", new { list, greaterThan });

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Did you spot an error in this example? The expression should return an IEnumerable<int> and not a List<int>! However, our library is smart enough and makes your life easy by automatically calling the ToList method to return the right type. That is one of the many advantages of using our library.

Evaluate a C# expression from a string

If you have taken the time to read and understand the first three sections of this tutorial, the rest should be straightforward to understand.

Our library adds a syntactic sugar execute extension method to extend string variables. Passing parameters and return type are similar to what we have previously seen.

In that example, we will execute an expression directly from a string.

var list = new List<int>() { 1, 2, 3, 4 };
var greaterThan = 2;

var expressionToExecute = "list.Where(x => x > greaterThan)";
list = expressionToExecute.Execute<List<int>>(new { list, greaterThan });

// SIMILAR TO: list = Eval.Execute<List<int>>("list.Where(x => x > greaterThan)", new { list, greaterThan });

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Under the hood, we call the same method as Eval.Execute, but we simply set the string as the first parameter.

Evaluate a C# expression from a context

All the Execute methods we learned in previous examples use the global EvalContext stored in the EvalManager.DefaultContext. In other words:

  • using the Eval.Execute method is similar to doing EvalManager.DefaultContext.Execute.
  • using the "the_string_expression".Execute method is similar to doing EvalManager.DefaultContext.Execute.

So anyway, what exactly is an EvalContext? That is a class you instancies to specify some configuration. You can set configuration globally by:

  • Using EvalManager.DefaultContext context
  • Creating a new context and storing it in a static variable you re-use later
  • Creating a new context for every evaluation, by example inside a method

In this example, we will create an instance context, add a new extension method named GreaterThan and use the Execute method.

var list = new List<int>() { 1, 2, 3, 4 };
var greaterThan = 2;

var context = new EvalContext();

context.AddMethod(@"
bool GreaterThan(this int x, int y)
{
	return x > y;
}
");

list = context.Execute<List<int>>("list.Where(x => x.GreaterThan(greaterThan))", new { list, greaterThan });

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The method GreaterThan only exists for this context. That means the Eval.Execute is unaware and cannot use this method.

Conclusion

In this getting started tutorial, you learned how to use the Execute method to dynamically evaluate some code, use parameters, and a return type.

Using the Execute method is straightforward, and most developers should already be able to use it without a problem. However, only time will allow you to master this method and exploit all the potential through all options of the EvalContext and the possibility that the library offers, so make sure to continue to read other tutorials and articles.


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