How do I return a value from a C# function using jint?

Question

I'm currently using Jint (https://github.com/sebastienros/jint) to process JavaScript.

I would like to be able to use a custom function in JavaScript that will execute a function created in C# and then return a value to the JavaScript.

For example if the JavaScript was:

var x = MultiplyByTwo(100);

And in my C# I had:

private static int MultiplyByTwo(int obj)
{
    return obj * 2;
}

Then the above would give the variable x the value of 200.

According to the Documentation there is the following option:

var engine = new Engine()
.SetValue("log", new Action<object>(Console.WriteLine));

engine.Execute(@"
    function hello() { 
        log('Hello World');
    };
 
    hello();
");

I can replace the "Console.Writeline" with a function name but it will only accept a void function. In other words, this can call a C# function from JS but not return a value.

Does an option like what I am looking for exist in Jint?

`Action` only accepts void functions, the see [Func](https://learn.microsoft.com/en-us/dotnet/api/system.func-2?view=net-6.0) if you want a return value. However, the linked page has a complete section about "Accessing .NET assemblies and classes", have you tried that? — JonasH, Mar 15 '22 at 15:49
You would have to be looking for maybe an overload of `SetValue` or maybe a different method that takes a `Func<>` or similar. In fact, I'd just boldly try to pass a Func and see what happens. — Fildor, Mar 15 '22 at 15:50

score 3 · Accepted Answer · answered Mar 15 '22 at 20:41

Here is a fully functional example. Casting Console.WriteLine is required since there are multiple overloads and we need to tell the compiler which one to use:

using System;
using Jint;

var engine = new Engine()
    .SetValue("multiplyByTwo", MultiplyByTwo)
    .SetValue("log", (Action<string>)Console.WriteLine)
    ;
    
engine.Execute(@"
    var x = multiplyByTwo(3);
    log(x);
");

static int MultiplyByTwo(int obj)
{
    return obj * 2;
}

David784 · Answer 2 · 2022-03-15T16:08:38.500

You can certainly reference a class, which can involve setting properties, and methods can return values. For example:

class Program
{
    static void Main(string[] args)
    {
        var myobj = new MyClass();
        var engine = new Engine()
            .SetValue("log", new Action<object>(Console.WriteLine))
            .SetValue("myobj", myobj);

        engine.Execute(@"
        function hello() { 
            myobj.One = 'asdf';
            var x = myobj.TwoTimes(4);
            log(x); // logs 8
        };
    
        hello();
    ");
        Console.WriteLine($"{myobj.One}, {myobj.Two}");
        // output => "asdf 8"
    }

}

public class MyClass
{
    public string One { get; set; }
    public int Two { get; set; }
    public int TwoTimes(int value)
    {
        Two = 2 * value;
        return Two;
    }
}

score 1 · Answer 3 · answered Mar 15 '22 at 20:22

1

As mentioned in some comments, use

SetValue("multiplyByTwo", new Func<int>(MultiplyByTwo))

answered Mar 15 '22 at 20:22

Sébastien Ros - MSFT

5,091
29
31

1

Or even just `.SetValue("MultiplyByTwo", MultiplyByTwo);` - since (like in most other cases), the C# compiler will infer the type of the method and add the creation of the required delegate implicitly. – JimmiTh Mar 15 '22 at 20:29

How do I return a value from a C# function using jint?

3 Answers3