ScriptengineException when string methods are called

Question

When I am using the following snippet, I get a ScrptEngineException, that the object does not support the Property or Method.

var engine = new JScriptEngine();
engine.AddHostType("String", typeof(System.String));
engine.ExecuteCommand("test = 'variabel'; test.StartsWith('nice');");

I have tried some other Stringfunctions like IndexOf, ToArray (Extension) and some others, but the do not seem to work.

Can someone help me out?

score 1 · Accepted Answer · answered Jul 04 '18 at 18:23

In your example, test is a JavaScript string, which has no StartsWith method. Even if it had been returned from .NET, it would have been converted to a JavaScript string for convenience.

You can add a method that converts a JavaScript string to a .NET string:

engine.AddHostObject("host", new HostFunctions());
engine.Execute(@"
    String.prototype.toHost = function() {
        return host.newVar(this.valueOf());
    }
");

And then this should work:

engine.AddHostType(typeof(Console));
engine.Execute(@"
    test = 'variable';
    Console.WriteLine(test.toHost().StartsWith('var'));
    Console.WriteLine(test.toHost().StartsWith('vaz'));
");

BTW, be careful with this:

engine.AddHostType("String", typeof(System.String));

That hides the built-in JavaScript String function and is likely to break things.

ScriptengineException when string methods are called

1 Answers1