How do you set a .NET enum from JavaScript?

[JohnLudlow]

How can I set a .NET enum from JavaScript. The common use case is an object with a property of an enum type or method which takes an enum, but here is a simplified (slightly contrived) use case: setting an enum variable.

enum Status
{
     Succeeded,
     Failed,
     InProgress,
 }

using (var host = new V8ScriptEngine(V8ScriptEngineFlags.EnableDebugging, 9002))
{
   host.AddHostType(typeof (Status));
   host.Script.status = Status.Succeeded;
   var a = (host.Evaluate("status"));
   var b = (host.Evaluate("status = Status.Failed"));
}

In this case, b and host.Script.status are undefined. How can we create the correct value from the enum here?

[ClearScriptLib]

Hi @JohnLudlow,

The issue here is that Status isn't a public type. ClearScript strictly enforces member visibility even for explicitly exposed types.

There are several ways to make the Status fields visible to script code. For example, you could make Status a public type. Another possibility is to expose it with the private access option:

host.AddHostType(HostItemFlags.PrivateAccess, typeof(Status));

See ScriptEngine.AccessContext for another alternative.

Good luck!

[JohnLudlow]

Dang how did I miss that?

Thanks

[ClearScriptLib]

Please reopen this issue if you have additional questions or concerns about this topic. Thank you!

[JohnLudlow]

Will do, but I think that answers it in terms of enums.

Thanks

[Brain2000]

I noticed that I wasn't able to find a function to set an enum from a plain number being passed in, which surprised me. Byte, int32, int64, nullable, string, ScriptObject... so much just works! Since an enum is backed by an int32 by default, I thought it would have been simple to use just a number.

[ClearScriptLib]

Hi @Brain2000,

Since an enum is backed by an int32 by default, I thought it would have been simple to use just a number.

The behavior here is defined by C#, which doesn't implicitly convert integers to enums. Consider this simple example:

public enum Kingdom { Plant, Animal };
public class Being { public Kingdom Kingdom; }

And then:

engine.AddHostType(typeof(Kingdom));
engine.Script.me = new Being();

To assign an integer value to me.Kingdom, you have to perform an explicit cast, just like in C#. ClearScript provides HostFunctions.cast for this purpose. To summarize:

engine.Script.host = new HostFunctions();
engine.Execute(@"
    me.Kingdom = Kingdom.Animal;         // OK
    me.Kingdom = host.cast(Kingdom, 1);  // OK; explicit conversion
    me.Kingdom = 0;                      // OK; SPECIAL CASE: zero is implicitly convertible to all enums
    me.Kingdom = 1;                      // ERROR
");

Good luck!

[EtienneLaneville]

Is there a way to get the integer value of an enum on the script side? I can get the name using .ToString but I am looking for something like .ToInt32. I tried using HostFunctions's toInt32 method but I get a 'Type mismatch' exception. Since the enum is converted to an object, this makes sense but I am hoping there is a way to somehow get the integer value.

Ideally the Is keyword would be supported in VBScript Case statements (Case Is Kingdom.Plant for example) and there would be no need to convert the enum to string or integer.

[ClearScriptLib]

Hello @EtienneLaneville,

Is there a way to get the integer value of an enum on the script side?

Sure. You can use HostFunctions.cast:

engine.AddHostObject("host", new HostFunctions());
engine.AddHostType(typeof(Int32));
engine.Execute("MsgBox host.cast(Int32, Kingdom.Animal)");

Another possibility is expose a managed function that performs the conversion:

engine.Script.EnumToInt = new Func<Enum, int>(Convert.ToInt32);
engine.Execute("MsgBox EnumToInt(Kingdom.Animal)");

I tried using HostFunctions's toInt32 method but I get a 'Type mismatch' exception.

Right. Those functions are designed for a different purpose – namely, to allow script code to specify the exact numeric type of a host method argument in order to avoid ambiguity in specific scenarios.

Ideally the Is operator would be supported in VBScript Case statements

Unfortunately, VBScript's Select Case statement only works with numbers and strings. However, it should work in conjunction with one of the suggestions above.

Cheers!

[EtienneLaneville]

EnumToInt function works great and you don't even need to add the enum as a host type if you are just converting an object's property:

engine.Execute("MsgBox EnumToInt(me.Kingdom)");

Instead of using engine.Script.EnumToInt = new Func<Enum, int>(Convert.ToInt32);, is there a way to add a host function like one would add an object? If I have a function delegate, is there something like engine.AddFunction("EnumToInt", delegate);. I couldn't find anything like this in the API documentation.

[ClearScriptLib]

Hi @EtienneLaneville,

Instead of using engine.Script.EnumToInt = new Func<Enum, int>(Convert.ToInt32);, is there a way to add a host function like one would add an object?

A delegate is an object, so AddHostObject is the right call:

engine.AddHostType(typeof(Kingdom));
engine.AddHostType(typeof(Console));
engine.AddHostObject("EnumToInt", new Func<Enum, int>(Convert.ToInt32));
engine.Execute("Console.WriteLine(EnumToInt(Kingdom.Animal))");

The following all do pretty much the same thing:

engine.AddHostObject("foo", obj);
engine.Script.foo = obj;
engine.Script["foo"] = obj;

AddHostObject is special in that it creates immutable items and supports HostItemFlags.

Good luck!