Dynamic variable serialization

[BlackGad]

Is it possible to extract dynamic value (V8ScriptItem), serialize it and later insert as host item into another V8Engine instance (right way)?

I can explore all properties via Engine.Script property, can enumerate whole structure of dynamic object. Even determine array types via

engine.Script.Array.isArray(value);

(Hope there is better way to do this)

But I cannot create object instance of the same V8ScriptItem type in another engine programmatically from host side. (with out reflection)

[ClearScriptLib]

Hi @BlackGad,

If the object is simple data, you can use JSON:

using (var engine = new V8ScriptEngine())
using (var engine2 = new V8ScriptEngine()) {
    engine.Execute("foo = { bar: 123, baz: 'qux' }");
    var fooJson = engine.Script.JSON.stringify(engine.Script.foo);
    engine2.Script.foo = engine2.Script.JSON.parse(fooJson);
}

For more complicated JavaScript scenarios, something like HydrateJS might work for you.

If your object graph contains references to host objects or types, some form of custom serialization is unavoidable, although JSON.stringify/parse do provide some hooks that might be useful.

Good luck!

[BlackGad]

My data is not simple :) I figured out how to process my case but found 5 different issues.

Here is example: https://github.com/BlackGad/ClearScriptIssue

Issue 1: Invalid dynamic object constructor parsing on Evaluate

engine.Evaluate("{a: 'b'}")

Issue 2: Invalid not typed array constructor parsing on Evaluate

engine.Evaluate("['a','b']")

Issue 3: Strange behavior when 'let' was used between 2 Evaluate calls.

engine.Evaluate("let test = ['a','b']; test;")
Console.WriteLine($"Test variable after first evaluate run: {engine.Script.test}"); //variable is undefined
engine.Evaluate("let test = ['a','b']; test;") //this will throw exception 'test object is already defined'

Issue 4: AddHostObject will throw 'Invalid host item' if variable was constructed by this engine. Interesting that same variable constructed by another engine will be hosted successfully

var s = engine.Evaluate("let test = ['a','b']; test;")
engine.AddHostObject("test2", s); //will throw exception
engine2.AddHostObject("test2", s); //will not throw exception

Exception throwed here
File: ClearScript\V8\V8ScriptEngine.cs
Line: 852
In example I bypass check

if (!(marshaledItem is HostItem))

And everything works as expected (by me at least)

Issue 5: not typed array information missed between 2 engines calls

var s = engine.Evaluate("let test = ['a','b']; test;")
var isArray = engine.Script.Array.isArray(s); //Is true
engine2.AddHostObject("test2", s);
var isArray = engine2.Script.Array.isArray(engine2.Script.test2); //Is false

[BlackGad]

Could you please comment post above? Maybe I just do something in wrong way.

[ClearScriptLib]

Hi @BlackGad,

Issue 1

The JavaScript syntax { a: 'b' } is ambiguous. You're expecting it to be interpreted as an object literal, but the parser sees it as a block containing a labeled expression. Try enclosing it in parentheses.

Issue 2

We're not seeing any incorrect behavior. In your test program, both

var simpleNotTypedArray = engine.Evaluate("['a','b']");

and

var simpleNotTypedArray = engine.Evaluate("let test2 = ['a','b']; test2;");

produce the same result – a JavaScript array with two elements.

Issue 3

Using let with the same variable name more than once in the same scope is a syntax error.

Issue 4

AddHostObject doesn't work with fundamental values such as numbers as strings. See Issue #90 for an explanation.

Normally it doesn't work with script objects for the same reason; a script object, when handed back to its engine of origin, is stripped of its .NET proxy and therefore can't support some features of AddHostObject.

Instead of using AddHostObject you can simply do this:

engine.Script.test = deserialized;

or this:

engine.Script["test"] = deserialized;

As you've noticed, AddHostObject can successfully expose a foreign script object, and that's because in that case the managed proxy is retained and all access to the object continues to go through ClearScript.

Note that exposing script objects across engine instances – via AddHostObject or the dynamic Script property – is supported but not recommended. Accessing a foreign script object is extremely expensive and can lead to deadlocks unless very carefully managed.

Issue 5

As is the case with host objects, the engine doesn't have direct access to a foreign script object. Instead, it has a proxy that routes access through ClearScript. That proxy is an instance of a special ClearScript-defined JavaScript prototype, and the actual type information of its external target is inaccessible to the local implementation of Array.isArray.

Let us know if you have any additional questions.

Good luck!

[BlackGad]

Issue 1
You are right, missed code blocks %) replaced with

dynamic result = engine.Evaluate($"new Object()");

Issue 2
Same, I made a mistake somewhere, sorry :)

Issue 3
Please clarify my understanding
ClearScript scope structure looks like this:

outer scope <- engine.Script bridge here
{
processing scope <- Evaluate executing here asynchronously
}

That's why let variable already defined on second call via Evaluate and not accessible via engine.Script bridge

Issue 4 and Issue 5 Thank you for detailed explanation!

[ClearScriptLib]

Hi @BlackGad,

Regarding Issue 3:

1. Script is a managed reference to the global object.

2. Evaluate and Execute run code within the global scope, where this refers to the global object.

3. In the global scope, let declares a global variable, but it does not add a property to the global object (see here). That's why you get the syntax error even though the variable isn't accessible via Script.

Cheers!

[BlackGad]

Thank you!

[hyzx86]

In the latest version (7.3.7), does the unit test work?

https://github.com/microsoft/ClearScript/blob/5d3484292270867ab70ae5191fb4ba3ea9023da7/ClearScriptTest/BugFixTest.cs#L1319-L1324

Not working in my asp.net core project

var engine = new V8ScriptEngine();
var obj = engine.Evaluate("({foo:123,bar:'baz'})");
Console.WriteLine(JsonConvert.SerializeObject(obj));

error:

Microsoft.CSharp.RuntimeBinder.RuntimeBinderException:“'Microsoft.ClearScript.ScriptObject' does not contain a definition for 'foo'”

[ClearScriptLib]

Hello @hyzx86,

Yes, the test and your sample code still work. You can see it in action here.

Any idea what you might be doing differently?

[hyzx86]

Hi @ClearScriptLib ,
Thanks for your confirm

Notice that your sample is used in the Microsoft. ClearScript.Linux-x64

But I'm referring to these two packages

		<PackageManagement Include="Microsoft.ClearScript.V8.Native.linux-x64" Version="$(ClearScriptVersion)" />
		<PackageManagement Include="Microsoft.ClearScript.V8.Native.win-x64" Version="$(ClearScriptVersion)" />

I am using ClearScript in Asp.net core project
If I want to be compatible with windows and linux servers.
Which packages should I refer to?

thanks

[hyzx86]

Updated to this, trying

<PackageReference Include="Microsoft.ClearScript.V8"/>
<PackageReference Include="Microsoft.ClearScript.Core"  />
<PackageReference Include="Microsoft.ClearScript.V8.ICUData" />

[hyzx86]

Now the error message is :
Cannot load ClearScript V8 library. Load failure information for ClearScriptV8.win-x64.dll