Blazor WASM unmarshalled interop with JSImport

[Hooterr]

Hi all,
I need to use JS interop without JSON overhead (too slow, too much data).
I was following examples in the docs that use IJSUnmarshalledRuntime, but is deprecated now. and the docs say I should use JSImportAttribute. Ok fair enough but I can't find the same example for JSImport as there was for the deprectaed interface. Namely a managed object in .NET (struct) passed to JS via a pointer and then read with Blazor.platform.readStringField using offsets.

Here is what I have

[JSImport("globalThis.test2")]
public static partial void Test2([JSMarshalAs<JSType.Any>] object param);
...
JsUnmarshalled.Test2(new InteropStruct()
{
    Name = "Brigadier Alistair Gordon Lethbridge-Stewart",
    Year = 1968,
});

JS

function test2(flights){
    console.log(flights);
    const name = Blazor.platform.readStringField(flights, 0);
    const year = Blazor.platform.readInt32Field(flights, 8);
    console.log(`${name} (${year})`)
}

But I'm getting null when reading name value. Looks like readStringField expects a pointer as a number but instead generated interop code passes ManagedObject Symbol(wasm js_owned_gc_handle): 118 into test2 function.
Not sure how to get an actual heap pointer from that in JS.

I tried another way

[JSImport("globalThis.test2")]
public static partial void Test([JSMarshalAs<JSType.Number>] int param);

and call it like this:

var a = new InteropStruct()
{
    Name = "Brigadier Alistair Gordon Lethbridge-Stewart",
    Year = 1968,
};
IntPtr tr = (int)&a;

JsUnmarshalled.Test(tr.ToInt32());

That worked. But I'm pretty sure that's not how it's supposed to be used. On the other hand the function I wan to call is supposed to decode fields from my managed .net object, copy then, and create JS objects to be used in JS. The function would be synchronous. If I'm not mistaken I don't really need to worry about GC moving my object somewhere else since GC can only do that after my JS function completes.

I know that's a niche usecase but maybe someone has some ideas on how to do this correctly?

The struct for completness

[StructLayout(LayoutKind.Explicit)]
public struct InteropStruct
{
    [FieldOffset(0)]
    public string Name;

    [FieldOffset(8)]
    public int Year;
}

[maraf]

cc @pavelsavara

[pavelsavara]

At the moment we don't support marshaling structures with [JSImport].

The [JSMarshalAs<JSType.Any>] will marshal it as a proxy JS objects, just a wrapper around GCHandle.
That's why Blazor's deprecated unsafe pointer based platform.readStringField would not work on it.
We deprecated this because it was really unsafe, because GC could move the managed objects in memory.

You said that you have lot of data.
Possible solutions how you can transfer it:

1. marshal each member of the structure as different parameter of JSImport method. Like void Method(string name, int year)
2. if you have array of InteropStruct[] you can transform it into multiple arrays of simple types. And then marshal it as void Method(string[] names, int[] years). This is what we do in the HTTP client implementation, when we pass HTTP headers to JS.
3. you can use JSON serializer yourself and marshal string. This could be slower. This is what Blazor does.
4. convert it to byte[] and pass that. In your case, the field Name makes it difficult because of the dynamic size.
5. pin it in memory and pass IntPtr. This is tricky, because you will need to pin the string too and deal with the details.

Hope this helps.

[Hooterr]

It's a geojson with a lot of data mostly strings and numbers. Serializing it to either JSON or binary is too slow anyway.

I went with option 1&2. It's still miles faster than normal interop which is great.

Thanks for the response!