message を汚染せず匿名関数でやりくりするタイプ

JavaScript

addEventListener('ExeLoaded',e=>{

	var url = `https://script.google.com/macros/s/SampleDeployId/exec`;
	var text = `SampleText: Fixed newly exposed boat not showing up in the boat yard menu 
	Changed contraptions to drop resources and inventory objects (crates) to drop resources even 
	when killed with fire. Previously anything killed by fire would drop nothing.`

	chrome.webview.hostObjects.exe.PostError(url,text).then(r=>{
		console.log(r)// null、async失敗
	})
	_exe.post(url,text).then(r=>{
	 	console.log(r)// async成功
	})
})

var _exe = new class {
	constructor(){
		addEventListener('HostObjectAdded',e=>{
			this.Promise.data = {}
			this.Promise.id = 0
			e.value.methods.forEach(s=>this.addMethod(e.value.name,...s.split(',')))
			dispatchEvent(new Event('ExeLoaded'))
		})
	}
	addMethod(h,m,attr){
		var s = m[0].toLowerCase() + m.slice(1);
		if(this[s]) return
		if(attr=='async'){
			this[s] = (...a)=>new this.Promise.Script(h,m,...a)
		}else{
			this[s] = (...a)=>new this.Promise.Json(h,m,...a)
		}
	}
}
_exe.Promise = class{
	constructor(){this.hook = ()=>{}}
	then(resolve){
		this.hook = resolve
		return this
	}
}
_exe.Promise.Script = class extends _exe.Promise{
	constructor(h,s,...a){super()
		var id = ++_exe.Promise.id
		_exe.Promise.data[id] = this
		chrome.webview.hostObjects[h][s](`
			(function(v,i){
				v[i].hook(/*value*/)
				delete v[i]
			})(_exe.Promise.data,${id})
		`,...a)
	}
}
_exe.Promise.Json = class extends _exe.Promise{
	constructor(h,s,...a){super()
		chrome.webview.hostObjects[h][s](...a).then(r=>{
			try{this.hook(JSON.parse(r))}
			catch(e){this.hook(r)}
		})
	}
}

HostObject.cs

public class HostObject {
	protected CoreWebView2 core = null!;
	public HostObject(WebView2 view) {
		view.NavigationCompleted+=(s,e) => {
			core = view.CoreWebView2;
			core.AddHostObjectToScript("exe",this);
			IIFE(@$"
				var e = new Event('HostObjectAdded')
				e.value = {{
					name:'exe',
					methods:{JsonSerializer.Serialize(GetMethods())},
				}}
				dispatchEvent(e)
			");
		};
	}
	
	//nullが返る
	public async Task<string> PostError(string url,string data) {
		var content = new StringContent(data,Encoding.UTF8);
		using(var client = new HttpClient()) {
			var response = await client.PostAsync(url,content);
			var s = await response.Content.ReadAsStringAsync();
			return s;
		}
	}
	//手動でResolve 
	public async void Post(string script,string url,string data) {
		var content = new StringContent(data, Encoding.UTF8);
		using (var client = new HttpClient()){
		    var response = await client.PostAsync(url, content);
			var s = await response.Content.ReadAsStringAsync();
			Resolve(script,s);
		}
	}

	protected async void Resolve(string script,object v) {
		var s = script.Replace("/*value*/",JsonSerializer.Serialize(v));
		var r = await core.ExecuteScriptAsync(s);
	}
	protected async void IIFE(string script) {//即時実行関数
		var r = await core.ExecuteScriptAsync($"(function(){{{script}}})()");
	}
	public string[] GetMethods(){
		var objectMethods = typeof(Object).GetMethods();
		var methods = new List<string>();
		Type async = typeof(AsyncStateMachineAttribute);
		foreach(MethodInfo m in GetType().GetMethods()) {
			if(!Array.Exists(objectMethods,v => { return v.Name==m.Name; })){
				if(m.GetCustomAttribute(async)==null) {
					methods.Add(m.Name);
				} else {
					methods.Add($"{m.Name},async");
				}
			}
		}
		return methods.ToArray();
	}
}