无法将JSON写入文件（Xamarin C#）

我尝试将JSON写入文件，如下所示：

string url2 = "http://papajohn.pp.ua/?mkapi=getProductsByCat&cat_id=83";
JsonValue json = await FetchAsync(url2);

var path = System.Environment.GetFolderPath(
    System.Environment.SpecialFolder.ApplicationData);
var filename = Path.Combine(path, "cache.txt");

File.WriteAllText(filename, json);

public async Task<JsonValue> FetchAsync(string url)
{
    System.IO.Stream jsonStream;
    JsonValue jsonDoc;

    using (var httpClient = new System.Net.Http.HttpClient())
    {
        jsonStream = await httpClient.GetStreamAsync(url);
        jsonDoc = JsonObject.Load(jsonStream);
    }

    return jsonDoc;
}

---

我的错误在哪里？

从URL获取JsonString的代码片段

public async Task<string> FetchAsync(string url)
    {
        string jsonString;

        using (var httpClient = new System.Net.Http.HttpClient())
        {
            var stream = await httpClient.GetStreamAsync(url);
            StreamReader reader = new StreamReader(stream);
            jsonString = reader.ReadToEnd();
        }

        return jsonString;
    }

稍后从该文件中读取JsonString并将其转换为Json的代码段

JsonValue readJson; 
            var jsonString =File.ReadAllText(filename);
            readJson = JsonObject.Parse(jsonString);

---

如何将此文件中的文本显示到console？console.WriteLine（jsonString）；

string url2 = "http://papajohn.pp.ua/?mkapi=getProductsByCat&cat_id=83";
            var json = await FetchAsync(url2);

            var path = System.Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments);
            var filename = Path.Combine(path, "cache.txt");
            File.WriteAllText(filename,json);

JsonValue readJson; 
            var jsonString =File.ReadAllText(filename);
            readJson = JsonObject.Parse(jsonString);