如何在Xamarin的后台执行函数？

当我的应用程序将打开时，我试图在后台执行一个函数，为此，我试图使用task创建一个异步函数，但我无法做到这一点

我该如何修理它

Xamarin OnStart

protected override void OnStart(){
     DefineConcursoSession();
}

//execute in background
private async void DefineConcursoSession(){
        ConcursoDTO dto = await ConcursoService.GetConcursoAtivo();
        //save session
    }

public async static Task<ConcursoDTO> GetConcursoAtivo(){
        String ENDPOINT = "/GetConcursoAtivo";
        String URL_FINAL = URL_CONTROLLER + ENDPOINT;

        ConcursoDTO dto = new ConcursoDTO();

        try
        {
            HttpClient hClient = new HttpClient();
            hClient.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
            hClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Basic", BasicAuth.getHash());

            //get
            HttpResponseMessage response = hClient.GetAsync(URL_FINAL).GetAwaiter().GetResult();
            var resultContent = response.Content.ReadAsStringAsync().GetAwaiter().GetResult();
            //Console.WriteLine("Content: " + resultContent);


            if (response.StatusCode == HttpStatusCode.OK){                
                JObject jo = JObject.Parse(resultContent);
                dto = JsonConvert.DeserializeObject<ConcursoDTO>(jo["Concurso"].ToString());                
                
            }
        }
        catch (Exception e)
        {
            Debug.WriteLine("Erro: GetConcursoAtivo:-> " + e.Message);
        }

        return dto;

    }

HttpClient

protected override void OnStart(){
     DefineConcursoSession();
}

//execute in background
private async void DefineConcursoSession(){
        ConcursoDTO dto = await ConcursoService.GetConcursoAtivo();
        //save session
    }

public async static Task<ConcursoDTO> GetConcursoAtivo(){
        String ENDPOINT = "/GetConcursoAtivo";
        String URL_FINAL = URL_CONTROLLER + ENDPOINT;

        ConcursoDTO dto = new ConcursoDTO();

        try
        {
            HttpClient hClient = new HttpClient();
            hClient.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
            hClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Basic", BasicAuth.getHash());

            //get
            HttpResponseMessage response = hClient.GetAsync(URL_FINAL).GetAwaiter().GetResult();
            var resultContent = response.Content.ReadAsStringAsync().GetAwaiter().GetResult();
            //Console.WriteLine("Content: " + resultContent);


            if (response.StatusCode == HttpStatusCode.OK){                
                JObject jo = JObject.Parse(resultContent);
                dto = JsonConvert.DeserializeObject<ConcursoDTO>(jo["Concurso"].ToString());                
                
            }
        }
        catch (Exception e)
        {
            Debug.WriteLine("Erro: GetConcursoAtivo:-> " + e.Message);
        }

        return dto;

    }

public异步静态任务GetConcursoAtivo（）{
字符串ENDPOINT=“/GetConcursoAtivo”；
字符串URL\u FINAL=URL\u控制器+端点；
ConcursoDTO dto=新ConcursoDTO（）；
尝试
{
HttpClient hClient=新的HttpClient（）；
hClient.DefaultRequestHeaders.Accept.Add（新的MediaTypeWithQualityHeaderValue（“应用程序/json”）；
hClient.DefaultRequestHeaders.Authorization=newauthenticationHeaderValue（“Basic”，BasicAuth.getHash（））；
//得到
HttpResponseMessage response=hClient.GetAsync（URL_FINAL.GetAwaiter（）.GetResult（）；
var resultContent=response.Content.ReadAsStringAsync（）.GetAwaiter（）.GetResult（）；
//Console.WriteLine（“内容：+resultContent”）；
如果（response.StatusCode==HttpStatusCode.OK）{
JObject jo=JObject.Parse（resultContent）；
dto=JsonConvert.DeserializeObject（jo[“Concurso”].ToString（））；
}
}
捕获（例外e）
{
Debug.WriteLine（“Erro:getconcursoative:->”+e.Message）；
}
返回dto；
}

如果你认为这是Hi的翻版，iOS和Android都有自己的背景技术来实现这一点，你可以看看和。