Question

我是C＃编程新手。我想使用openweathermap API开发一个简单的天气应用程序。我想从URL下载和读取文件的内容。

这是我下载文件内容的代码：

 class WebClientToDownload
{
    string webresponse;

    public async void DownloadFile(string url)
    {
        string baseurl = "http://api.openweathermap.org/data/2.5/forecast/daily?q=";
        StringBuilder sb = new StringBuilder(baseurl);
        sb.Append(url + "&mode=json&units=metric&cnt=7");
        string actual = sb.ToString();
        HttpClient http = new System.Net.Http.HttpClient();
        HttpResponseMessage response = await http.GetAsync(actual);
        webresponse = await response.Content.ReadAsStringAsync();           
    }

    public string StringReturn()
    {
        return webresponse;
    }

传递给函数的

字符串是city的名称。这是MainPage代码，我称之为这些函数：

 string JSONData;
    private void GetWeatherButton_Click(object sender, RoutedEventArgs e)
    {
        WebClientToDownload Cls = new WebClientToDownload();
        Cls.DownloadFile(GetWeatherText.Text);
        JSONData = Cls.StringReturn();
        JSONOutput.Text = JSONData;
    }

我在最后一行代码中收到错误，如

An exception of type 'System.ArgumentNullException' occurred in mscorlib.dll but was not handled in user code

Additional information: Value cannot be null.

Answer 1

看起来好像是等待你的使用。基本上，await会将控制权传递回调用函数并允许它继续运行，直到等待它为止，这在您的情况下不会发生，因此它在返回数据之前调用Cls.StringReturn（）。您可以更改如下：

以您的形式：

string JSONData;
// Note the async keyword in the method declaration.
private async void GetWeatherButton_Click(object sender, EventArgs e)
{
    WebClientToDownload Cls = new WebClientToDownload();
    // Notice the await keyword here which pauses the execution of this method until the data is returned.
    JSONData = await Cls.DownloadFile(GetWeatherText.Text);
    JSONOutput.Text = JSONData;
}

在您的下载课程中：

class WebClientToDownload
{
    // Notice this now returns a Task<string>.  This will allow you to await on the data being returned.
    public async Task<string> DownloadFile(string url)
    {
        string baseurl = "http://api.openweathermap.org/data/2.5/forecast/daily?q=";
        StringBuilder sb = new StringBuilder(baseurl);
        sb.Append(url + "&mode=json&units=metric&cnt=7");
        string actual = sb.ToString();
        HttpClient http = new System.Net.Http.HttpClient();
        HttpResponseMessage response = await http.GetAsync(actual);
        // Return the result rather than setting a variable.
        return await response.Content.ReadAsStringAsync();
    }
}

我已经过测试并返回了有效数据但如果其中任何一项不清楚，请告知我们。

无法从网址C＃Windows 8.1下载字符串

1 个答案: