无法从 Url C# Windows 8.1 下载字符串

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.

下载并读取文件的内容

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城市的名称。这是我调用这些函数的 MainPage 代码：

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

我在代码的最后一行遇到错误

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

看来是因为你使用了 await。基本上， await 会将控制权传回给调用函数并允许它继续，直到它被等待，这在你的情况下不会发生，所以它在返回数据之前调用 Cls.StringReturn() 。您可以更改如下：

在您的表单中：

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;
}

在您的下载中class：

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();
    }
}

我已经测试并返回了有效数据，但如果有任何不清楚的地方请告诉我。

无法从 Url C# Windows 8.1 下载字符串

Cannot Download String From An Url C# Windows 8.1

c#

json

windows-8.1