为什么在创建System.Net.Http.HttpClient()的实例时会出现NullReferenceException？

Question

在我们的Xamarin Forms App中,我们必须从多个API加载大量数据.最简单的方法是直接从PCL使用HttpClient,但是当有数百个它时我得到了这个错误.

有了这个页面,我可以在一个新的PCL项目中重现这个问题.

using System;
using Xamarin.Forms;

namespace HTTPTest
{
public class HTTPTestPage : ContentPage
{
    int Index = 0;

    public HTTPTestPage()
    {
        Content = new Label
        {
            Text = "HTTP Test"
        };
    }

    protected override void OnAppearing()
    {
        base.OnAppearing();
        StartLoading();
    }

    void StartLoading()
    {
        for (int i = 0; i < 400; i++)
        {
            if (string.IsNullOrEmpty(LoadStringFromEndpoint("https://google.de")))
            {
                break;
            }
        }
    }

    public string LoadStringFromEndpoint(string url)
    {
        System.Diagnostics.Debug.WriteLine("LoadFromEndpoint: " + url);
        try
        {
            Index++;
            var client = new System.Net.Http.HttpClient();
            return client.GetStringAsync(url).Result;
        }
        catch (NullReferenceException ex)
        {
            System.Diagnostics.Debug.WriteLine($"{ex.Message} at Index: {Index}");
            System.Diagnostics.Debug.WriteLine(ex.StackTrace);
        }
        return string.Empty;
    }
 }
}

这就是结果:

LoadFromEndpoint: https://google.de
Object reference not set to an instance of an object at Index: 123
  at System.Net.Http.RuntimeOptions.Read () [0x0002c] in /Users/builder/data/lanes/3985/9d6e1ab1/source/xamarin-macios/src/ObjCRuntime/RuntimeOptions.cs:229 
  at System.Net.Http.RuntimeOptions.GetHttpMessageHandler () [0x00000] in /Users/builder/data/lanes/3985/9d6e1ab1/source/xamarin-macios/src/ObjCRuntime/RuntimeOptions.cs:265 
  at System.Net.Http.HttpClient.GetDefaultHandler () [0x00000] in /Library/Frameworks/Xamarin.iOS.framework/Versions/10.4.0.121/src/mono/mcs/class/System.Net.Http/HttpClientEx.cs:36 
  at System.Net.Http.HttpClient..ctor () [0x00000] in /Library/Frameworks/Xamarin.iOS.framework/Versions/10.4.0.121/src/mono/mcs/class/System.Net.Http/HttpClientEx.cs:24 
  at HTTPTest.HTTPTestPage.LoadStringFromEndpoint (System.String url) [0x00020] in [...]HTTPTest/HTTPTestPCL/HTTPTestPage.xaml.cs:41 

如果我得到此异常一次并在之后加载文件,我也会得到一个System.IO.Exception(太多打开的文件).有没有办法解决这个问题(最好不要不使用它)？

Answer 1

你不应该创建那么多的实例HttpClient.这是一个允许同时进行多个连接的对象.所以在你的外面定义你的对象LoadStringFromEndpoint.

"默认的HttpClient是您可以开始发送请求的最简单方式.单个HttpClient可用于同时发送任意数量的HTTP请求,因此在许多情况下您只需创建一个HttpClient,然后将其用于所有请求".

更多信息