如何将同步的VB.NET Web请求函数转换为异步方法？

我来帮你把这个同步的Web请求函数改成异步版本，其实现在更推荐用HttpClient（原生支持异步，API更简洁），不过我也会提供基于你原有HttpWebRequest的异步改造方案，同时附上C#版本供参考~

一、VB.NET 实现方案 #

1. 推荐：使用 HttpClient 异步版本 #

HttpClient是.NET中官方推荐的HTTP请求工具，异步支持更完善，而且建议复用同一个实例（不要每次请求都新建，避免耗尽套接字资源）。

Imports System.Net.Http
Imports System.Threading.Tasks

' 建议把HttpClient作为类的静态成员复用，避免频繁创建释放
Private Shared ReadOnly _httpClient As New HttpClient()

Public Async Function GetPostWebServiceAsync(urlget As String, stringdata As String) As Task(Of String)
    Try
        ' 先检查网络状态
        If Not internet_on_line Then
            Return "[ERROR]"
        End If

        ' 构建请求URL，和原逻辑一致
        Dim milliseconds = CLng(DateTime.UtcNow.Subtract(New DateTime(1970, 1, 1)).TotalMilliseconds)
        Dim uncache = milliseconds.ToString()
        Dim url = $"https://{apiUrl}/webservice/{urlget}&data={stringdata}&uncache={uncache}"

        Console.WriteLine("URL: " & url)

        ' 设置HttpClient的相关配置（比如TLS1.2、UserAgent）
        _httpClient.DefaultRequestHeaders.UserAgent.ParseAdd("Mozilla/4.0 (compatible)")
        ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12

        ' 异步发送GET请求并获取响应内容
        Dim result = Await _httpClient.GetStringAsync(url)
        Return result
    Catch ex As Exception
        Console.WriteLine($"GetPostWebServiceAsync Falhou: {ex.Message}")
        ' 修正原代码的顺序：先处理异常再返回错误标识
        Dim funcName = New StackTrace(ex).GetFrame(0).GetMethod().Name
        If funcName = "GetStringAsync" Then
            writeExeption(ex, False)
        Else
            writeExeption(ex, True)
        End If
        Return "[ERROR]"
    End Try
End Function

2. 基于原有 HttpWebRequest 的异步改造 #

如果你想保留原有的HttpWebRequest逻辑，只需要把同步方法替换为对应的异步版本（比如GetResponseAsync代替GetResponse，异步读取流）：

Imports System.Net
Imports System.IO
Imports System.Threading.Tasks

Public Async Function GetPostWebServiceAsync(urlget As String, stringdata As String) As Task(Of String)
    Dim result As String = ""
    Try
        If Not internet_on_line Then
            Return "[ERROR]"
        End If

        Dim milliseconds = CLng(DateTime.UtcNow.Subtract(New DateTime(1970, 1, 1)).TotalMilliseconds)
        Dim uncache = milliseconds.ToString()
        Dim url = $"https://{apiUrl}/webservice/{urlget}&data={stringdata}&uncache={uncache}"

        Console.WriteLine("URL: " & url)

        ServicePointManager.Expect100Continue = True
        ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12

        Dim targetURI As New Uri(url)
        Dim fr = DirectCast(WebRequest.Create(targetURI), HttpWebRequest)
        fr.UserAgent = "Mozilla/4.0 (compatible)"
        fr.Timeout = 15000

        ' 异步获取响应
        Using response = Await fr.GetResponseAsync()
            ' 异步读取流内容
            Using stream = response.GetResponseStream()
                Using str As New StreamReader(stream)
                    result = Await str.ReadToEndAsync()
                End Using
            End Using
        End Using

        Return result
    Catch ex As Exception
        Console.WriteLine($"GetPostWebServiceAsync Falhou: {ex.Message} - {result}")
        ' 修正原代码的顺序：先处理异常再返回错误标识
        Dim funcName = New StackTrace(ex).GetFrame(0).GetMethod().Name
        If funcName = "GetResponseAsync" Then
            writeExeption(ex, False)
        Else
            writeExeption(ex, True)
        End If
        Return "[ERROR]"
    End Try
End Function

二、C# 实现方案 #

1. HttpClient 异步版本 #

using System;
using System.Net.Http;
using System.Threading.Tasks;

public class WebServiceHelper
{
    // 复用HttpClient实例，避免频繁创建
    private static readonly HttpClient _httpClient = new HttpClient();

    public async Task<string> GetPostWebServiceAsync(string urlget, string stringdata)
    {
        try
        {
            if (!internet_on_line) // 确保internet_on_line变量在当前上下文可用
                return "[ERROR]";

            long milliseconds = (long)DateTime.UtcNow.Subtract(new DateTime(1970, 1, 1)).TotalMilliseconds;
            string uncache = milliseconds.ToString();
            string url = $"https://{apiUrl}/webservice/{urlget}&data={stringdata}&uncache={uncache}";

            Console.WriteLine($"URL: {url}");

            _httpClient.DefaultRequestHeaders.UserAgent.ParseAdd("Mozilla/4.0 (compatible)");
            System.Net.ServicePointManager.SecurityProtocol = System.Net.SecurityProtocolType.Tls12;

            string result = await _httpClient.GetStringAsync(url);
            return result;
        }
        catch (Exception ex)
        {
            Console.WriteLine($"GetPostWebServiceAsync Failed: {ex.Message}");
            var funcName = new System.Diagnostics.StackTrace(ex).GetFrame(0).GetMethod().Name;
            if (funcName == "GetStringAsync")
                writeExeption(ex, false); // 确保writeExeption方法在当前上下文可用
            else
                writeExeption(ex, true);
            return "[ERROR]";
        }
    }
}

2. HttpWebRequest 异步版本 #

using System;
using System.IO;
using System.Net;
using System.Threading.Tasks;

public async Task<string> GetPostWebServiceAsync(string urlget, string stringdata)
{
    string result = "";
    try
    {
        if (!internet_on_line)
            return "[ERROR]";

        long milliseconds = (long)DateTime.UtcNow.Subtract(new DateTime(1970, 1, 1)).TotalMilliseconds;
        string uncache = milliseconds.ToString();
        string url = $"https://{apiUrl}/webservice/{urlget}&data={stringdata}&uncache={uncache}";

        Console.WriteLine($"URL: {url}");

        ServicePointManager.Expect100Continue = true;
        ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;

        Uri targetURI = new Uri(url);
        HttpWebRequest fr = (HttpWebRequest)WebRequest.Create(targetURI);
        fr.UserAgent = "Mozilla/4.0 (compatible)";
        fr.Timeout = 15000;

        // 异步获取响应
        using (var response = await fr.GetResponseAsync())
        {
            using (var stream = response.GetResponseStream())
            {
                using (var str = new StreamReader(stream))
                {
                    result = await str.ReadToEndAsync();
                }
            }
        }

        return result;
    }
    catch (Exception ex)
    {
        Console.WriteLine($"GetPostWebServiceAsync Failed: {ex.Message} - {result}");
        var funcName = new System.Diagnostics.StackTrace(ex).GetFrame(0).GetMethod().Name;
        if (funcName == "GetResponseAsync")
            writeExeption(ex, false);
        else
            writeExeption(ex, true);
        return "[ERROR]";
    }
}

关键注意点 #

• 异步方法需要用Async（VB.NET）/async（C#）关键字修饰，返回类型为Task(Of T)（VB.NET）/Task<T>（C#）
• 所有同步的阻塞调用都要替换为对应的异步版本，并用Await（VB.NET）/await（C#）等待完成
• 修正了原代码的逻辑错误：原Catch块中Return语句会跳过后续的异常处理代码，现在调整为先处理异常再返回错误标识
• HttpClient建议复用实例，不要每次请求都新建，避免耗尽套接字资源

备注：内容来源于stack exchange，提问作者Luiz