c# .net6 下载文件提示The SSL connection could not be established，给出处理范例

在C# .NET 6中，遇到"The SSL connection could not be established"这种错误通常是由于SSL/TLS证书的问题，可能是服务器证书不受信任、过期，或者是系统证书存储设置问题。以下是处理这种情况的一个常见范例：

using System;
using System.Net.Http;
using System.Net.Security;
using System.Security.Cryptography.X509Certificates;

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
    }

    private async void btnDownload_Click(object sender, EventArgs e)
    {
        try
        {
            // 自定义认证策略，允许不受信任的证书
            ServicePointManager.ServerCertificateValidationCallback += (_, _, _, _) =>
            {
                return true; // 假设这里接受所有证书，生产环境应替换为实际的信任检查逻辑
            };

            // 设置需要下载的URL
            string url = "https://example.com/file-to-download.pdf";
            using HttpClient httpClient = new HttpClient();

            // 开始异步请求
            HttpResponseMessage response = await httpClient.GetAsync(url);

            // 处理响应...
        }
        catch (HttpRequestException ex) when (ex.InnerException is SslPolicyErrorsException sslEx)
        {
            // 处理SSL连接错误
            switch (sslEx.Status)
            {
                case SslPolicyErrors.RemoteCertificateNameMismatch:
                    MessageBox.Show("远程证书名称与预期不符。");
                    break;
                case SslPolicyErrors.RemoteCertificateChainErrors:
                    MessageBox.Show("远程证书链存在问题。");
                    break;
                default:
                    MessageBox.Show($"SSL连接错误：{sslEx.StatusDescription}");
                    break;
            }
        }
        catch (Exception ex)
        {
            MessageBox.Show($"下载过程中发生错误：{ex.Message}");
        }
    }
}

这段代码首先允许所有证书通过自定义的验证回调函数，然后捕获`SslPolicyErrorsException`来识别具体的SSL错误，并向用户显示适当的错误消息。