.NET Framework 4.5.2 C#桌面应用调用REST API遇连接错误求助

我用.NET Framework 4.5.2开发C#桌面应用，调用API获取Token时遇到以下错误，该请求在Postman中可正常运行：

One or more errors occurred.System.Net.Http.HttpRequestException: An error occurred while sending the request. --->
System.Net.WebException: The underlying connection was closed: An unexpected error occurred on a send. --->
System.IO.IOException: Unable to read data from the transport connection: An existing connection was forcibly closed by the remote host. --->
System.Net.Sockets.SocketException: An existing connection was forcibly closed by the remote host

我的代码如下：

string username = "username";
string password = "password";
string apiUrl = "https://.../auth/token";

var httpClientHandler = new HttpClientHandler
{
    AllowAutoRedirect = false,
    UseCookies = false
};

using (var httpClient = new HttpClient(httpClientHandler))
{
    httpClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue(
        "Basic",
        Convert.ToBase64String(System.Text.Encoding.ASCII.GetBytes($"{username}:{password}"))
    );

    var requestContent = new StringContent(
        "{ \"terminal_id\": \"95000059\" }",
        System.Text.Encoding.UTF8,
        "application/json"
    );


    try
    {
        var response = httpClient.PostAsync(apiUrl, requestContent).Result;
        if (response.IsSuccessStatusCode)
        {
            var responseContent = response.Content.ReadAsStringAsync().Result;
            richTextBox1.Text = responseContent;
        }
        else
        {
            richTextBox1.Text = $"Failed to get token. Status code: {response.StatusCode}";
        }
    }
    catch (HttpRequestException ex)
    {
        richTextBox1.Text = ex.Message;
    }
}

解决方法 #

1. 启用TLS 1.2支持 #

.NET Framework 4.5.2默认未启用TLS 1.2，而多数现代API服务器仅支持TLS 1.2及以上版本，Postman默认兼容该协议，这是最可能的问题根源。在创建HttpClient前添加以下代码：

System.Net.ServicePointManager.SecurityProtocol = System.Net.SecurityProtocolType.Tls12;

修改后的完整代码示例：

string username = "username";
string password = "password";
string apiUrl = "https://.../auth/token";

// 启用TLS 1.2
System.Net.ServicePointManager.SecurityProtocol = System.Net.SecurityProtocolType.Tls12;

var httpClientHandler = new HttpClientHandler
{
    AllowAutoRedirect = false,
    UseCookies = false
};

using (var httpClient = new HttpClient(httpClientHandler))
{
    httpClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue(
        "Basic",
        Convert.ToBase64String(System.Text.Encoding.ASCII.GetBytes($"{username}:{password}"))
    );

    var requestContent = new StringContent(
        "{ \"terminal_id\": \"95000059\" }",
        System.Text.Encoding.UTF8,
        "application/json"
    );

    try
    {
        var response = httpClient.PostAsync(apiUrl, requestContent).Result;
        if (response.IsSuccessStatusCode)
        {
            var responseContent = response.Content.ReadAsStringAsync().Result;
            richTextBox1.Text = responseContent;
        }
        else
        {
            richTextBox1.Text = $"Failed to get token. Status code: {response.StatusCode}";
        }
    }
    catch (HttpRequestException ex)
    {
        richTextBox1.Text = ex.Message;
    }
}

2. 对齐请求头与Postman配置 #

对比Postman中的请求头，确保代码包含所有必要字段。例如Postman会自动添加User-Agent，可手动补充：

httpClient.DefaultRequestHeaders.UserAgent.ParseAdd("Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/114.0.0.0 Safari/537.36");

3. 改用异步调用模式（可选） #

使用.Result同步调用异步方法可能引发死锁或异常，建议改为异步写法：

private async void GetTokenAsync()
{
    string username = "username";
    string password = "password";
    string apiUrl = "https://.../auth/token";

    System.Net.ServicePointManager.SecurityProtocol = System.Net.SecurityProtocolType.Tls12;

    var httpClientHandler = new HttpClientHandler
    {
        AllowAutoRedirect = false,
        UseCookies = false
    };

    using (var httpClient = new HttpClient(httpClientHandler))
    {
        httpClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue(
            "Basic",
            Convert.ToBase64String(System.Text.Encoding.ASCII.GetBytes($"{username}:{password}"))
        );

        var requestContent = new StringContent(
            "{ \"terminal_id\": \"95000059\" }",
            System.Text.Encoding.UTF8,
            "application/json"
        );

        try
        {
            var response = await httpClient.PostAsync(apiUrl, requestContent);
            if (response.IsSuccessStatusCode)
            {
                var responseContent = await response.Content.ReadAsStringAsync();
                richTextBox1.Text = responseContent;
            }
            else
            {
                richTextBox1.Text = $"Failed to get token. Status code: {response.StatusCode}";
            }
        }
        catch (HttpRequestException ex)
        {
            richTextBox1.Text = ex.Message;
        }
    }
}

内容的提问来源于stack exchange，提问作者Orgil