使用C#调用NowPayments API发送HTTP Get请求时出现404错误

问题重现 #

我编写了C#代码调用NowPayments API查询支付发票状态，但返回404错误，Postman调用相同接口却能成功。代码如下：

var client2 = new HttpClient();
var request2 = new HttpRequestMessage(HttpMethod.Get, "https://api.nowpayments.io/v1/payment/" + paymentId);
request2.Headers.Add("x-api-key", apiKey);
var response2 = await client2.SendAsync(request2);
response2.EnsureSuccessStatusCode();
Console.WriteLine(await response2.Content.ReadAsStringAsync());

响应信息：

Response: "{StatusCode: 404, ReasonPhrase: 'Not Found', Version: 1.1, Content: System.Net.Http.StreamContent, Headers:
{
Connection: keep-alive
x-dns-prefetch-control: off
x-frame-options: SAMEORIGIN
strict-transport-security: max-age=15552000; includeSubDomains
strict-transport-security: max-age=15768000
x-download-options: noopen
x-content-type-options: nosniff
x-xss-protection: 1; mode=block
vary: Origin
vary: Accept-Encoding
CF-Cache-Status: DYNAMIC
Report-To: {"endpoints":[{"url":"https://a.nel.cloudflare.com/report/v3?s=OdTo2JBrpASej8JsgICuTwKgyGmvadtradnnwieH8OI6lPFXH38c%2F5YwSg6jUfMQEnKGoTa6Q7TLI4rhrqYt8K0XkdOK0qX%2BsSFmmkF6qfkQqAK66fA46l6ci8gVXusvmuzjoQ%3D%3D"}],"group":"cf-nel","max_age":604800}
NEL: {"success_fraction":0,"report_to":"cf-nel","max_age":604800}
CF-RAY: 7a330879af87b75b-AMS
Date: Sun, 05 Mar 2023 14:26:17 GMT
Server: cloudflare
Content-Length: 82
Content-Type: application/json; charset=utf-8
}}"

已确认API URL和API Key正确，Postman调用正常，目标是获取已创建支付发票的状态。

排查与解决步骤 #

1. 检查paymentId有效性与URL拼接问题 #

手动拼接URL时，若paymentId包含特殊字符（如斜杠、空格、编码字符）会导致URL无效。建议用UriBuilder构建合法URL：

var baseUrl = new Uri("https://api.nowpayments.io/v1/payment/");
var requestUrl = new Uri(baseUrl, paymentId);
var request2 = new HttpRequestMessage(HttpMethod.Get, requestUrl);

同时确认paymentId非空、无多余空格，与Postman中使用的完全一致。

2. 添加必要请求头 #

Postman默认会发送Accept: application/json头，部分API会因缺少该头返回404，补充该头：

request2.Headers.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));

3. 复用HttpClient而非每次实例化 #

频繁创建HttpClient会导致连接池耗尽或请求异常，建议复用单例实例：

// 全局或类级别声明
private static readonly HttpClient _httpClient = new HttpClient();

// 调用时
var request2 = new HttpRequestMessage(HttpMethod.Get, requestUrl);
request2.Headers.Add("x-api-key", apiKey);
request2.Headers.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
var response2 = await _httpClient.SendAsync(request2);

4. 对比Postman请求细节 #

在Postman中点击「Code」按钮选择C# HttpClient代码，对比以下差异：

• 是否有额外请求头（如User-Agent）
• URL是否完全一致（包括末尾斜杠）
• API Key的大小写和格式是否匹配

5. 确认API环境一致性 #

NowPayments分测试环境（https://api-sandbox.nowpayments.io）和生产环境，确保代码与Postman使用同一环境。

修正后的完整代码示例 #

private static readonly HttpClient _httpClient = new HttpClient();

public async Task<string> GetPaymentStatus(string apiKey, string paymentId)
{
    var baseUrl = new Uri("https://api.nowpayments.io/v1/payment/");
    var requestUrl = new Uri(baseUrl, paymentId);
    
    var request = new HttpRequestMessage(HttpMethod.Get, requestUrl);
    request.Headers.Add("x-api-key", apiKey);
    request.Headers.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
    
    var response = await _httpClient.SendAsync(request);
    var responseContent = await response.Content.ReadAsStringAsync();
    if (!response.IsSuccessStatusCode)
    {
        Console.WriteLine($"Error response: {responseContent}");
        response.EnsureSuccessStatusCode();
    }
    return responseContent;
}

内容的提问来源于stack exchange，提问作者user19951222