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在 C# 中调用 ChatGPT API，您可以使用 .NET 内置的 HTTPClient 类或第三方的 RestSharp 类库。

以下是使用 .NET 内置的 HTTPClient 类调用 ChatGPT API 的示例代码：

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

class Program
{
    static async Task Main(string[] args)
    {
        string url = "https://api.openai.com/v1/engines/davinci-codex/completions";
        string apikey = "YOUR_API_SECRET_KEY";
        string prompt = "Hello, how are you?";
        double temperature = 0.7;
        int max_tokens = 100;

        using (var httpClient = new HttpClient())
        {
            httpClient.DefaultRequestHeaders.Authorization = new System.Net.Http.Headers.AuthenticationHeaderValue("Bearer", apikey);
            httpClient.DefaultRequestHeaders.Accept.Add(new System.Net.Http.Headers.MediaTypeWithQualityHeaderValue("application/json"));

            var requestData = new { prompt = prompt, temperature = temperature, max_tokens = max_tokens };
            var content = new StringContent(Newtonsoft.Json.JsonConvert.SerializeObject(requestData), System.Text.Encoding.UTF8, "application/json");

            var response = await httpClient.PostAsync(url, content);

            if (response.IsSuccessStatusCode)
            {
                var responseContent = await response.Content.ReadAsStringAsync();
                Console.WriteLine(responseContent);
            }
            else
            {
                Console.WriteLine("Failed to call API. StatusCode={0}", response.StatusCode);
            }
        }
    }
}