c# .net6 使用multipart/form-data POST上传文件，参数有sp1和sp2，给出范例

在C# .NET 6 中，如果你需要使用`multipart/form-data`格式POST上传文件并附带额外的数据（如sp1和sp2），你可以使用HttpClient或者第三方库如Microsoft.AspNetCore.Http.Client。下面是一个基本的示例，展示如何通过HttpClient发送这样的请求：

using System.Net.Http;
using System.IO;
using System.Text;

public async Task UploadFileWithFormDataAsync(string url, string sp1, string sp2, Stream fileStream)
{
    // 创建一个HttpClient实例
    var client = new HttpClient();

    // 创建MultipartContent对象作为主体
    var content = new MultipartFormDataContent();
    
    // 添加文本参数
    var formContent = new StringContent($"sp1={sp1}&sp2={sp2}", Encoding.UTF8, "application/x-www-form-urlencoded");
    content.Add(formContent);

    // 添加文件参数
    var fileContent = new StreamContent(fileStream);
    fileContent.Headers.ContentType = new MediaTypeHeaderValue("multipart/form-data");
    fileContent.Headers.ContentDisposition = new ContentDispositionHeaderValue("form-data")
    {
        Name = "file", // 文件名字段
        FileName = Path.GetFileName(fileStream.Name) // 获取流对应的文件名
    };
    content.Add(fileContent);

    try
    {
        // 发送POST请求
        HttpResponseMessage response = await client.PostAsync(url, content);
        
        // 检查响应状态码
        if (response.IsSuccessStatusCode)
        {
            Console.WriteLine($"Upload successful with status {response.StatusCode}");
        }
        else
        {
            Console.WriteLine($"Upload failed with status {response.StatusCode} and reason {response.ReasonPhrase}");
        }
    }
    catch (Exception ex)
    {
        Console.WriteLine($"Error occurred during upload: {ex.Message}");
    }
}

// 调用函数时提供文件流、sp1和sp2值
string filePath = "path_to_your_file";
FileStream fileStream = File.OpenRead(filePath);
string sp1 = "value1";
string sp2 = "value2";

await UploadFileWithFormDataAsync("http://your-api-url.com/upload", sp1, sp2, fileStream);