Python POST 请求的 C# 等效语法是什么？

Question

作为（重新）熟悉 C# 的编程练习，我一直在将我的程序从 Python 移植到 C#。到目前为止一切顺利，但该程序的核心功能之一是获取构建的信息，将其传递给给定的 API，然后取回图像。据我从我找到的教程中可以看出，我已经编写了应该等效的代码，但到目前为止我收到了 400 个错误。

在 Python 中，我的解决方案如下所示：

import requests

#Simple program:
#Create a request to the API and return an image.

url = "https://givenurl.com/api/"
headers = {'Content-Type': 'application/x-www-form-urlencoded'}

dataFile = open("Some Data.txt",'r')
metaFile = open("Some Metadata.txt",'r')

data = dataFile.read()
metadata = metaFile.read()

dataFile.close()
metaFile.close()


totaldata = {'data': data, 'metadata': metadata}
param1 = "foo"
param2 = "bar"

r = requests.post(url=url, data=totaldata , headers=headers, params={"param1": param1, "param2": param2})
print(r.status_code)

file = open("Some Data Picture.jpg", "wb")
file.write(r.content)
file.close()

等效的 C# 代码如下所示：

internal class ThisClass{
    private static HttpClient client;

public ThisClass(){
    client = new HttpClient();
}


public async void imageCreate(dataStruct data)
        {
            //Take a dataStruct data from elsewhere in the program, use the HTTPClient set up at the 
        //beginning of this class and write the file to data.filepath

            string uri = "https://givenurl.com/api/";

            var payload = new Dictionary<string,string> 
        {
            {"data", data.data},
            {"metadata", data.metadata},
            {"param1", data.param1},
            {"param2", data.param2}
        };

            var info = new FormUrlEncodedContent(payload);

            var res = await client.PostAsync(uri, info);

            System.Diagnostics.Debug.WriteLine("Response Code: " + res.StatusCode);

            if (res.IsSuccessStatusCode) {
                Image picture = Image.FromStream(await res.Content.ReadAsStreamAsync());
                picture.Save(data.filename);
            }

    }
        

虽然 Python 脚本每次都有效，但我的 C# 实现每次都返回 400 错误。