我已经找到了问题的答案。

client.DefaultRequestHeaders.Add("X-Version","1");

这将为您的请求添加一个自定义标头

var request = new HttpRequestMessage {
    RequestUri = new Uri("[your request url string]"),
    Method = HttpMethod.Post,
    Headers = {
        { "X-Version", "1" } // HERE IS HOW TO ADD HEADERS,
        { HttpRequestHeader.Authorization.ToString(), "[your authorization token]" },
        { HttpRequestHeader.ContentType.ToString(), "multipart/mixed" },//use this content type if you want to send more than one content type
    },
    Content = new MultipartContent { // Just example of request sending multipart request
        new ObjectContent<[YOUR JSON OBJECT TYPE]>(
            new [YOUR JSON OBJECT TYPE INSTANCE](...){...}, 
            new JsonMediaTypeFormatter(), 
            "application/json"), // this will add 'Content-Type' header for the first part of request
        new ByteArrayContent([BINARY DATA]) {
            Headers = { // this will add headers for the second part of request
                { "Content-Type", "application/Executable" },
                { "Content-Disposition", "form-data; filename=\"test.pdf\"" },
            },
        },
    },
};

HttpRequestMessage类中有一个Headers属性。您可以在那里添加自定义报头，它将随每个HTTP请求一起发送。另一方面，HttpClient类中的DefaultRequestHeaders设置使用该客户端对象发送的每个请求一起发送的报头，因此称为默认请求报头。

希望这能让事情更清楚，至少对将来看到这个答案的人来说。

以下是基于Anubis的回答(这是一个更好的方法，因为它不修改每个请求的头)，但更相当于原始问题中的代码:

using Newtonsoft.Json;
...

var client = new HttpClient();
var httpRequestMessage = new HttpRequestMessage
    {
        Method = HttpMethod.Post,
        RequestUri = new Uri("https://api.clickatell.com/rest/message"),
        Headers = { 
            { HttpRequestHeader.Authorization.ToString(), "Bearer xxxxxxxxxxxxxxxxxxx" },
            { HttpRequestHeader.Accept.ToString(), "application/json" },
            { "X-Version", "1" }
        },
        Content = new StringContent(JsonConvert.SerializeObject(svm))
    };

var response = client.SendAsync(httpRequestMessage).Result;

我在HttpClient头文件中添加了x-api-version，如下所示:

var client = new HttpClient(httpClientHandler)
{
    BaseAddress = new Uri(callingUrl)
};
client.DefaultRequestHeaders.Accept.Clear();
client.DefaultRequestHeaders.Accept.Add(new System.Net.Http.Headers.MediaTypeWithQualityHeaderValue("application/json"));
client.DefaultRequestHeaders.Add("x-api-version", v2);

如果有人想知道如何调用httpClient.GetStreamAsync()，它没有重载，需要HttpRequestMessage来提供自定义头，您可以使用@Anubis给出的上述代码并调用

等待response.Content.ReadAsStreamAsync ()

特别有用，如果你返回一个blob url范围头作为一个FileStreamResult

我的意见。我同意heug的观点。公认的答案是令人费解的。让我们后退一步。

默认头应用于特定HttpClient发出的所有请求。因此，您将为共享头文件使用默认头文件。

_client.DefaultRequestHeaders.UserAgent.ParseAdd(_options.UserAgent);          

然而，我们有时需要特定于某个请求的头文件。因此，我们将在方法中使用类似这样的东西:

public static async Task<HttpResponseMessage> GetWithHeadersAsync(this 
    HttpClient httpClient, string requestUri, Dictionary<string, string> headers)
    {
        using (var request = new HttpRequestMessage(HttpMethod.Get, requestUri))
        {
            foreach(var header in headers)
            {
                request.Headers.Add(header.Key, header.Value);
            }

            return await httpClient.SendAsync(request);
        }
    }

如果你只需要一个额外的非默认头，你可以简单地使用:

request.Headers.Add("X-Version","1")

更多帮助: 如何添加请求头时使用HttpClient

你也可以使用HttpClient。PostAsync方法:

using (HttpClient http_client = new HttpClient())
{
   StringContent string_content = new StringContent(JsonSerializer.Serialize("{\"field\":\"field_value\"}"));
   string_content.Headers.Add("X-Version","1");
   using (HttpResponseMessage response = await http_client.PostAsync("http://127.0.0.1/apiMethodName", string_content))
   {
      if (response.IsSuccessStatusCode)
      {
         Console.WriteLine($"Response status code: {response.StatusCode}");
      }
      else
      {
         Console.WriteLine($"Error status code: {response.StatusCode}");
      }
   }
}