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\"" },
            },
        },
    },
};

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\"" },
            },
        },
    },
};

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

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。

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);