.Net HttpClient中如何设置Content-Type为application/x-www-form-urlencoded？

错误原因 #

Content-Type属于内容头，而非请求级别的头部，必须绑定到HttpContent对象上，不能直接添加到HttpRequestMessage.Headers集合中，这就是错误提示的核心原因。

解决方法 #

方法1：使用FormUrlEncodedContent（推荐）

该类会自动设置正确的Content-Type头，同时方便构造表单参数：

using (var httpClient = new HttpClient())
{
    // 构造表单参数（根据接口需求添加，无参数可留空）
    var formParams = new Dictionary<string, string>
    {
        // {"paramName", "paramValue"}
    };

    var request = new HttpRequestMessage
    {
        Method = HttpMethod.Post,
        RequestUri = new Uri(path),
        // 用FormUrlEncodedContent作为请求内容，自动携带Content-Type
        Content = new FormUrlEncodedContent(formParams)
    }; 

    request.Headers.Add("Accept", "application/json");

    HttpResponseMessage response1 = await httpClient.SendAsync(request);
    var token = await response1.Content.ReadAsStringAsync();
}

方法2：手动为HttpContent设置Content-Type头

如果接口仅需要该头而无需请求体，可手动创建空内容并设置头：

using (var httpClient = new HttpClient())
{
    var request = new HttpRequestMessage
    {
        Method = HttpMethod.Post,
        RequestUri = new Uri(path),
        Content = new StringContent(string.Empty) // 空请求内容
    }; 

    request.Headers.Add("Accept", "application/json");
    // 为内容对象设置Content-Type头
    request.Content.Headers.ContentType = new System.Net.Http.Headers.MediaTypeHeaderValue("application/x-www-form-urlencoded");

    HttpResponseMessage response1 = await httpClient.SendAsync(request);
    var token = await response1.Content.ReadAsStringAsync();
}

内容的提问来源于stack exchange，提问作者Niranga