如何以form-data类型调用API？请求发送后抛出格式无效异常

我向API发送POST请求，要求以form-data类型提交，但收到响应后抛出异常：

值'{"Id":"1111111","Message":"something","Description":"something"}'的格式无效。

原代码 #

SendingRequestModel requestModel = new SendingRequestModel();

requestModel.Id = "1111111";
requestModel.Message = "something";
requestModel.Description= "something";

string message = JsonSerializer.Serialize(requestModel);

var serviceSettings = appSettings.ServiceSettings;

var options = new RestClientOptions(serviceSettings.ServiceUrl)
{
    Authenticator = new HttpBasicAuthenticator("username", "password")
};
var client = new RestClient(options);

var request = new RestRequest() { Method = Method.Post, RequestFormat = DataFormat.Json };
request.AddParameter("Content-Type", "multipart/form-data", ParameterType.RequestBody);

using (MultipartFormDataContent formDataContent = new MultipartFormDataContent())
{
    KeyValuePair<string, string>[] keyValuePairs = new[]
    {
        new KeyValuePair<string, string>("message", message),
        new KeyValuePair<string, string>("queue-name", "notification")
    };
    foreach (var keyValuePair in keyValuePairs)
    {
        request.AddParameter(keyValuePair.Value, keyValuePair.Key);
    }
}

var response = await client.ExecuteAsync(request);

问题分析 #

1. 参数顺序完全错误：request.AddParameter的标准参数顺序是(字段名, 值)，但代码里写反成了(值, 字段名)，导致API收到的字段名和内容完全错位，无法识别JSON格式的message值。
2. 错误设置请求格式：RequestFormat = DataFormat.Json会强制RestSharp按JSON格式处理请求体，和form-data的提交需求冲突。
3. 手动设置Content-Type冗余且错误：RestSharp处理form-data请求时会自动生成正确的multipart/form-data请求头，手动添加反而会破坏请求结构。
4. 冗余代码：实例化了MultipartFormDataContent但未实际使用，属于无效代码。

修正后的代码 #

SendingRequestModel requestModel = new SendingRequestModel();

requestModel.Id = "1111111";
requestModel.Message = "something";
requestModel.Description= "something";

string message = JsonSerializer.Serialize(requestModel);

var serviceSettings = appSettings.ServiceSettings;

var options = new RestClientOptions(serviceSettings.ServiceUrl)
{
    Authenticator = new HttpBasicAuthenticator("username", "password")
};
var client = new RestClient(options);

// 无需设置RequestFormat，RestSharp会自动处理form-data类型
var request = new RestRequest() { Method = Method.Post };

// 按正确顺序添加form-data字段：(字段名, 值)
request.AddParameter("message", message);
request.AddParameter("queue-name", "notification");

var response = await client.ExecuteAsync(request);

如果API要求message字段的JSON需明确标注媒体类型，可改用AddStringBody：

request.AddStringBody(message, "application/json");
request.AddParameter("queue-name", "notification");

请求相关截图 #

内容的提问来源于stack exchange，提问作者Celal Sunnetci