HttpClient扩展方法 PostAsJsonAsync 找不到

升级到.Net5后HttpClient多了一些扩展方法，

命名空间:
System.Net.Http.Json
程序集:
System.Net.Http.Json.dll
签名如下：

public static System.Threading.Tasks.Task<System.Net.Http.HttpResponseMessage> PostAsJsonAsync<TValue> (this System.Net.Http.HttpClient client, string? requestUri, TValue value, System.Threading.CancellationToken cancellationToken);

public static System.Threading.Tasks.Task<object?> GetFromJsonAsync (this System.Net.Http.HttpClient client, string? requestUri, Type type, System.Threading.CancellationToken cancellationToken = default);

public static System.Threading.Tasks.Task<System.Net.Http.HttpResponseMessage> PutAsJsonAsync<TValue> (this System.Net.Http.HttpClient client, string? requestUri, TValue value, System.Threading.CancellationToken cancellationToken);

其中我试用了GetFromJsonAsync ,正如我认为的那样，并没有发现什么问题，使用方法如下：

 var response = await httpClient.GetFromJsonAsync<mymodel>(url);

这里重点说一下PostAsJsonAsync，用它来提交数据的示例：

HttpClient httpClient = new HttpClient();
            string url1 = @"https://lebang2020.cn/api/siteinfo";
            var postArgs = new { VIN = "00152F3B1895355561317EAEAFD778843C864520" };
            var response = await httpClient.PostAsJsonAsync(url1, postArgs);
            if (response.IsSuccessStatusCode)
            {
                var resStr = await response.Content.ReadAsStringAsync();
            }

上面代码足矣。还有以下几种示例：

 HttpClient httpClient = new HttpClient();
            string url1 = @"https://lebang2020.cn/api/siteinfo";
            var postArgs = new { VIN = "00152F3B1895355561317EAEAFD778843C864520" };
	  var source = new CancellationTokenSource();
            var response = await httpClient.PostAsJsonAsync(url1, postArgs, source.Token);
            if (response.IsSuccessStatusCode)
            {
                var resStr = await response.Content.ReadAsStringAsync();
            }

或者以下写法：

 HttpClient httpClient = new HttpClient();
            string url1 = @"https://lebang2020.cn/api/siteinfo";
            var postArgs = new { VIN = "00152F3B1895355561317EAEAFD778843C864520" };
	  var source = new CancellationTokenSource();
	 var postContent = JsonContent.Create(postArgs);
            await postContent.LoadIntoBufferAsync();
            var response = await httpClient.PostAsJsonAsync(url1, postContent, source.Token);
            if (response.IsSuccessStatusCode)
            {
                var resStr = await response.Content.ReadAsStringAsync();
            }

最后说一下它的代替方案：以上方法只能用于.Net 5及以上版本，下面这个可以用到.Net Core 3中

 HttpClient httpClient = new HttpClient();
            string url1 = @"https://lebang2020.cn/api/siteinfo";
            var postArgs = new { VIN = "00152F3B1895355561317EAEAFD778843C864520" };
            var response = await httpClient.PostAsync(url1, new StringContent(JsonConvert.SerializeObject(postArgs), Encoding.UTF8, "application/json"));
            if (response.IsSuccessStatusCode)
            {
                //other code .... 
            }

lebang2020.cn